the art of encoding flash video: about the flv format

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The Art of Encoding Flash Video: About the FLV Format

By: Derrick Ypenburg

The process of encoding quality streaming video for Flash (or for any video streaming playback deliveryfor that matter) is the most important process in creating video streaming applications. Any slick-lookingvideo player that you build will only be as good as the quality of the video streaming through it.

This article provides a comprehensive overview of FLV encoding by the encoding software. Understanding this article will help you create higher quality FLV files when it comes to applying settings in an encoder and creating video for encoding.

About Flash Video

Video for Flash basically comes in one flavor: the FLV format. Flash will only stream a video that hasbeen encoded into this format. Videos using the FLV format, however, do come in a couple of differentversions. These versions are defined by the CODEC (method of compression/decompression) that wasused to encode a video to and FLV.

The two different CODECs used to encode video to FLV are:

Sorenson Spark (developed by Sorenson Media (http://www.sorensonmedia.com/) )

On2VP6 (developed by On2 Technologies (http://www.on2.com/) )

The above CODECs are not necessarily just an "option" when deciding what CODEC you should use to encode a video, they are Flash Player version specific CODECs.

The Sorenson Spark CODEC was around for Flash player 6 and 7. The On2VP6 CODEC is new to Flash Player 8 (although Flash player 8 will stream Sorenson Spark encoded videos). For more information on how to choose a CODEC for streaming delivery, you can find more details in Rob Reinhardt's article Choosing a Video Codec for Your Flash 8 Movie (http://www.communitymx.com/abstract.cfm?cid=BD063) .

For a long definition of what a CODEC is, you find it in the Wikipedia here (http://en.wikipedia.org/wiki/Codec) .

About Compression for Flash Video

Both the Sorenson Spark and On2VP6 CODECs use interframe and intraframe encoding methods.These methods result in a lower bandwidth for high-quality video. These methods are also known as Spatial/Temporal compression.

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Spatial Compression:

Reducing data within one frame

Image representing one frame can be compressed using lossless or lossy type compression

A spatially compressed frame is referred to as an intraframe or a keyframe

Temporal Compression:

Frames that only store the difference between one or more frames

Video data stored in the succeeding frame is the difference between frames

Keyframes are frames that act as a frame for comparison

The difference between frames are called interframes

Flash encoders use both spatial and temporal compression.

In a nutshell, an intraframe takes a high quality snapshot of what is currently in a frame. An interframe only encodes any differences that occur between the preceding intraframe and the current frame being encoded. This results in only encoding differences that are happening in the video from frame-to-frame and not compressing redundant images across a video. This frees up file space to focus on more complicated sequences, or, it just frees up file space and it is not used (making parts of the video faster to download and result in a less congested stream delivery).

Keyframes

When I discussed intraframes previously, I also referred to them as keyframes. Keyframes are a terminology you might be familiar with. Not the keyframes you use on the timeline in Flash though. The keyframes I'm talking about are a setting in any Flash video encoder.

Keyframes (or intraframes) are baseline frames that subsequent frames (interframes) are compared against (only the differences between keyframes and interframes are encoded).

The keyframe interval settings in any Flash encoder provides the option to specify how often you'd likekeyframes to occur in the video. Keyframes intervals are defined by frames. For example, a keyframeinterval of 60 for a 30fps video will insert a keyframe every 60 frames (or 2 seconds).

Increasing the number of keyframes (smaller frame interval) will:

increase quality of video

produce a larger file size and can exceed desired data rate

create possible performance issues on less powerful systems

Decreasing the number of keyframes will:

decrease the quality of video

result in a smaller file size

result in lower data rates

So how do you choose a data rate?

Higher motion, color, edits video should have a more frequent keyframe rate.



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Slower video, talking head shots can have less keyframes (less frequent keyframe interval).

Other Things to Know About Keyframes

Keyframes affect seeking of a video. Flash video will only pause, and seek to, a keyframe. If the user pauses at, or seeks to a frame that is not a keyframe, Flash will jump to the next keyframe and pause thereor start playing from it. If accurate pausing and seeking is important for your video, a more frequent keyframe interval will be required.

Cue points encoded into a video are inserted as keyframes. These keyframes also have embedded datathat was entered when the cue point was created in the encoding software. If you want to have a video that has chapters, and you want to have navigation that seeks the video to exact times for each chapter, insert an empty cue point at the precise times to ensure each chapter starts in the exact spot you want it start at when seeked to.

A Review of Data Rates

A data rate is a measure of bandwidth, i.e. how much can be delivered through a medium.

Data rates are measured in:

Kilobits per second (Kbps) ~ 1000 bits/sec

Megabits per second (Mbps) ~ 1,000,000 bits/sec

A bit is 8x smaller than a byte (don't mistake the two)

How to Choose Target Data Rates

When choosing the best data rates for your streaming applications, you ned to know your audience.

