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DIGITAL AUDIO Basics and Overview Digital Audio Signal Processing Syracuse University, Fall 2007 Instructor: Jayant Datta A day for BASICS Basics of Audio Interconnects Basics of Sampling Analog vs Digital + Aliasing + Quantization + Jitter + Dither Basics of Audio

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digital audio basics and overview

DIGITAL AUDIOBasics and Overview

Digital Audio Signal Processing

Syracuse University, Fall 2007

Instructor: Jayant Datta

a day for basics
A day for BASICS
  • Basics of Audio Interconnects
  • Basics of Sampling
    • Analog vs Digital + Aliasing + Quantization + Jitter + Dither
  • Basics of Audio
  • Basics of Digital Systems
    • LTI + Convolution + Digital Filters
  • Basics of Pointers + AGU
audio interconnects
Audio Interconnects
  • Think about the audio interconnects between the DVD Player and the Receiver.
  • Please consider the analog and the digital connectivity.
  • Now, try to figure out how information is being conveyed in each case (analog and digital).
basics of sampling
Basics of Sampling
  • Sampling
    • Continuous Time/Amplitude {Analog}
    • Discrete Time {Sampled}
  • Sampling Theorem
    • Aliasing -- spoked wheel, {Alias, NormAlias} spectra, {CreatePlayAlias(fDuration, fGain, iFreqOrig, iFs)}
    • Require an anti-aliasing filter before ADC stage
  • Quantization
    • Discrete Amplitude {Quant}
    • Effect at different signal levels {Quantizer, NormQuantErr [2,5,8]}
    • How good is 16-bits of resolution? 24-bits?
    • Dither {DispDither}
basics of sampling7
Basics of Sampling
  • Analog / Digital Conversion
    • Multi-bit Systems
    • Low level signals -- could benefit from dithering
    • Oversampled Systems / Single Bit Systems
      • Noise Shaping
      • Faster Clock Speed
      • Low tolerance to Jitter -- {Jitter} [no quantization]
      • Better anti-aliasing
      • More linear
analog versus digital
Analog versus Digital
  • Cost and Complexity versus Flexibility
    • Analog
      • inexpensive
      • well understood technology
      • natural intertia against change
    • Digital
      • almost by definition greater flexibility
      • example of parametric EQ and reverb -- code change
      • system design could be simpler and flexible
  • Programming Expertise -- bundled software
  • Just mimicking analog underutilizes digital
analog versus digital9
Analog versus Digital
  • Nature of Errors
    • SNR (small versus large signals)
      • Jitter
      • Masking
      • Distortion
    • Non-linearity of magnetic tape -- soft versus hard clipping
    • Quantization / Aliasing / Jitter / Dither / Psychoacoustic Masking
    • Humans are analog; digital artifacts are not intuitive
outline
Outline
  • Introduction
  • Consumer Audio Home Theater Overview
  • Guessing the Future
introduction
Introduction
  • How Technology advances
    • Taking things for granted
    • Recorded Sound
    • Pursuing perfection
    • Law of diminishing returns
early entertainment
First ‘recording’ made in 1855 with a “Phonautograph” – mouthpiece horn, membrane, stylus, rotating cylinder with smoke-blackened paper – research and analysis

Wax cylinders

“Listening studios” where a dozen people could listen to the same recording, each through a tube in the ear

Already claims of “tell the difference”

Early entertainment
phonograph
Whole variety of media: Vertical, horizontal bumps; wax and metal, cylinders and platters, hand cranked, electrical, different sizes

If in perfect condition, some people prefer the “richer” analog sound, also people find noise “warmer”

Incidentally, early recordings were done from the center outwards

Phonograph
open reel decks
Open Reel Decks
  • Open Reel Decks at one point defined the state of the art in audio recording
8 track
All 1966 Fords offered a factory installed in-dash 8-track player. In the 1967 model year, Chrysler and GM offered the same. By the late 1960s, several companies were making players for the other tape loop systems, including 4-track, but the only serious competition came from cassette tapes (which appeared at around the same time as 8-tracks) and the almighty vinyl records.

