1 chapter 3 1. audio interfacing standard - digital audio - analog audio - musical instrumental...

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Chapter 3Chapter 3

1. Audio Interfacing Standard

- Digital audio

- Analog audio

- Musical instrumental digital interface (MIDI)

2. Audio Application

- Sound Card

- MP3 Audio Player

ITECC/TP/ECE/SimBNEC5103PA_Apr2008

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Audio Interfacing StandardAudio Interfacing Standard

– Digital audio– Analog audio– Musical instrumental digital interface

(MIDI)

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Digital AudioDigital Audio

• In digital audio, the purpose of binary numbers is to express the values of samples which represent analog sound velocity or pressure waveforms.

• All digital audio technology is based on sampling technology.

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Sampling Sampling

• When a digital recorder takes a sample, it basically takes a snapshot of the audio wave form and turns it into bits which can be stored and manipulated

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Sample BitsSample Bits

• The more bits used to describe something, the better the clarity and fidelity.

• The bit resolution of a system defines the dynamic range of the system. 6dB is gained for every bit. For example, 8 bits equals 256 states = 48 dB, 16 bits equals 65,536 states = 96 dB.

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Sample Rate Sample Rate

• Sample Rate = Number of Samples (measurements) taken of an analog signal per second

• The sample rate determines the frequency range (bandwidth) of a system.

• The faster the sample rate, the better the accuracy of getting a true picture of higher frequencies.

• Sample rates examples:

22,050 @ 22.05 kHz - 22,050 samples per second. A sample every 1/22,050 of a sec.

24,000 @ 24 kHz - 24,000 samples per second. A sample every 1/24,000 of a sec.


44,100 @ 44.1 kHz - 44,100 samples per second. A sample every 1/44,000 of a sec.


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• High sample rates are better at capturing high frequency waveforms.

• However, if sampling at lower frequency sounds, such as kick drum, bass, one might consider sampling at the lower rate to save hard drive space.

Sample Rate Sample Rate

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Nyquist TheoremNyquist Theorem

• Nyquist theory states that a wave form must be sampled twice in order to get a true representation.

• The sampling frequency must be at least twice the highest signal frequency recorded in order to be effective.

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Sample Rate Vs StorageSample Rate Vs Storage

• Although faster sample rate gives better sample, one still needs to consider the amount of storage each sample rate demands.

• If audio is recorded in stereo at 44.1 kHz

44,100 samples/sec x 16 bits/sample x 2 channels x 60 sec

= 8,467,200 bits per minute. = 10,584,000 bytes per minute

• For 4 minutes of song,= 10,584,000 bytes per minute x 4 minutes 42 million bytes of space.

•

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Conversion ProcessConversion Process

• A digital audio signal starts with an analog-to-digital converter (ADC) that converts an analog signal to a digital signal.

An overview of the digital <-> analogue conversion process.

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• After being sampled with the ADC, the digital signal may then be altered in a process which is called digital signal processing where it may be filtered or have effects applied.

• The digital audio signal may then be stored or transmitted. – Digital audio storage can be on a CD, an MP3 player, a hard

drive, USB flash drive, CompactFlash, or any other digital data storage device.

– Audio data compression techniques — such as MP3, Advanced Audio Coding, Ogg Vorbis, or Flac — are commonly employed to reduce the file size.

– Digital audio can be streamed to other devices.

• The last step for digital audio is to be converted back to an analog signal with a digital-to-analog converter (DAC).


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Digital audio technologies:• DAB (Digital Audio Broadcasting) • Digital audio workstation • Digital audio player

Storage technologies:• Digital Audio Tape (DAT) • Compact disc (CD) • DVD DVD-A • MiniDisc • Super Audio CD • various audio file formats


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Analog AudioAnalog Audio

• Whatever devices are used to make the analog to digital conversion, proper level of input is ensure to feed it the proper level.

– Sending a microphone signal (or too low of a line level signal) into a line-level input will result in noisy, coarse sound, encoded at effectively a lower bit-depth. Sending a line level input into a mic level input will usually create distortion and clipping.