You basically have broadband users, dial-up users and connection speeds in between such as ISDN.

A good reference chart with the connection speeds (in order) as well as clocking your bandwidth and see where you fit in, check out this great app at InternetFrog (http://www.internetfrog.com/mypc/speedtest/) .com.

I find a lot of developers are dropping the dial-up market completely. I'm not kidding. I have to convincesome clients to re-consider the dial-up market. Especially with the new On2VP6 CODEC, Flash video fordial-up connections looks better than ever.

It's not only the dial-up market that gets data rates of 56Kbps and less. I've clocked my connection slowerthan that when I was hooked-up to a slow WiFi connection at O'Hare airport in Chicago. There is agrowing wireless market out there that gets a full range of low-to-high data rates.

In other words, consider all users and demographics. They are also purchasers of products and viewers ofadverting $$$.

When encoding Flash video for different data rates, you have to encode a separate file for each data rate. You can then give the user an option of which data rate they want to play, or, you can program some bandwidth detection (I will write an article on bandwidth detection later this fall).



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Below is a list of common data rates for different connection speeds (The ones bolded are my preferredrates. I'll explain why after):

Encoded at 1 Mbps - 1024 Kbps connections

Encoded at 700 Kbps - 768 connections

Encoded at 450 Kbps - 512 Kbps connections (any connection fast enough)

Encoded at 350 Kbps - 384 Kbps connections (Satellite, slow high-speed connections)

Encoded at 208 Kbps - 256 Kbps connections

Encoded at 80 Kbps - 128 Kbps connections (ISDN)

Encoded at 48 Kbps - 46 Kbps connections (Dial-up)

So if all the data rates listed above are the most common ranges, why not encode a video and detect the data rate to cover all of them? Why not make one higher than 450 Kbps?

I'm definitively a proponent of accommodating dial-up/mobile-connected users still so I encode down at 48 Kbps.

The 80 Kbps range takes care of the not-quite high-speed crowd and the 350Kbps takes care of the usersthat have high-speed but not very fast high-speed.

The 450 Kbps data rate takes care of everything else above 450 Kbps. In most cases, the common video Iencode is 320 x 240 in display size. I can pretty much get any video at that size looking terrific at 450 Kbps. If the video is not going to be blown-up full screen, there is no reason to encode at a higher data rate. You are just delivery unnecessary bandwidth from your streaming server that is not needed as the video won't look much better at the cost of higher bandwidth.

Note: Overall data rates listed above include both audio and video data rates combined.

Video Properties That Affect Encoding

We now have a better understanding of what goes into encoding an FLV, now let's look at other factors that affect the encoding process.

There are many video properties that affect the quality of encoded video. I'll break a list down into two categories: Properties that affect encoding quality, and, properties that are affected by encoding.

Properties that Affect Encoding Quality (during encoding)

Color and movement - The more color and movement a video has, the more information that needs to be stored to represent all the changes and color ranges.

Edits, Transitions, Pans and Zooms - All of these make changes in the video. Not only a change, but a gradual change over time. This kind of stuff drives the encoder nuts. Although we might not see everything that is changing, the computer sees everything.

Frame rate - The more frames that are encoded, the more information that needs to be stored and less data rate toshare over time.

Display size - The larger the display size, the more pixels and colors that need to be encoded.

Sound ranges - Encoding unnecessary sound ranges (deep vibrating bass's that are not heard and very high pitches) take up file size and are not really heard unless the volume is cranked. Some encoding software offers a range selector for omitting high and low audio frequencies.

Interlacing - Video that was intended for television or that was filmed without a progressive scan camera is



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interlaced video. Interlacing is the method of 2 fields of lines that are combined to create a single frame. Frames of interlaced video appear with a "combing" effect at times are when paused. This is one of the most important things you need to take care of when encoding video.

Properties That are Affected By Encoding (during playback)

Image quality

Smoothness of playback

Audio quality

Transitions and effects

Text and images in video

Methods of Improving Quality of Video

Aside from above-mentioned considerations, there are a lot of ways to improve encoded video quality. Below is a list of different methods of improving video quality that go above and beyond playing with data rates and keyframes.

The Flash 8 Video Encoder that ships with Flash 8 Professional does not perform all of the items mentioned below. I will mention which ones the Flash 8 Video Encoder does not do. I will also discuss software that will do all of these things afterwards.

2-Pass vs. single pass encoding - Single pass encoding analyses a video and encodes it in the same pass. 2-passencoding has a first pass just for analyzing a video and a second pass just for the encoding process. Single pass encoding is faster, but is less in quality as where 2-pass is higher quality but takes longer to encode. 2-pass encoding in other words, increases encoded video quality. (The Flash 8 Video Encoder is single pass only).