Eight-track tapes were with us for quite a long time. 8-track was the preeminent portable and car audio format of the 1970s

8 Track
8 track19
8 Track
  • Head had to be perfectly calibrated; bulky; no recordable ones
cassette tape
Was originally intended as a dictating format

Very convenient format, fits in a shirt pocket

On introduction, had three main problems: speed stability, frequency response, and background hiss

The first two were surmountable, through better tape drive mechanisms, heads, and tape formulations

Cassette Tape
dolby alphabet soup
Dolby Alphabet Soup
  • Dolby A [1965]
  • Dolby B [1968]
  • Dolby C
  • Dolby HX
  • Dolby HX Pro
  • Dolby SR [1986]
  • Dolby S
  • Dolby AC-1 [1985]
  • Dolby AC-2 [1989]
  • Dolby AC-3 [1992]
  • Dolby E
dolby stereo surround
Dolby Stereo / Surround
  • 1975: 35 mm stereo optical release print format Dolby Stereo.
  • Conventional mono optical soundtrack replaced by two soundtracks carrying not only left and right, but also a third center-screen and a fourth surround channel for ambient sound and special effects -- compatible with mono.
  • 1982: Consumer version of Dolby stereo called Dolby Surround
compact disc
Compact Disc
  • In 1978 Sony teamed up with Philips to develop a standard, universal compact disc to hold audio. Two years later a Philips/Sony Compact Disc Digital Audio standard disc was officially announced.
  • The disc was 120 millimeters, made from a polycarbonate substrate, and molded with a groove that provides timing and tracking information for the compact disc player. It was released formally in Europe and Japan in 1982 and in the United States the following year.
  • Sampling rate: 44.1 kHz
  • Maximum duration: 74 ~ 80 minutes
  • Quantization:16-bit linear
  • Rotational speed:1.2–1.4 m/sec. (constant linear velocity)
  • Error correction code:Cross Interleave Reed-Solomon Code (with 25% redundancy)
compact disc25
Compact Disc
  • A CD is a fairly simple piece of plastic, about four one-hundredths (4/100) of an inch (1.2 mm) thick. Most of a CD consists of an injection-molded piece of clear polycarbonate plastic. During manufacturing, this plastic is impressed with microscopic bumps arranged as a single, continuous, extremely long spiral track of data. Once the clear piece of polycarbonate is formed, a thin, reflective aluminum layer is sputtered onto the disc, covering the bumps. Then a thin acrylic layer is sprayed over the aluminum to protect it. The label is then printed onto the acrylic. A cross section of a complete CD (not to scale) looks like this:
dolby surround pro logic
Dolby Surround / Pro Logic
  • Dolby Surround only decoded left, right and surround
  • 1987: Birth of Home theater -- now enjoyed by millions of consumers worldwide -- was made possible by the inclusion of four-channel Dolby Surround Pro Logic decoding in products such as A/V receivers
digital audio tape dat
Developed in the 1980's as a successor to the analog cassette tape.

Information is recorded using the "helical scan" recording technique which is the same method used in VHS, Beta, and 8mm videocassette recorders.

Compared to these other formats, DAT tapes are much smaller and the information is encoded digitally.

Used for studio recording … expensive…consumer versions were less expensive

Old favorite of bootleg trading

Digital Audio Tape (DAT)
slide29
ADAT = Alesis DAT

ADATs use S-VHS tapes. A 120 minute tape provides 40 minutes of recording on the ADAT.

Storage of up to 999 address locations – automation

A greatly expanded dynamic range

Elimination of tape hiss and frequency loss – generation loss

Backups of master tapes sound like the first generation master.

Perfect track separation (no track "bleed" on multi-channel mixes).

Lower cost of recording tape.

Less archiving space required.

ADAT
dolby digital 5 1
Dolby Digital 5.1
  • 1992: Dolby Digital (AC-3) is a multichannel digital audio coding technology first used for cinema sound. Today it is also used to bring multichannel sound into the home via a wide variety of digital formats, including DVD, DTV, and digital cable.
dcc digital compact cassette
DCC players are backward compatible with analog tapes

Track and time codes are on the tape. DCC decks can locate a chosen track on either side of the tape.