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The Connectors • The most common audio connectors are cables that

transmit several different signal strengths, and some are used for both digital and analog connections.

• The most common types are 1/8-inch, 1/4-inch, RCA and XLR cables.

Levels - Line/Mic• three ranges most likely to deal with:

– Microphone level;– -10dB line-level; and – +4 dB line level.

Gain Staging• regulates the sensitivity of the input to that channel,

the amount of pre-amplification or attenuation that is applied to the incoming signal.

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Tracks/Channels

• Most current recorders or editors will record two channels, which can be two separate, discrete signals (such as one mic for the interviewer and one for the interviewee) or a stereo signal, which is made up of two related, synchronous signals (such as the output of a stereo mic, or a CD player.)

• Not everything that has two channels is stereo.

• While in the digital world a stereo signal is carried on a single cable, in the analog realm one generally needs one cable per channel (with the exception of stereo headphone cables).

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Analog Sound Versus Digital SoundAnalog Sound Versus Digital Sound

Disadvantages of analog system– the sensitivity of analog media to physical

degradation. – require comparatively bulky, high-quality playback

equipment to capture the signal from the media as (or more) accurately as digital.

Advantages of digital systems– very uniform source fidelity, – inexpensive media duplication (and playback) costs, – direct use of the digital 'signal' in today's popular

iPod-type ultra-portabe playback devices.

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Musical instrumental digital interface (MIDI)

Musical instrumental digital interface (MIDI)

• MIDI stands for Musical Instrument Digital Interface.

• The objective was to connect or interface instruments of different manufacture to control common functions, such as note events, timing events, pitch bends, pedal information, etc.

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• A simple MIDI network

•

Summary of the MIDI hardware specification

Summary of the MIDI hardware specification

http://en.wikipedia.org/wiki/Image:DIN-5_Diagram.svg

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MIDI Data Format MIDI Data Format

• The majority of MIDI communication consists of multi-byte packets beginning with a status byte followed by one or two data bytes.

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Standard MIDI FilesStandard MIDI Files

• Created in 1988 due to an explosion of different types of MIDI software, a standard, transportable file format for saving MIDI sequences and opening them with other programs was adopted.

• There are three main types of Standard MIDI Files (SMFs), type 0, 1 and 2.

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General MIDIGeneral MIDI

• A 'standard' in which certain programs of either General MIDI instruments or computer sound cards would contain specific instrumental or sound effects patches.

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MIDI Sequencing SoftwareMIDI Sequencing Software

• The most common type of software used to link MIDI instruments and computers is a large variety of MIDI sequencing programs.

• Current sequencing software allows recording and playback of notes, patch changes, controller information, and even SysEx codes to the limits of the program's memory allocation, usually hundreds of thousand of events.

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Audio ApplicationsAudio Applications

1.Sound card2.MP3

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3.1 Sound Card3.1 Sound Card

• A sound card (audio card) is a computer expansion card that facilitates the input and output of audio signals to/from a computer under control of computer programs.

• Typical uses of sound cards include:-– providing the audio component for multimedia

applications such as music composition, – editing video or audio, – presentation/education, and– entertainment (games).

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• Sound cards usually feature a digital-to-analog converter that converts recorded or generated digital data into an analog format.

• The output signal is connected to an amplifier, headphones, or external device using standard interconnects, such as a TRS connector or an RCA connector.

• More advanced cards usually include more than one sound chip to provide for higher data rates and multiple simultaneous functionality,

• eg between digital sound production and synthesized sounds (usually for real-time generation of music and sound effects using minimal data and CPU time).

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• Digital sound reproduction is usually done with multi-channel DACs, which are capable of multiple digital samples simultaneously at different pitches and volumes, or optionally applying real-time effects like filtering or distortion.

• Multi-channel digital sound playback can also be used for music synthesis when used with a digitized instrument bank, typically a small amount of ROM or Flash memory containing samples corresponding to MIDI instruments.

• A contrasting way to synthesize sound on a PC uses "audio codecs", which rely heavily on software for music synthesis, MIDI compliance, and even multiple-channel emulation.

