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Digital Audio CompressionDigital Audio Compression

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FormatsFormats

There are many different formats for storing and communicating digital audio:CD audioWavAiffAu MP3

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The Storage ProblemThe Storage Problem

CD quality recording44100 sampling rate16 bit quantization2 channels (stereo)

176.4 Kbytes per second1 minute is ~ 10.5 MBytes74 minutes is ~780 MB

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PsychoacousticsPsychoacoustics

The study of the psychological and physiological principles of sound perception

CDs try to accurately reproduce the original audio signalBut we do not hear all of this signalThe parts that we don’t hear are redundantIf we remove these parts we can store the

signal using less data but without effecting the perceived sound

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Threshold of Hearing & Masking

Threshold of Hearing & Masking

The threshold of hearing curve describes the minimum level at which the ear can detect a tone at a given frequency

Fletcher-Munson curves

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Amplitude MaskingAmplitude Masking

Amplitude masking occurs when a tone shifts the threshold curve upwards in the frequency region that surrounds it

0.

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Critical BandCritical Band

Hair cells on the Basilar membrane respond to the strongest stimulation in their local region

This local region is called the critical band

Critical bands are smaller for low frequency signals than they are for high frequency signals

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Critical BandsCritical Bands

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Amplitude Masking & Thresholds

Amplitude Masking & Thresholds

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Temporal MaskingTemporal Masking

Masking can also occur when tones are sounded at slightly different timesPremasking – signal A is masked by signal B

which occurs laterPostmaking – signal A is masked by signal B

which ends before signal A has startedTemporal masking increases as time

differences reduce

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Temporal MaskingTemporal Masking

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MaskingMasking

Amplitude and temporal masking form a masking area in the time-frequency domain

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Perceptual CodingPerceptual Coding

Perceptual coders analyse the frequency and amplitude content of the input signal and compare it to a model of human auditory perception

Parts of the input signal which are inaudible are removed

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Perceptual CodingPerceptual Coding

A perceptual coder uses a digital filter bank to split a short duration of audio signal into multiple frequency bands

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Perceptual CodingPerceptual Coding

The coder analyses the energy in each of these subbands to determine which subbands contain audible information

Subbands which are not audible are not coded

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Perceptual CodingPerceptual Coding

Quantization bits are assigned according to signal strength above the audibility curve

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Perceptual CodingPerceptual Coding

The purpose of perceptual coding is to reduce the data rate

Perceptual coders maintain sampling frequency, selectively decrease word length

Coders reduction ratio is the ratio of input bit rate to output bit rateRatios of up to 6:1 are often transparent

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Perceptual CodingPerceptual Coding

Because the inaudible content of the signal is removed the playback system’s ability to convey audible music should improveIn theory it is possible to get better

reproduction after perceptual coding than the original! (In theory…)

Perceptual coders more properly code an audio signal for passage through an audio system

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MP3MP3

Mpeg 1 Audio Layer 3Developed to support audio coding for

playback with videoUses :

A filterbank producing 32 subbands from 24ms of audio data

Perceptual coder originally produced by the Fraunhofer Institut Integrierte Schaltungen

Lossless Huffman coding

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MP3MP3

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MP3MP3

Sound quality is highly dependent on the performance of the encoder

Most encoders use constant-bitrate (CBR) encoding. In this mode you choose a target bitrate (e.g. 128kBit/s)

CodecsFraunhoferXing MP3 encoderEtc…

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Joint Stereo CodingJoint Stereo Coding

Takes advantage of interchannel redundancy between stereo channels

Some sounds and some components are equal in both channelsLow frequencies: Bass instruments, strings,

low components of drumsCentrally placed signals: typically vocals

Removing duplication reduces data without effecting perceived sound

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FinFin

Fin