The DCC 900 can digitally record music in 16-bit resolution and supports sampling frequencies of 32, 44.1, and 48 kHz

Precision Adaptive Sub-band Coding (PASC) compression to code the digital information onto tape. (4:1 ratio)

DCC: Digital Compact Cassette
md mini disc
MiniDisc is like a floppy disk – you can record and erase files on a MiniDisc just as easily as you can on a floppy disk. The big difference between the a MiniDisc and a floppy disk is that a MiniDisc can hold about 100 times more data

Durable because in diskette…no scratching

ATRAC encoding (Adaptive Transform Acoustic Coding), lossy, compression 5:1

MD: Mini Disc
dts digital theater system
DTS (Digital Theater System)
  • 1993, Steven Spielberg's Jurassic Park introduced the crisp, clear sound of DTS
  • Simple future-proof decoder
    • Intelligence is in encoding stage
    • Decoder follows instructions within coded bit stream
    • Encoding algorithm may be updated and modified, automatically benefiting every consumer decoder
    • Syntax of data stream specification designed to provide room for additional audio data – improvements in audio quality or changes in audio format
  • DTS – CD
dvd digital versatile disc
DVD: Digital Versatile Disc
  • Emerging technology that is extremely powerful.
  • Looks like a CD – but is a lot more.
  • Can store up to eight hours of CD quality sound.
  • Can store up to 133 minutes of high resolution video data.
  • Soundtrack is presented in up to eight different languages, and uses 5.1 channel Dolby Digital surround sound/DTS.
  • A single sided, single layer DVD can store 4.38GB of data. A double sided, double layer DVD can store 15.9GB of data. Most CDs can only store 700MB of data.
  • Definitely revolutionizing home theater.
  • Bonus footage; director’s/actor’s commentary; different camera angles.
multichannel enhancements
Multichannel “Enhancements”
  • 5.1 / 6.1 / 7.1 /10.2
  • 1999: Dolby Digital Surround EX in Star Wars: Episode I: The Phantom Menace – pseudo 6.1
  • DTS-ES (Extended Surround), ES 6.1 discrete, Neo:6, 96k/24-bit, 7.1
  • THX is a technology developed by Lucasfilm to allow the home viewer to experience the sound of a movie theater at home
slide36
mp3
  • Invented in 1989 in Erlangen, Germany, mp3 has quickly come to symbolize a paradigm shift in the way many people access their music.
  • The home computer revolution, along with the Internet, has allowed millions of Net-connected music fans to take advantage of the latest audio medium.
  • Short for Moving Picture Experts Group, Audio Layer III, MP3 is a compression format that shrinks digital audio files with negligible sound-quality degradation.
  • In 1997, the format truly realized its potential, thanks to a man named Tomislav Uzelac, who created the AMP MP3 playback engine.
  • The first mp3 player was invented just in time for the Napster revolution in the form of 1998's Winamp – widely regarded as the first free, consumer-ready mp3 player.
dvd audio
Sampling rates: 96 / 192 kHz

24-bit resolution

Up to 6-channels of audio

Uses MLP (Meridian Lossless Packing) technology

Not compatible

Difference is fairly subtle

Competing against SACD

Audio only format

DVD Audio
sacd super audio cd
1-bit Direct Stream Digital (DSD)

Sampling rate: 2.822 MHz

SACD player can play CD

4 times CD capacity

With this extra capacity, a standard Super Audio CD will provide space for 2-channel stereo data, as well as an area for up to 6-track multi-channel data, storage capacity for text and images, disc variations, copyright protection and much more.

