School of Computer Science & Information Technology

School of Computer Science & Information Technology

Digital Audio Digital Audio

G6DPMMG6DPMM

School of Computer Science & Information Technology

School of Computer Science & Information Technology

G6DPMM - Lecture 14G6DPMM - Lecture 14

Digital AudioDigital Audio

Properties of SoundProperties of Sound

AcousticsAcoustics Pressure waves in airPressure waves in air Pitch - frequency - time (kHz)Pitch - frequency - time (kHz) Amplitude - level - energy (dB)Amplitude - level - energy (dB) Subjective Volume - frequency and energySubjective Volume - frequency and energy

Analogue vs DigitalAnalogue vs Digital

Analogue technologiesAnalogue technologies PhysicalPhysical MagneticMagnetic

Digital technologiesDigital technologies Sample soundSample sound Approximation of analogueApproximation of analogue Must be captured (a2d conversion)Must be captured (a2d conversion)

Sound in MultimediaSound in Multimedia

MIDIMIDI Defines the synthesis of soundDefines the synthesis of sound Analogous to vector graphicsAnalogous to vector graphics

Digital AudioDigital Audio Captured waveformCaptured waveform Analogous to bitmap graphicsAnalogous to bitmap graphics

MIDIMIDI

Musical Instrument Digital Interface.Musical Instrument Digital Interface. Standard for synthesisers and electronic Standard for synthesisers and electronic

instruments and devices.instruments and devices. MIDI “score” describes time stamped sequence MIDI “score” describes time stamped sequence

of notes.of notes. Reproduction dependent upon hardware (MIDI Reproduction dependent upon hardware (MIDI

device).device).

Advantages of MIDIAdvantages of MIDI

Very compact (up to 1,000 times smaller than Very compact (up to 1,000 times smaller than CD audio)CD audio)

Sounds produced by hardware - therefore low Sounds produced by hardware - therefore low system overhead.system overhead.

Low bandwidth requirements.Low bandwidth requirements. High quality - dependent upon hardware.High quality - dependent upon hardware. Scaleable, Editable & cross platform.Scaleable, Editable & cross platform.

Disadvantages of MIDIDisadvantages of MIDI

Hardware dependent - only a faithful reproduction if the Hardware dependent - only a faithful reproduction if the equipment is constant.equipment is constant.

Cannot record “real world” sounds.Cannot record “real world” sounds. Difficult to represent speech.Difficult to represent speech.

Creation of MIDICreation of MIDI Via editing software (sequencer)Via editing software (sequencer) Via instruments / peripheralsVia instruments / peripherals Usually requires musicians or sound effects specialists.Usually requires musicians or sound effects specialists.

Digital AudioDigital Audio

Sampled SoundSampled Sound At regular time intervals a sample is taken, and the At regular time intervals a sample is taken, and the

information is stored digitally.information is stored digitally.

ApplicationsApplications CDCD Digital tape (DAT)Digital tape (DAT) Digital broadcast (radio / TV)Digital broadcast (radio / TV) Most multimedia soundMost multimedia sound

Sampling (capturing)Sampling (capturing)

Sampling hardware - from any analogue source (usually line in).Sampling hardware - from any analogue source (usually line in).

Sampling rateSampling rate frequency of samplesfrequency of samples often called “frequency”often called “frequency”

Sample sizeSample size amount of information stored.amount of information stored. often called “resolution”often called “resolution”

Sampling RateSampling Rate

Sample SizeSample Size

Sampling ParametersSampling Parameters

Common Sample Rate (frequency)Common Sample Rate (frequency) 44.1 kHz (CD audio)44.1 kHz (CD audio) 22.05 kHz22.05 kHz 11.025 kHz11.025 kHz

Common Sample Sizes (resolution)Common Sample Sizes (resolution) 8 bit (256 amplitude states)8 bit (256 amplitude states) 16 bit (65,536 amplitude states)16 bit (65,536 amplitude states)

ExamplesExamples

Frequency(kHz)

Resolution(bits)

Tracks Storage(1 min)

44.1 16 2 10.5 Mb “CD quality”

44.1 16 1 5.25 Mb High quality mono

22 16 1 2.5 Mb Reasonable speech

22 8 2 2.6 Mb Medium qualitystereo

22 8 1 1.3 Mb “TV quality”

11 8 1 650 Kb “Telephone quality”

DistortionDistortion

Distortion is caused when the reconstruction of the waveform is Distortion is caused when the reconstruction of the waveform is unacceptable.unacceptable.