– become common as manufacturers seek to simplify the design and the cost of sound cards.


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• Most sound cards have a line in connector for signal from a cassette tape recorder or similar sound source.

• The sound card digitizes this signal and stores it (under control of appropriate matching computer software) on the computer's hard disk for storage, editing, or further processing.

• Another common external connector is the microphone connector, for use by a microphone or other low level input device.

– Input through a microphone jack is often used by speech recognition software or for Voice over IP applications.


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Methods of Sound CreationMethods of Sound Creation

• Frequency modulation (FM) synthesis, in which the computer overlaps multiple sound waves to make more complex wave shapes.

• Wavetable synthesis, which uses samples of real instruments to replicate musical sounds. – uses several samples of the same instrument played at

different pitches to provide more realistic sounds. – In general, wave table synthesis creates more accurate

reproductions of sound than FM synthesis.

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CODECCODEC

• A codec is a device or program capable of encoding and/or decoding a digital data stream or signal.

• Codecs are often used in videoconferencing and streaming media applications.

• A video camera's analogue-to-digital converter (ADC) converts its analogue signals into digital signals, which are then passed through a video compressor for digital transmission or storage.

• A receiving device then runs the signal through a video decompressor, then a digital-to-analogue converter (DAC) for analogue output.

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What is MP3 format?What is MP3 format?

• MP3 file format is a way to store sounds and music using MPEG Audio Layer 3 compression.

• MP3 format is usually used to store music in computer environment because it provides about 1:10 compression.

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Perceptual Audio CodingPerceptual Audio Coding

• Perceptual encoding is a lossy compression technique

• This is achieved by adapting the encoding process to the characteristics of the human perception of sound: – the parts of the audio signal

that humans perceive distinctly are coded with high accuracy,

– the less distinctive parts are coded less accurately, and

– parts of the sound which we do not hear at all are mostly discarded or replaced by quantization noise.

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Perceptual Audio CodingPerceptual Audio Coding

• An MP3 file is made up of multiple MP3 frames, which consist of the MP3 header and the MP3 data.

• MP3 works on both mono and stereo audio signals.

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MP3 Player/Recorder (Portable Audio)


• Being portable typically implies that the devices are battery-operated and therefore system power consumption is a key concern.

• Portable audio ICs for the consumer audio market include:-– integrated audio codecs or– audio ADCs and DACs, – headphone or – speaker drivers for earpieces, headphones or – built-in speakers, – programmable DSPs for encoding/decoding compressed data

in such formats as MP3, WMA or AAC, – microcontrollers for handling system-level control and user

interface functions, and– wireless transceivers for transmission/reception of data over

RF.

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The core subsystems include:

• DSP -performs the audio/encode functions, executes post-processing algorithms like eq and bass management and system related tasks like file management and the user interface control.

• Memory- stores executing code and data/parameters.

• Peripheral Interface- allows user to control I/Os and display.

• Audio/voice CODEC- interface with the phone lines, audio input, microphone, headphone, and speaker for digitizing the audio in the DSP.

• Power Conversion – converts the battery power to run various functional

blocks.

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MP3 SummaryMP3 Summary

• The increasing popularity of MP3 portables triggers dozens of companies to launch similar portable devices for compressed music playback, and stimulates the development of additional audio codecs for use in PCs and consumer electronics.

• The possibility to store thousands of MP3 encoded songs on a small portable player, search them by album, artist, title, genre or even to have play lists generated automatically has reawakened millions of people's love for music. Whole music collection is available at the push of a button. In fact, inexpensive 40GB MP3 players today can store over 10,000 songs of CD quality readily available. MP3 is more than just a technology. It is a sensational development that connects musicians with music lovers, speakers with their listeners, artists with their audience.

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Review QuestionsReview Questions

1. All digital audio technology is based on ______________________ technology to capture an analog audio waveform.

2. The number of samples taken of an analog signal per second is called _________.

3. A 16 bit sound card will have a sampling rate of _________ KHz.

4. Give two advantages of digital sound systems as compared to analog sound.

5. What does MIDI stand for?6. What is the main objective for creating MIDI standard?

7. Briefly describe the function of a sound card.8. What is a codec?9. What does MP3 stand for?10.Name the compression technique used by MP3.