SACD: Super Audio CD
aac advanced audio coding
AAC : Advanced Audio Coding
  • AAC is an audio compression technology.
  • This standard, developed by Dolby, the Fraunhofer Institute, and others, may become the major ingredient in 21st century digital music distribution.
  • The AAC codec was formally introduced to the world at the CES 2001, along with dozens of new digital audio players able to play AAC files.
  • Currently, companies such as Liquid Audio distribute audio using AAC.
mp3 versus aac
mp3 versus AAC
  • Lower complexity of mp3, makes it the system of choice for current CD-quality applications.
  • AAC is the designated successor – providing up to 50% more playing time, while maintaining the same quality and allowing encoding of higher quality audio (96 kHz)
  • AAC also features built-in copyright protection – making it the system of choice for Electronic Music Distribution (EMD)
key recent consumer audio developments
Key Recent Consumer Audio Developments
  • Audio Completes the Home Theater Experience
  • Digital Surround Formats become Popular
  • Home Audio and Computer Technologies Converge
  • Music Companies Authorize Paid Downloads
  • Sales of Internet Audio Portables Increase
the future
The Future
  • Future of listening to music -- expect the unexpected
  • What do we want?
    • More channels
    • Fewer channels
    • Instant access
    • Flexibility
  • How do we get it?
    • HRTF
    • Psychoacoustics
    • DSP
    • Metadata
want more channels realism
Want: More channels => Realism
  • For home theater applications, the 5.1 format's two discrete digital surround tracks are a major improvement over the original Dolby Surround's single mono surround track.
  • But more discrete digital surrounds would be much better yet. The ability to truly envelope the listener in a soundfield is a function of the number of available channels.
  • Virtual reality, according to a German study, requires at least 18 channels to correctly reproduce the directionality of the diffuse field. Of course, that amounts to an impractical data stream during encoding and decoding and also a horrendous mess of speakers and amplifiers for playback.
  • A more practical solution is a 10.2 system – in existence today
want fewer channels ease
Want: Fewer channels => Ease
  • More than anything else, we seek ease of use and convenience.
    • We would want to have the experience of being surrounded by sound
    • We would want to experience this at any time
    • Even when we are moving around
    • We would like this to be inexpensive (not too many speakers)
    • Portable application using headphones
  • Challenge is to squeeze in multiple channels into a stereo pair without losing the directionality of the original 360 degree soundfield
  • Lake Technology, Sensaura, WaveSurround, MIT Media Labs
want instant access demand
Want: Instant Access => Demand
  • Digital Jukebox
  • We could all be "pierced" with microchips instead of earrings.
  • Those chips may enable you to walk into a music "station" and call out the name of the song you want to hear, only to have it start to play right into your inner ear.
  • The future of music no doubt involves convenience, and some form of an “always on” jukebox
  • Access to every song ever recorded will be at your fingertips – or more likely your vocal chords, as voice-recognition technology will likely become seamlessly integrated into your music-selection process – better yet, we may just want to think about a particular song, and it will start playing in our ear!
want flexibility control
Want: Flexibility => Control
  • Consider recording raw audio with control information relating to its processing contained as part of the ‘audio packet’
  • This has no analogy in the analog world
  • Example 1: raw audio file accompanied by EQ information
  • Example 2: a multi-channel recording of a choir maybe heard from the vantage of the audience or from that of a performer
  • Requires more real time processing capability
  • Allows ‘Final audio’ to be determined at a very late stage -- depending on user preference or the visual scene the audio is accompanying – for example, use a raw audio file with or without ‘telephone’ filter
  • This is done by embedding metadata into the audio
  • MPEG streams; parameters in DCinema, THX Blackbird
enabling technologies
Enabling Technologies
  • HRTF:To find the sound pressure that an arbitrary source produces at the ear drum, we need the impulse response from the source to the ear drum. This is called the Head-Related Impulse Response (HRIR), and its Fourier transform is called the Head Related Transfer Function (HRTF). The HRTF captures all of the physical cues to source localization. Once you know the HRTF for the left ear and the right ear, you can synthesize a direction for a monaural source.
  • Psychoacoustics: Goal is to understand how the human brain processes sound. Mathematical models of the perception process are tested in experiments on human listeners.
  • DSP:Efficient Digital Signal Processing / Faster Digital Signal Processors
flexibility of digital systems
Flexibility of Digital Systems
  • Digital Filters
    • General Equation
    • Hardware Elements
    • Software Implementation
  • Coefficients
  • {FreqResp}
  • Hardware -- more dedicated HW available
  • Software -- always striving to do more; more parallel processing, harnessing the power of PCs