Pre-processing filters - Other encoding software (sorry, not the Flash 8 Video Encoder) ships with pre-processing filters. Pre-processing filters allow you to make adjustments to a video that will increase encoding quality.

One example is a filter called Black Restore. This filter corrects all blacks in a video that have become shades ofgray when it was digitized. Multiple values of gray instead of black affect image quality as well as consume more file size. restoring all dark grays to blacks improves the quality of black images and requires less file size.

De-interlacing - Video that is interlaced can give a "combing" effect when played back digitally. De-interlacing interlaced video will combine to two fields of lines together to make a solid image for each frame. This is one of the most important factors in getting good quality video for digital playback. This method is not included with the Flash 8 Video Encoder.

Keyframes - As mentioned before, analyzing movement and edits and transitions in a video will help you determine lower or higher keyframe intervals for better quality video vs. data rate. You can adjust keyframe intervals in the Flash 8 Video Encoder.

Smaller display size - Reducing the frame size of a video to 320 x 240 for higher bandwidths and smaller for lower bandwidths will result in a higher quality image. Frame sizes can be adjusted in the Flash 8 Video Encoder.

Reducing Frame Rates - Reducing the frame rate of a 30 fps video to 15 fps will result in higher quality images. The downside however is that the video can appear to be choppy for high motion video.

When you drop the frame rate of a video, it is best to reduce the frame rate using good math. Reduce frames ratesby half or quarters. Arbitrary frame rate reduction can result in audio synchronization problems and choppy playback.

As I mentioned, the free Flash 8 Video Encoder does not perform much of the methods above, and more methods that are offered by 3rd party encoding applications such as Sorensen Squeeze(http://www.SorensonMedia.com) and the On2 Flix encoder (http://www.on2.com/) . By Adobe's own admittance, the free Flash 8 Video Encoder was included with Flash 8 Professional to get people going



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on flash video, but recommends going with Sorenson or On2 for higher end encoding.

Conclusion

Now that you are experts in what happens behind the scenes for encoding video to the FLV format, some of the settings that you see in your encoders will make more sense and we can make better decisions on what settings to adjust to achieve higher quality encoded video.

Part 2 of this article will cover the encoding process using the Flash 8 Video Encoder vs. Sorenson Squeeze 4.3 (and all of its added features). I will demonstrate side-by-side comparisons with videos encoded in both the Flash 8 Video Encoder and Sorenson Squeeze using the exact same settings (but withthe extra features offered in Squeeze) and see the differences in quality.

Keywordsflv, encoding, sorenson, video, audio

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The Art of Encoding Flash Video: Sorenson Squeeze


If you want to dramatically increase the quality of encoded video, you need to go beyond the free Flash 8Video Encoder that ships with Flash 8 Professional. Third-party software such as On2 Flix Exporter forFlash (http://on2.com/consumer/flix-exporter) and Sorenson Squeeze 4.5(http://www.sorensonmedia.com/pages/?pageID=3) are the best options for encoding video with superior quality. Third-party encoding software also has many additional options and settings that the standardFlash encoder does not include: de-interlacing options, pre-processing filters and greater control ofaudio/video data rate and compression settings. These are all enhanced options that any encodingprofessional needs to make the best video they can for streaming delivery.

If you want to shake your "money-maker" with top-notch video, then this article is for you. This articlewill go into detail of all the advanced settings of Sorenson Squeeze and make quality comparisons tovideo encoded with the Flash 8 Video Encoder.

If you'd like to educate yourself more about Flash video encoding,, please read part 1 of this series, The Art ofEncoding Flash Video: About the FLV Format (http://www.communitymx.com/abstract.cfm?cid=9AEA3) . You may also want to take a look at Tom Green's article, FLV Creation in Flash Professional 8(http://www.communitymx.com/abstract.cfm?cid=553e4) .

The article will use Sorenson Squeeze 4.5 for encoding Flash video. If you'd like to know more about SorensonSqueeze,you can visit the Sorenson Squeeze info page (http://www.sorensonmedia.com/pages/?pageID=3) . You can also download a free limited trial here (http://www.sorensonmedia.com/pages/?pageID=8) .

About Sorenson Squeeze

Sorenson Media (http://www.sorensonmedia.com/pages/?pageID=3) has been in the encoding business for a long time now. Squeeze has always been the standard in 3rd-party video encoding applications for Flashand other streaming formats. Before the advent of the On2VP6 codec (created by On2 Technologies(http://on2.com/consumer/flix-exporter) ) for Flash 8 Video, the Sorenson Spark codec was used forencoding video into FLV format. Sorenson supports the On2VP6 codec in its Squeeze applicationsversion 4.2 and up. The current release of Sorenson Squeeze is version 4.5.

