Week 12 Lecture 1

Audio and Video File Formats

Audio on the web

• Can be linked to and downloaded like any other file

• Traditional audio files are quite large

• Long download time

• Advances– Steaming audio– MP3 format – lossy compression- shrinks files

to 1/10 their original size

Digital Audio

• An analog signal must be converted to digital information

• To convert an analog sound wave into a digital description of that wave, samples of the wave are taken at timed intervals

• The number of samples taken per second is called the sampling rate

• The more samples taken per second, the more accurately the digital description can recreate the original shape of the sound wave, and therefore the better the quality of the digital audio

Digital Audio

• Sample rates are typically measured in kilohertz (KHz). • CD-quality audio has a sampling rate of 44.1 KHz (or

44,100 samples per second) • The higher the sampling rate, the more information is

contained in the file, and therefore the larger the file size• Like images, audio files are also measured in terms of their

bit depth (also called sampling resolution or word length). • The bit depth corresponds to the resolution of the

amplitude (or volume) of the sound file • Some common bit depths are 8-bit and 16-bit, which is

required to describe music of CD quality

Commonly used Audio qualities

• 8 KHz, telephone

• 32 KHz miniDV camcorders

• 44.1 KHz CD, MP3

• 88.2 & 176.4 KHz Professional recording equipment

• 96-192 KHz DVD-Audio, HD DVD,Blu-ray

Audio File Formats

• AIFF - Audio Interchange File Format

• WAV - WAVE File Format

• MIDI - Musical Instrument Digital Interface

• AU - audio format originally by SUN

• MP3 - MPEG Audio Layer 3 format

• RA - RealAudio

WAV and AIFF file formats

• The WAV and AIFF audio formats are very similar in peformance

• The Waveform Audio File Format (.wav) was originally developed as the standard audio format for the Microsoft Windows operating system, but it is now supported on the Macintosh as well

• The Audio Interchange File Format (.aif or .aiff) was developed as the standard audio format for the Macintosh platform, but it is now supported by Windows and other platforms

• WAV and AIFF: 8 KHz and 11.025 KHz at 8- or 16-bit are/were most common for Web use

WAV and AIFF

• Now less commonly used on the Web than they once were, now that we have MP3 or formats designed specifically for the Web (streaming formats).

• WAV and AIFF files are typically used as the source format for audio that then gets compressed into more web-friendly formats

• They sound good when uncompressed, but they suffer drastic loss of quality when compressed to small file sizes

• They are useful for very short, downloadable audio clips, such as short greetings

WAV and AIFF

• Good for: Storing high-quality source audio before converting to web formats, delivering short audio clips.

• Creation tools: The majority of sound editing tools can save files in WAV and AIFF format.

• Player: WAVs and AIFFs generally play using the browser's default function for sound handling

MIDI – Musical Instrument Digital Interface

• MIDI (.mid) was originally developed as a standard way for electronic musical instruments to communicate with each other

• A MIDI file contains no actual audio information (the digital representation of analog sound), but rather numeric commands that trigger a series of notes (with instructions on each note's length and volume).

• These notes are played by a MIDI player using the available "instrument" sounds on a computer's sound card.

• As a result, MIDI files are incredibly compact and ideal for low-bandwidth delivery. They are capable of packing a minute of music into just 10K, which is 1,000 times smaller than a one-minute WAV file (approximately 10 MB).

Au file Format

• “AU” was developed by Sun Microsystems and is a standard for Unix computers. 

• Most Web browsers include the capability to play “AU” files directly, so it makes the format a good choice for Internet work that will be received by a larger Net audience.

• This type of file can be larger than other types of audio files so it is best used for short sound clips for effective download times. 

MP3

• MP3  is the most popular file format on the Web for distributing music

• The key to its success is MP3's ability to maintain excellent sound fidelity at very small file sizes.

• Its compression scheme can reduce an audio source to just one-tenth of its original size

• Four minutes of high-quality music:

– in WAV format - 40 MB of disk space

– in MP3 format - 3.5 MB

MP3

• MPEG is a family of multimedia standards created by the Moving Picture Experts Group. It supports three types of information: video, audio, and streaming (synchronized video and audio).

• MPEG uses a lossy compression scheme that is based on human auditory perception.

• Sounds that are not discernible to the human ear are thrown out in the compression process

• The resulting file sounds nearly the same, but contains much less data than the original

RealAudio - RA

• RA is a proprietary streaming audio format developed by RealNetworks (once Progressive Networks)

• RealAudio is a server-based streaming audio solution

• It has grown to be the standard for streaming audio, including live broadcasts

• To listen to RealAudio files, users must have RealPlayer, which is available for Windows, Mac, and Unix systems.

• For more info see http://www.real.com.

Audio and Video• Delivering video via the Web is especially problematic

because video files require huge amounts of data to describe the video and audio components, making for extremely large files.