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Creative Live!Drive

Fig 4Fig 4

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Sound Card may be connected toSound Card may be connected to

Headphones Amplified speakers analogue input source Microphone Radio Tape deck A digital input Digital audiotape (DAT) CD-ROM drive An analogue output device-tape deck A digital output device CD-R four-speaker output digital interface through a jack.

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A digital sound card

digital sound input and output CD-R, DVD and DAT Staying digital without conversion to or from

analog prevent "generational loss"

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Function of a sound card

play pre-recorded music, games or DVDs record audio in various media from external

sources synthesize sounds process existing sounds

Pre-recorded - CDs or sound files, such as wav or MP3

DAC and ADC getting audio in and out of sound card while DSP oversees the process.

DSP focuses on audio processing

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MIDIMIDI

Fig 5Fig 5

• Musical Instrument Digital Interface

• a standard adopted by electronic music industry for controlling devices that emits music, e.g. synthesizers and sound cards

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MIDIMIDI

- at minimum, a MIDI sound includes values for note's pitch, length, and volume. It can also include additional characteristics, such as attack and delay time.

- most synthesizers support the MIDI standard.

- computers with MIDI interface can record sounds created by a synthesizer and then manipulate the data to produce new sounds. - For example, you can change the key of a

composition with a single keystroke.

- use software programs to compose and edit music.- Eg. when you play a tune on a keyboard connected

to a computer, a music program can translate what you play into a written score.

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Producing SoundProducing Sound

Process of converting input sound to wav file and recorded to the hard disk :

1.sound card receives analog-input signal from microphone jack.

2.Software selects which input(s) to use, depend on whether the microphone sound is being mixed with a CD in the CD-ROM drive.

3.The input signal is processed in real-time by an ADC, creating a digital output.

4.DSP compress digital data to save space.

5.Output from DSP is fed to the computer's data bus.

6.Digital data is processed by computer's processor and recorded in hard-disk as a wav file.

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Producing SoundProducing Sound

To listen to wav file, the process is reversed:

1. digital data is read from hard disk and passed to computer’s processor.

2. central processor passes data to the DSP. 3. DSP uncompressed the digital data.4. data is processed in real-time by DAC chip, creating an analog

signal to hear in headphones or through speakers

To test for full-duplex capability by launching two copies of Sound Recorder:

1. Start menu 2. Programs 3. Accessories 4. Entertainment 5. Sound recorder

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List of audio formatsList of audio formats

• 1870s: Phonograph cylinder • 1895: Gramophone record • 1930s: Wire recording • 1940s:Reel-to-reel audio tape recording, Magnetic Tape (audio) • 1948: Vinyl record • 1960s: 8-track • 1963: Compact cassette • 1969: Microcassette • 1970s: Elcaset • 1979: Compact Disc Digital Audio System • 1985: AIFF • 1987: Digital audio tape (DAT) • 1990s: Digital Compact Cassette • 1991: Minidisc • 1992: WAV • 1995: MP3 • 1996: DVD • 1999: Windows Media Audio • 2000: FLAC • 2001: AAC • 2002: Ogg Vorbis

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Digital Audio- reproduction and transmission of sound stored in a digital format. - CDs and any sound files stored on a computer.

WAV- developed by Microsoft and IBM. - Supported from Windows 95 - de facto standard for sound on PCs. - .wav extension

RealAudio• The de facto standard for streaming audio data over the World

Wide Web. • developed by RealNetworks and supports FM-stereo-quality sound. • To hear a Web page that includes a RealAudio sound file, you

need a RealAudio player or plug-in, a program that is freely available from a number of places.

• included in current versions of both Netscape Navigator and Microsoft Internet Explorer.