Usually arises from incorrect settings of disparate equipment Usually arises from incorrect settings of disparate equipment (amplitude / levels).(amplitude / levels).

Also caused by insufficient sampling frequency (Nyquist theory).Also caused by insufficient sampling frequency (Nyquist theory).

The Nyquist RuleThe Nyquist Rule

Implications of NyquistImplications of Nyquist

Each half of the waveform must be recordedEach half of the waveform must be recorded there must be 2 samples per periodthere must be 2 samples per period the sampling frequency must be at least twice the highest the sampling frequency must be at least twice the highest

signal frequency.signal frequency.

For example:For example:if the highest frequency is 14,080 Hz, then at least 2x14.08 = 28.16 kHz must be if the highest frequency is 14,080 Hz, then at least 2x14.08 = 28.16 kHz must be used. Thus 22.05 would not suffice - 44.1 would be needed.used. Thus 22.05 would not suffice - 44.1 would be needed.

Editing Digital SoundEditing Digital Sound

Wide range of softwareWide range of software Commercial (eg Sonic Foundry Sound Forge)Commercial (eg Sonic Foundry Sound Forge) Shareware/FreewareShareware/Freeware Bundled with hardwareBundled with hardware

Common OperationsCommon Operations TrimmingTrimming Splicing and assemblySplicing and assembly Volume adjustmentsVolume adjustments DownsamplingDownsampling Fades & other effectsFades & other effects

Sound CompressionSound Compression

In principle very similar to image or moviesIn principle very similar to image or movies

Usually (not always) lossyUsually (not always) lossy

Requires codec for compression and playbackRequires codec for compression and playback

Streaming is commonplace and well developed (eg Streaming is commonplace and well developed (eg RealAudio, QuickTime, MS Media Player).RealAudio, QuickTime, MS Media Player).

Common CodecsCommon Codecs

GSM - mostly for voice (3.5-28 Mb per hour)GSM - mostly for voice (3.5-28 Mb per hour) ADPCM (Microsoft or IMA) - high quality, but low compression (14-ADPCM (Microsoft or IMA) - high quality, but low compression (14-

152 Mb per hour)152 Mb per hour) Lernout & Hauspie - voice only (3.5-7 Mb per hour)Lernout & Hauspie - voice only (3.5-7 Mb per hour) CCIT A-Law - European TAPI devices, very high quality (24-302 Mb CCIT A-Law - European TAPI devices, very high quality (24-302 Mb

per hour)per hour) CCIT CCIT -Law - American TAPI devices, very high quality (24-302 Mb -Law - American TAPI devices, very high quality (24-302 Mb

per hour)per hour)

Common File FormatsCommon File Formats

Very large numberVery large number Audio IFF (AIFF) - developed by Apple for Macintosh, also used by SGI and various software.Audio IFF (AIFF) - developed by Apple for Macintosh, also used by SGI and various software. AU Audio - uncompressed format developed by Sun, widely used on the Internet.AU Audio - uncompressed format developed by Sun, widely used on the Internet. WAV - developed by Microsoft for Windows.WAV - developed by Microsoft for Windows. MP3 - highly compressed, high quality spin-off of the MPEG projectMP3 - highly compressed, high quality spin-off of the MPEG project RealAudio RAM - developed for streaming by RealMedia.RealAudio RAM - developed for streaming by RealMedia.

Differ in terms of:Differ in terms of: Compression algorithmsCompression algorithms MetadataMetadata Security & EncryptionSecurity & Encryption