• Many of the same technologies that have improved the experience of receiving audio over the Web have been applied to video as well.

• As with audio, you have the option of simply linking a video to your web page for download and playback, or you can choose from a number of streaming solutions (begins playing almost immediately continues playing as the data is transferred; however, the file is never downloaded to the user's machine)

• Many of the principles for developing and delivering video content for the Web are the same as those for audio.

Digital Video

• Movie length : Limiting the length of your video clip limits its file size. Videos longer than a minute or two may cause prohibitively long download times. If you must serve longer videos, consider one of the streaming video solutions.

• Frame size: Obviously, the size of the frame has an impact on the size of the file. "Full-screen" video is 640 X 480 pixels. The most common frame size for web video is 160 X 120 pixels.

Digital Video

• Frame rate: The frame rate is measured in number of frames per second (fps). Standard TV-quality video uses a frame rate of 30 frames per second to create the effect of smooth movement. For the Web, a frame rate of 15 or even 10 fps is more appropriate and still capable of producing fairly smooth video playback.

• Quality: Many video-editing applications allow you to set the overall quality of the video image. The degree to which the compression algorithms crunch and discard data is determined by the target quality setting.

Digital Video

• Color bit depth : The size of the video is affected by the number of pixel colors in each frame. Reducing the number of colors from 24- to 8-bit color will drastically reduce the file size of your video, just as it does for still images. Of course, you also sacrifice image quality.

• Data rate (bit rate): This is the rate at which data must be transferred in order for the video to play smoothly without interruption. The data rate (also called "bit rate") for a movie is measured in kilobytes per second (K/sec or Kbps).

Digital Video Compression

• Digital video wouldn't be possible without methods for compressing

• Compression can be "lossless," which means no information is lost and the final file is identical to the original

• Most compression schemes use forms of lossy compression. Lossy compression sacrifices some data from the file to achieve much higher compression rates.

• Lossy compression schemes, such as MPEG, use complicated algorithms that remove sound and image data details that are not discernible to the human ear or eye.

Compression Algorithms

• The decompressed file is extremely similar in character to the original, yet is not identical. This is similar to the way JPEG handles still images.

• Spatial (or intraframe) compression takes place on each individual frame of the video, compressing the pixel information as though it were a still image.

• Temporal (or interframe) compression happens over a series of frames and takes advantage of areas of the image that remain unchanged from frame to frame, throwing out data for repeated pixels.

• There are a number of compression/decompression algorithms (codecs) that can be used to compress video files for the Web. Many of these codecs can be applied to several different file formats

Digital Video File Formats

• MPEG - Moving Picture Experts Group Video File

• MOV - QuickTime (Apple Computer) file format

• AVI - Audio Video Interleave

• RM - Real Media

• ASF - Advanced Systems Format

QuickTime Movie (.mov)

• QuickTime, a system extension that makes it possible to view audio/video information on a computer, was introduced by Apple Computer in 1991

• Although developed for the Macintosh, it is also supported on PCs via QuickTime for Windows

• Most hardware and software offer QuickTime support.

• While originally developed as a video format, it has evolved into a container format capable of storing all sorts of media (still images, audio, video)

RealMedia (.rm)

• RealMedia is the industry standard streaming media format.

• RealNetworks (Progressive Networks) first launched its streaming video capabilities in Version 3.0 of its RealMedia line of products (of which RealAudio is the star component).

• RealMedia files (.rm) are viewed using RealPlayer 3 and higher.

• The wide distribution of RealPlayer and a proven track record of effective playback have made RealNetworks' products the de facto standard for adding streaming media to a web site.

Windows Media (.wmv or .asf)

• Windows Media is the new standard for audio and video, created by Microsoft and therefore very closely integrated with the Windows OS.

• The Windows Media Player is capable of playing Microsoft's proprietary Windows Media Video (.wmv) and Advanced Streaming Format (.asf ), as well as a number of other formats such as AVI, MPEG, MP3, and QuickTime.

• Windows Media movies are encoded using the proprietary Windows Media Video algorithms

AVI format

• AVI (Audio/Video Interleaved) was introduced by Microsoft in 1992 as the standard movie format to work with its Video for Windows (VFW) multimedia architecture for Windows 95.

• The AVI format has been replaced by the more robust Windows Media as the standard media format for Windows.

• AVI movies are not as common as they once were for web distribution. More often, they serve as the high-quality source file for the video which is then converted into a more web-friendly format.

MPEG-4

• MPEG-4, introduced in late 1998, is the designation for a group of audio and video coding standards and related technology agreed upon by the ISO/IEC Moving Picture Experts Group (MPEG).

• The primary uses for the MPEG-4 standard are web (streaming media) and CD distribution, conversational (videophone), and broadcast television.

• MPEG-4 absorbs many of the features of MPEG-1 and MPEG-2 and other related standards

• Most of the features included in MPEG-4 are left to individual developers to decide whether to implement them