If any of the terminologies above seem new to you, please read part 1 of this series, The Art of Encoding Flash Video:About the FLV Format (http://www.communitymx.com/abstract.cfm?cid=9AEA3) before you move on.



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Sorenson Squeeze vs Flash 8 Video Encoder

There are 2 main advantages of using Sorenson Squeeze over the free Flash 8 Video Encoder:

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Constant vs. Variable bitrates: A video can be encoded utilizing the data rate in two (2) ways:

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Single-pass vs. 2-pass encoding:

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Even though a video might look fine to the eye, the encoder notices everything. The smallest pixel of color that movesdifferently from the rest can cause extra encoding work and loss of quality. Think about how gradients in imagescompress when they are saved as GIFs and JPEGs. The pixel difference throughout the gradient starts banding and thequality of the gradient suffers. This is the same when video frames are encoded.

Advanced Encoding Practices

Now that you are familiar with Sorenson Squeeze, we are going to create some high-quality encodedvideo. To show the immediate difference between Squeeze and the Flash 8 Video Encoder, preview thetwo example videos below (Comparison 1). Both have the exact same settings but they are encoded withdifferent software. The first time you may not see a difference but a few comparisons should show youthat the colors in the video on the right are better.



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Video Comparison 1 - Comparison between Flash 8 Video Encoder and Sorenson

To achieve a much more profound difference between the two videos, the full power of Squeeze must beused.

If you have Flash 8 installed, and you'd like to use the same video employed in this article, you can find it in the Flash 8 install directory. Use the following directory path (PC) to find the QuickTime version of the video:C: Program files > Macromedia > Flash 8 > Samples and Tutorials > Tutorial Assets > Cafe Townsend >cafe_townsend_chef.mov

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A frame preview of the video and the job setting for the video will appear in the 2 right large panels in theinterface (Figure 1).

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Figure 1 - Overview of Squeeze 4.5 interface

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Although presets provide good guidelines for encoding settings, all videos are different, and a single preset maynot achieve sufficient encoding quality on all types of videos. Trial and error is one of the best ways of learninghow to encode different types of videos. In most cases, presets are a good starting point.

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Figure 3 - Audio/Video Compression Settings panel

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Audio SettingsCodec: Fraunhofer MP3 option is the only one

Data Rate: 48Kbps

Sample Size: 16

Sample Rate: 22050

Channels: Stereo

Video SettingsCodec: On2VP6 Pro (Flash 8+)

Method: 2-Pass CBR (2-pass encoding with a constant bitrate)

Date Rate: 350Kbps

Frame Size: 320 x 240 (reducing frame size to 320 x 240 for streaming is a big factor in encodingquality)

Display Aspect Ratio Policy: Maintain Aspect Ratio (will scale the video proportionately)

Frame Rate: 15 (dropping the frame rate will increase the quality of encoded video frames)

Keyframe Every: 30 (will insert a keyframe every 2 seconds)

Many of these options are available in the Flash 8 Video Encoder with exception to the following:

2-pass VBR, 2-pass CBR (Flash 8 Video Encoder is only single-pass)

Display Aspect Ratio Policy

Less control over the audio settings data rates

The above settings go a long way to getting good looking video

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Below is a new side-by-side (Comparison 2) example with the new settings and the original Flashencoded video from the start of this article.

Encoder: Flash 8 Video EncoderData-rate:Video: 350Kbps, 1-pass, CBRAudio: 48Kbps (stereo)Frame-rate: 30fps Keyframe rate: 60 (2 secs)

Encoder: Sorenson Squeeze Data-rate:Video: 350Kbps, 2-pass, CBR Audio: 48Kbps (stereo)Frame-rate: 15fps Keyframe rate: 30 (2 secs)

Video Comparison 2 - Comparison between Flash 8 Video Encoder and new Sorenson settings

The colors are definitely better on the right this time around and the brightness of the video is betteroverall. This may seem minor but it's a good start for such simple, extra settings.

The next part is the real money-maker. Squeeze's filters make a world of difference. I have even madesome videos look better color-and-brightness-wise than the original version, and that's encoding andcompressing for Flash.

Pre-processing Filters

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Figure 4 - Filters panel

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Figure 5 - Filter applied to video settings

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Figure 6 - Filter settings panel

Below is a list of pre-processing filters available in Squeeze. This article won't go into complete detail onall of them, but it will point out some of the key filters and their settings.

Contrast

Brightness

Gamma

White Restore

Black Restore

De-interlace

Video Noise Reduction

Cropping

Video / Audio fade in and out

Normalize audio

Key filter settings:

De-interlace - if the video you are encoding is in NTSC format (or was not shot on a progressive scan camera),de-interlacing a video is a MUST.