GlossaryGlossary

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MP3 PlayerMP3 Player

2.1 MP3 Format2.2 Using MP3 Format2.3 Parts of a MP3 Player

Fig 6Fig 6

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MP3 FormatMP3 Format

• MPEG Audio Layer-3.• MP3 movement started by of music lovers on internet. • A CD stores songs as digital information. • Music sampled 44,100 times/sec• samples are 2 bytes long for each second of music:

44,100 samples/sec x 16 bits/sample x 2 channels = 1,411,200 bits per second.

= 176,000 bytes per second

• For 3 minutes of song,= 176,000 bytes x 180 secs. 32 million bytes of space.

. for 56K modem, it takes two hours to download one song.

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• Compression system for music.

• Reduce number of bytes without hurting the quality of the sound.

• Compress a CD-quality song by a factor of 10 to 14.

• With MP3, a 32-MB song on a CD compresses down to about 3 MB.

• A GIF file is a compressed image.

• A ZIP files to compress text.

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MP3 uses perceptual noise shaping technique, which uses characteristics of the human ear to design the compression alogrithm.

- Certain sounds the human ear cannot hear, or hears much better than others.

- When two sounds are playing simultaneously, only the louder one can be heard

-Thus, certain parts of a song can be eliminated, and by compressing the rest of the song to get a "near CD quality" song.

Fig 7Fig 7

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Using MP3 Format.Using MP3 Format.

easy for anyone to:• distribute music at nearly no cost. • find music and access it instantly. • manipulate sound on computer.

Fig 8Fig 8

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Parts of a MP3 PlayerParts of a MP3 Player

Microprocessor- the brains of the player. - monitors user input through the playback controls, - displays information about the current song on LCD panel - sends directions to DSP chip that tells it exactly how to

process the audio.

DSP- pulls song data from memory, applies any special effects,

or EQ, and streams it to the amplifier. - DSP runs a decompression algorithm that undoes the

compression of the MP3 file - then a DAC turns the bytes back into waves.

Amplifier- boosts the strength of the signal and sends it to the audio

port, where a pair of headphones or ear buds is connected.

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Review QuestionsReview Questions

1 Which sound card is the de facto standard for PC sound?

2 Name two basic methods for sound cards to translate digital data into analog sounds.

3 What is the function of a digital signal processor on a sound card?

4 Which sound file format was developed jointly by Microsoft and IBM and becomes a de facto standard for sound on PCs?

5 Computers that have a MIDI interface can record sounds created by a synthesizer and then manipulate the data to produce new sounds. ( T / F )

6 Describe how “perceptual noise shaping” technique makes a good compression algorithm for sound.

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Quality of Sound Card

1. bit resolution- 16bit, 18bit, 20 bit, 24 bit

2. maximum sample rate capability - most card has max.sample rate of 48khz (CD audio is 44khz) - newer 24 bit resolution and max. sample rate of 96khz(DVD audio is 24bit/96khz)

3. quality of ADC & DAC

4. design of sound card

5. external breakout board for inputs and outputs.

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Components of a Sound Card

• digital signal processor (DSP) handles most computations

• digital to analog converter (DAC) for audio leaving the computer

• analog-to-digital converter (ADC) for audio coming into the computer

• ROM or Flash memory for storing data

• Musical Instrument Digital Interface (MIDI) for connecting to external music equipment – for many cards, the game port is also used

to connect an external MIDI adapter)

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Components of a Sound Card

• jacks for connecting speakers, microphones, line in and line out

• a game port for connecting a joystick or game pad

• PCI PCI slot -- current sound cardsslot -- current sound cards

• ISA ISA bus -- older or inexpensive bus -- older or inexpensive

• incorporate the incorporate the sound card as asound card as a chipset chipset on on motherboard. motherboard.

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FM Synthesis & Wavetable Wavetable SynthesisSynthesis

1. FM Synthesis– mimics different musical instruments according to

built-in formulas.

– takes tones at varying frequencies and combines them to create an approximation of a particular sound.

– Used by old sound cards

2. Wavetable SynthesisWavetable Synthesis– recordings of actual instruments to produce sound.

– produces more accurate sound.

– more expensive

– standard for most sound cards

1 chapter 3 1. audio interfacing standard - digital audio - analog audio - musical instrumental...

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