1. 1 Digital Audio Compression

2. 2 Formats  There are many different formats for storing and communicating digital audio:  CD audio  Wav  Aiff  Au  MP3

3. 3 The Storage Problem  CD quality recording  44100 sampling rate  16 bit quantization  2 channels (stereo)  176.4 Kbytes per second  1 minute is ~ 10.5 MBytes  74 minutes is ~780 MB

4. 4 Psychoacoustics  The study of the psychological and physiological principles of sound perception  CDs try to accurately reproduce the original audio signal  But we do not hear all of this signal  The parts that we don’t hear are redundant  If we remove these parts we can store the signal using less data but without effecting the perceived sound

5. 5 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

6. 6 Amplitude Masking  Amplitude masking occurs when a tone shifts the threshold curve upwards in the frequency region that surrounds it 0.

7. 7 Critical 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

8. 8 Critical Bands

9. 9 Amplitude Masking & Thresholds

10. 10 Temporal Masking  Masking can also occur when tones are sounded at slightly different times  Premasking – signal A is masked by signal B which occurs later  Postmaking – signal A is masked by signal B which ends before signal A has started  Temporal masking increases as time differences reduce

11. 11 Temporal Masking

12. 12 Masking  Amplitude and temporal masking form a masking area in the time- frequency domain

13. 13 Perceptual 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

14. 14 Perceptual Coding  A perceptual coder uses a digital filter bank to split a short duration of audio signal into multiple frequency bands

15. 15 Perceptual 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

16. 16 Perceptual Coding  Quantization bits are assigned according to signal strength above the audibility curve

17. 17 Perceptual 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 rate  Ratios of up to 6:1 are often transparent

18. 18 Perceptual Coding  Because the inaudible content of the signal is removed the playback system’s ability to convey audible music should improve  In 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

19. 19 MP3  Mpeg 1 Audio Layer 3  Developed to support audio coding for playback with video  Uses :  A filterbank producing 32 subbands from 24ms of audio data  Perceptual coder originally produced by the Fraunhofer Institut Integrierte Schaltungen  Lossless Huffman coding

20. 20 MP3

21. 21 MP3  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)  Codecs  Fraunhofer  Xing MP3 encoder  Etc…

22. 22 Joint Stereo Coding  Takes advantage of interchannel redundancy between stereo channels  Some sounds and some components are equal in both channels  Low frequencies: Bass instruments, strings, low components of drums  Centrally placed signals: typically vocals  Removing duplication reduces data without effecting perceived sound

23. 23 Fin