When a non-progressive scan camera shoots video , it shoots at 30 frames per second (fps). Each frame iscomposed of 2 intermixed fields. These fields are basically images that are "combed" together to create the imagefor the frame.

Have you ever paused a digital video on your computer and the video appears as a series of intermixed lines? Thisis interlacing in action. You are basically pausing on the video where the 2 fields are being merged together tocreate the frame. See Figure 6 below for an example of interlacing.

De-interlacing works by removing one of the fields from a frame of video and converting the remaining field into



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a full frame.

There are 5 methods of de-interlacing:

- 1) Auto remove interlacing, 2) Remove interlacing odd, 3) Remove interlacing even are the most common methods of de-interlacing. Auto remove interlacing leaves de-interlacing up to Squeeze's best judgment (removesodd or even fields depending on which one is better). If Auto remove does not provide a good result, try the oddor even methods (removes odd fields or even fields only).

- 4) Remove telecine odd, 5) Remove telecine even work similar to the above 3 methods but you use thesemethods in the case of up-sampling the frame rate of a video. If the source video was shot for motion picture film(or at 24 fps), telecine will de-interlace as well as adding frames where they most make sense to up-sample theframe rate of the video to 30fps. This is also used when converting PAL formatted video (25fps) to 30fps video.These methods are mostly used when encoding video for DVD playback and such.

Figure 7 - Interlacing artifacts

Black restore / White Restore - These filters makes your blacks more consistent and your whites whiter.Visually the video will look nicer, but it will also encode better as there are less shades blacks and whites to dealwith during encoding.

Contrast, Brightness and Gamma are common filters in any software that handles images and video. These canbe used to correct an associated imperfections in the video that need tweaking.

Video Noise Reduction - Use this filter when you see noise that occurred during the capture process. Whenvideo is captured, not all the data is easily understood by the capture process and noise will occur. Capturingvideo from a VHS tape would cause the worst case of video noise. use the settings depending on the quality of thevideo capture.

The above list is a summary of the most important filters to use in Squeeze. The rest of the filters can besearched from the Help menu for further definitions.

As you adjust the filter settings, Squeeze allows you to preview the results of the settings you areapplying to the video. The preview panel allows you to do a side-by-side comparison with the view of thesource video and the view of the video after it's encoded. Follow the steps in the list below to apply andpreview filters in your video.

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Figure 8 - Previewing filter settings with brightness setting adjusted

Filters that are applied to a video are applied to the entire video. You can not apply a filter to a specific range ofa video. As you apply filters, use the scrub bar at the bottom of the video preview panel to see the filter's effecton the rest of the video. Although a filter might make one part of a video look better, it might adversely affectanother part of a video.

Figure 9 below shows a preview of the de-interlacing filter applied to a standard NTSC video. De-interlacing is afundamental filter that should be applied to all videos that were not shot with a progressive scan camera.

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Figure 9 - Live preview of de-interlacing set to Auto remove interlacing

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Previewing and Comparing Videos

Below is a comparison of the final video complete with filter settings and the video that was encodedwith the Flash 8 Video Encoder. There wasn't anything more that could be done to the video on the left asthe Flash 8 Video Encoder and its settings had reach it's limitations. A thousand words later and a lot ofinfo, Sorenson Squeeze is still be explored.

To someone who doesn't work with video too much, the comparison below might not look like much. Tonotice the differences, a video has to be looked at beyond the main subject of a sequence or frame. Lookat the backgrounds (much more clear and better contrast eh?). The plate for the salad and near the end ofthe video looks much better. You can see the chefs face more clearly.



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Encoder: Sorenson Squeeze Data-rate:Video: 350Kbps, 2-pass, CBR Audio: 48Kbps (stereo)Frame-rate: 15fps Keyframe rate: 60 (2 secs)

Comparison 3 - Comparison of Flash 8 Video Encoder video and the final Squeeze encoded video.

Filter settings used for final Sorenson Squeeze video:

Figure 10 - Final filter settings applied to Cafe Townsend video



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Conclusion

At the end of the day, all the settings above are great, but, its an eye for detail, experience, andexperimentation with settings that provide the great result of well encoded video.

Create a table of different settings, create a web page full of Flash video for side-by-side-by-side-etc.comparisons.

Group types of video into groups (high-movement, talking head, medium motion, etc) and determineideal settings that can create good encoding quality for each video that falls within a certain category.

Settings, filters and Squeeze are great, but encoding video is more of a learned skill and an art form!

Approximate download size: 390k

KeywordsSorenson squeeze, On2, On2VP6, video, flash, FLV, deinterlace, filters, encoding, data rate, bit rate,

compression, streaming, progressive download

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The Art of Encoding Flash Video - Part 3: Advanced Compression Settings with Sorenson Squeeze


In the first two installments of the Art of Encoding Series, I covered the basics of the FLV Format (TheArt of Encoding Flash Video: About the FLV Format(http://www.communitymx.com/abstract.cfm?cid=9AEA3) ) and introduced everyone to encoding video usingSorenson Squeeze (The Art of Encoding Flash Video: Sorenson Squeeze(http://www.communitymx.com/abstract.cfm?cid=9AEA3) ).

This installment will go further into the compression settings of Sorenson Squeeze 4.5 by using itsadvanced compression settings. Getting familiar with and using these advanced settings will result inhigher encoded video quality and better playback performance of Flash video. If you are not familiar withSorenson Squeeze 4.5, please read the introduction article to Sorenson Squeeze, The Art of EncodingFlash Video: Sorenson Squeeze (http://www.communitymx.com/abstract.cfm?cid=9AEA3) .

The article will use Sorenson Squeeze 4.5 for encoding Flash video. If you'd like to know more about SorensonSqueeze,you can visit the Sorenson Squeeze info page (http://www.sorensonmedia.com/pages/?pageID=3) . You can also download a free limited trial here (http://www.sorensonmedia.com/pages/?pageID=8) .



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Sorenson Squeeze 4.5 Advanced Compression Settings

Figure 1 - Squeeze interface with advanced compression options menu

To start exploring the advanced settings as I describe each setting in detail, you first must import a fileand apply a Flash Video setting to it.

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Overview of Advanced Video Compression Settings

Below is a list and description of all the advanced compression settings you can apply to your video.Depending on the CODEC chosen and the encoding method you choose, not all of the settings will beavailable. I will notify you of what settings are available only for certain compression methods andCODECs. To make the first notification easy, select the Sorenson Spark Pro codec from the Codecdrop-down menu and you'll see you get maybe 20% of the advanced settings compared to using the On2VP6 Pro codec. I will be focusing on the available settings for the On2 VP6 Pro codec.



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Codec: - This is where you select the codec used to compress your Flash video. I will be using the On2 VP6 procodec. The following describes the two available codecs and what version of Flash player supports each forplayback.

On2VP6 Pro:Available in Flash Player 8+.

Encodes alpha channels

More advanced compression settings

Sorenson Spark ProFlash Player 6+

Method - The method setting has four (4) different options for how the video is encoded. The setting is acombination of 2-pass vs 1-pass encoding, and CBR (Constant bit rate) vs VBR (Variable bit rate).

1-pass vs 2-pass encoding are methods of analyzing and encoding the video in a single pass (1-pass), oranalyzing the video on the first pass and encoding the video based on the analysis of the first pass andthen applying it to the compressing that happens in the second pass (2-pass).

2-pass encoding always results in higher quality video compression as the tasks of analyzing the video andcompressing it are separate. This allows the encoder more time to focus and essentially do a better job.2-pass encoding does take longer to encode a video, but the end result is worth the wait.

CBR (constant bit rate) and VBR (variable bit rate) are methods of how the target data rate for the videois applied.

A Constant Bit Rate takes the target data rate for the video and applies the specified data rate preciselyto each second of video. i.e. A target video data rate of 276Kbps applies exactly 276Kb to each second ofvideo regardless if that much data is needed to encode a second of video.

A Variable Bit Rate on uses as much data as needed to encode a second of video. If a section of video isrelatively easy to encode such as a still moment in the video, the encoder uses only as much data asneeded to make the section look good. If only 100Kb is needed to make the section look good, it will onlyuse 100Kb. This helps save room in the pipeline for the rest of the video to stream/load faster, thuskeeping your buffer full and smoother playback results. This also helps save unnecessary output of datafrom your server.

The general rule of thumb is to use a Constant Bit Rate for streamed videos under 80Kbps. For streamingvideo above 80Kbps, variable bit rates are applied. Using a CBR method for low data rates is necessary asall bandwidth needs to be consumed for making the best quality video as possible.

Data Rate - To apply a data rate to your video, you can choose a preset from the drop-down menu or you cantype your own target video data rate in. The presets have common target data rate settings based on the averageconnection speeds of internet users, but you can get into typing in your own custom data rates as well to tweakyour video quality, as well as trying to get the lowest data rate as possible while still maintaining quality.

Note that the data rate for the video is separate from the audio data rate. This means that the selected video datarate is added to the audio data rate.

Frame Size - This is used to re-size your video. I generally use 320 x 240 (depending on the aspect ratio of thevideo) for general streaming. Adjusting the size of the video allows higher quality video when you go smallerthan the original display size of the video. This is because there are fewer pixels to compress and fewer pixelsmeans more data rate to go around to make smaller images look good.

Display Aspect Ratio Policy - This setting affects how your video is re-sized. If you select Unconstrained, yourvideo will be stretched or squashed on re-sizing. If you select Maintain Aspect Ratio, the video will be re-sized proportionately. Using the Letter Box or Pillar setting will maintain the aspect ratio of the video but the videowill be cropped by the width and height setting applied in the Frame Size option.

Frame Rate - Dropping the frame rate of streaming video is a given. The less frames that are encoded, the higherthe overall image quality will be. I generally cut my frame rates in half. i.e. 30fps gets cut to 15fps. For talkinghead shots, you can go as low as 10fps as there is not much movement going on.



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Less frames frees up data rate for image quality but results in more choppy motion

More frames provides smoother playback but results in more blurry/pixilated images

Use evenly divisible frame rates to reduce frame rates i.e. halves, thirds, quarters (this avoidssynchronization issues)

Key Frame Every - Keyframes for video are very different from the keyframes you use in Flash. Keyframes invideo are "snapshots" taken over time of what is going on in the video. When a keyframe is placed, all encodedframes after a keyframe are compared to what is going on the the last keyframe. This method encodes only whathas changed in a current frame compared to the last keyframe. Say if the background in a frame is the same as thelast keyframe and only part of a person talking has changed, only the person talking is encoded and theinformation stored for the background in the keyframe is re-used for that frame. This helps free up data rate andapplies it to only things that have changed. This results in higher quality images throughout the video.

To apply a keyframe rate, you can choose from the preset values in the drop down menu or you can type your ownvalues in. The values represent the number of frames between keyframes. If your frame rate is 15fps and thekeyframe rate is 30, a keyframe will be placed in the video every 2 seconds (or 30 frames).

Use lower values (more keyframes) for complex material (i.e. Sports, lots of effects and transitions).

Use higher value (less keyframes) for less complex material (i.e. talking head shot)

A high frequency of keyframes can result in the target data rate of the video being exceeded

Compress Alpha Data: - Checking this option encodes alpha channel channels

Alpha Data Rate: - Percentage of target data rate allocated to compression of alpha channel. This woulddepend on complexity of edges of alpha channels.

Auto Key Frames Enabled: - Squeeze will automatically determine the frequency of keyframes needed forsections of video. If two frames are drastically different, the encoder inserts a keyframe.

The Auto Key Frame Threshold determines how different the frames have to be before a keyframe is thrown. Ahigher number means fewer keyframes. �This option works in conjunction with the keyframe rate you may havepreviously set.

Minimum Distance to Key Frame:Minimum number of frames allowed between keyframes

Low value can result in increase to overall data rate

Higher value for less complex material (i.e. talking head shot)

Compression Speed: - This setting adjusts the speed at which the video encodes at. I only use Best for myencoding speed. The Best settings is marginally slower than the other settings. I'm not in that much of a hurry thatI want to compromise encoding quality to save seconds or a few minutes.

For high-production environments, you could invest in dedicated encoding machine and then encoding time willnot matter that much.

Speed: - If Compression Speed is set to Fastest, the encoder attempts to use percentage of processorcycles based on value set in slider

Minimum Quality:Worst allowed frame quality

Higher numbers mean you won't get bad looking frames

May interfere with compressor to maintain target data rate without dropping frames

Maximum Quality:The best allowed frame quality

Higher is better

Helps manage achievement of target data rate without dropping frames

Quality:Only available for 1-Pass VBR method

Uses a constant frame quality as specified in slide (0 – 100)



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Data rate is controlled by dropping frames or resizing frames if selected

VBR VariabilityUsed to control variability of bit rate of VBR method

0 creates a constant bitrate

Higher values create higher variability

The compressor uses data rate proportionate to complexity of section that is being encoded

Drop Frames to Maintain Data Rate: - Allows compressor to drop frames in order to hit target rate in difficultsections.

Drop Frames Watermark: - Sets a data rate threshold in which the encoder is allowed to start dropping framesto maintain target data rate

Sharpness: - If your source video is blurry or muddy, the sharpness setting can help sharpen your video. Thesetting values are between 0 and 10 (smoothest and sharpest, respectively).

Lower value results in fewer visible artifacts but can result in blurring

Higher value will provide a sharper image but with more visible artifacts

Can help with blurry text and graphics

Noise Pre-processing Level: - Helps reduce noise artifacts that result from video signal during video capture.Will not remove existing noise in source video. 0 is no noise removal, 6 is highest.

Start Buffer Level:Applies to CBR methods only

Amount of video data that is pre-loaded into client player in seconds

Compensates for consistent datarate

Minimizes playback interruption but results in slower initial video playback

Minimum 2-Pass VBR Data Rate:The lowest percentage of target data rate the encoder will allow for any section

Does not matter how easy section is

Prevents more difficult sections from stealing too much data rate from easy sections

Setting is based on selected percentage of overall data rate

Maximum 2-Pass VBR Data Rate: - The highest percentage of target data rate that can be streamed no matterhow difficult section is.

Data Rate Undershoot: - Averages a slightly lower overall data rate (based on target data rate) to allow moreavailable bits in buffer for more difficult sections.

Overview of Audio Compression Settings

Figure 2 - Audio compression settings

Audio compression settings are pretty straight forward once you know what all the settings mean.

Codec: - The options are Uncompressed or Fraunhofer MP3. To achieve good sound quality within a specifieddata rate. the MP3 option is mandatory.

Data Rate: - Select a target data rate for the audio from the drop-down menu. You cannot free-type in your own



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data rate here. For high data rates I generally choose 48Kbps and never exceed 96Kbps. Audio sounds great atlow data rates for streaming. Anything over 96Kbps is getting close to the range of CD quality audio which forstreaming over the web is getting frivolous. Remember what I mentioned earlier, the audio and video data ratesare separate and they are added together as the overall data rate so high audio data rates make you take away datarate for video quality.

As audio and video data rates are compounded, below is a rule of thumb for data rates of combined audio/videodata rates for dial-up connection speeds. Anything above that you can test for the best combination of combinedaudio and video data rates.

Dial-up connections:Overall data rate: 48Kbps (you need to consume basically all of the bandwidth for thisconnection speed)

Audio data rate: 16Kbps

Video data rate: 32Kbp

Sample Size: - The Sample Size is the number of bits used to store every second of audio information. Thesetting for Flash video is stuck at 16 bits.

Sample Rate: - The sample rate is how many times per second audio information is lifted out of the bit storage ofthe audio. A sample rate of 44100 means that 16 bits of audio information is sample 44,100 times per second.This results in higher quality but it can compromise the overall target data rate for the audio. The Sample Ratevalues are limited depending on the audio Data Rate selected.

Channels: - There are 2 channels options. Mono or stereo. This option varies on the selected target data rate.Whenever present, I select the stereo option. That means the audio is spread out over left and right speakerchannels.

A Couple of Final Settings

Figure 3 - Final Squeeze settings

The last few settings that I'll cover are pretty simple. At the top of the Compression Settings panel are 3settings options.

Total Data Rate: - This is a read-only value that displays the combined audio and video data rates. After youapply your settings, have a look at this value to make sure you have not exceeded any target data rate you aresetting out to achieve.

Constrain File Size: - This displays some math that multiplies the video in length (in seconds) by the appliedtarget data rate. If selected, you are guaranteeing the final file size of the video jives with the overall target rateapplied to the video. This prevents any overshots in data rate that may occur depending on the settings you haveapplied.

Name: - Re-type in a name for your settings and press the Export button at the bottom of the panel to export thesettings as a custom preset. This custom preset will appear in the compression settings panel. You can also sendthe settings file (you will see where is gets saved to when you export the settings) to colleagues or anyone whoneeds the same settings and they can import the settings file into Squeeze.

Summarizing the Audio and Video Settings

As you may have found in the audio compression settings section, the audio settings are pretty straightforward because of the limitations in place. Any audio settings over 48Kbps is going to sound prettygood. I always start with 48Kbps, test for quality and bump the data rate up if need be. I tend to focusmore on the video quality. I usually start with an overall data rate of 350Kbps for video and go from



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there. If the video looks great at 350Kbps, I'll lower the video data rate and test again. The goal is not tohave a standard data rate for different connection speeds but try to get the lowest data rate you can for allof your connection speeds while maintaining good quality video within compromise. Just because youhave a high-speed audience, it does not mean a video requires high data rates. Some videos still lookgood at lower data rates. Remember, you have a responsibility at your end to manage the amount ofinformation that streams from your server. Tweaking data rates to as low as they can go means money inyour pocket if you pay your service provider by the amount of data that gets pulled from your server.Further, if you have a large amount of users streaming from your server at one time, lower data rates areless taxing on your server if you are streaming a lot of videos at once.

For 56Kbps users, you will pretty much have to consume all of their bandwidth if you hope to have anykind of quality stream. Using a constant bit rate will also help achieve the best quality you can for lowbandwidth users.

Once you test and get familiar with what data rates and settings satisfy certain categories of video i.e.high motion, talking heads, medium motion, you can create custom settings to apply in batch processesfor specified video categories so you do not need to hand-tweak each video. Not all videos are the sameand you cannot satisfy every video with the same settings so there will be exceptions. But for a largeencoding environment, compromises need to be made in order to be efficient.

KeywordsSoresnon Squeeze, video, FLV, compression, data rates, frame rates, codecs, On2 VP6, encoding, audio

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the art of encoding flash video: about the flv format

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