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Howell Istance School of Computing De Montfort University

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1 Howell Istance School of Computing De Montfort University
Digital Audio Howell Istance School of Computing De Montfort University © De Montfort University, 2001

2 © De Montfort University, 2001
What is sound? Variation in air pressure caused by compression and decompression of molecules Caused by friction producing force (stick striking symbol, finger plucking guitar string) ‘Waves’ produced by cohesion of molecules, which fall on eardrum or microphone Directly and through reflection off surfaces in room Ear can detect frequencies in the range 20Hz to 20khZ Ear has very high dynamic response compared with eye (ie ability to detect changes in pressure) Requires much higher sampling rates to digitise audio compared with images © De Montfort University, 2001

3 © De Montfort University, 2001
Properties of sound… Waveform – distinctive pattern of variations in airpressure Musical instruments produce orderly repeating waveforms Noise produces random chaotic waveforms Fourier (French Mathematician) demonstrated how any wave form can be decomposed into a series of component sine waves of different frequencies different frequency components, or pure tones, which are added together to produce a complex waveform are called the frequency spectrum of that waveform © De Montfort University, 2001

4 Same note.. different waveforms
Both figures show an ‘A’ note, left played on an alto sax and the right on a tenor sax. Both have additional frequencies as well as the main 440Hz © De Montfort University, 2001

5 Physical and subjective attributes..
Important to distinguish between the properties of a stimulus and those of a subjective response to that stimulus A linear increase in the stimulus value does not necessarily produce a similar increase in the subjective response Stimulus value Subjective response (luminance) (brightness) Amplitude of wave Loudness of sound Frequency of wave Pitch of sound Several attributes (hard to define) Timbre of sound © De Montfort University, 2001

6 Amplitude and Frequency
Amplitude measured in decibels The louder a sound is, the more it will mask or dominate other other sounds adjacent to it in time Frequency measured in cycles per second (Hertz – Hz) More digital information required to encode higher frequency sounds, lower pitched sounds degraded less by low sample rates Timbre loosely defined by ‘tone’, ‘colour’, ‘texture’ of sound that enables brain to differentiate one tone from another Affected by acoustic properties of instruments and room © De Montfort University, 2001

7 © De Montfort University, 2001
Digitising sound Analogue signal is sampled and converted to a series of digital values (A to D converter) Digital values later converted back to analogue for playback through speakers (D to A conversion) Parameters are frequency at which samples are taken and the resolution of each sample (i.e number of bits used to encode analogue signal value Nyqvist’s theorem prescribes minimum sample rate in order to be able to re-construct analogue signal If maximum frequency in the waveform is n Hz, then minimum sample rate should be 2n Hz © De Montfort University, 2001

8 Sampling and Quantizing
Sampling – process of acquiring an analogue signal Quantizing – conversion of held signal into sequence of digital values © De Montfort University, 2001

9 © De Montfort University, 2001
Sample rates If upper range of ear is 20Khz, then there is no need to faithfully reproduce frequency components in signals higher than this. CD quality: at least 2 x 20KHz = 44.1KHz The human voice has few frequency components lower than 100Hz, or higher than 3000Hz - a bandwidth of 2900Hz Speech: at least 2 x 2.9KHz = 8KHz © De Montfort University, 2001

10 © De Montfort University, 2001
Sample data rates For CD quality, Rate = 44.1Khz (44100 samples per second) Resolution = 16 bits Stereo = 2 channels Data rate = * 16 * 2 bits/second = bits/sec (10Mb storage for 1 minute of recorded sound) © De Montfort University, 2001

11 Examples of data rates and quality
Sample Rate Resolution Stereo/Mono Bytes (1 min) 44.1 KHz 16 bit Stereo Mb 44.1 KHz 8 bit Mono Mb 22.05 KHz 16 bit Stereo Mb 22.05 KHz 8 bit Mono Mb 11 KHz 8 bit Mono Kb 5.5 KHz 8 bit Mono Kb CD quality audio As good as a TVs audio As good as a bad phone line © De Montfort University, 2001

12 Digitized vs Synthesized
Multimedia sound comes from two sources: Digitized – from an external (sampled) real life sound Synthesized – created from waveforms in a sound card for example Traditional analogue sound synthesis is achieved by Creating a waveform using an oscillator, which sets the basic frequency Adding an "envelope", by specifying parameters such as attack, decay, sustain, release Then sending through filter(s) to modify timbre The envelope properties define how quickly the sound starts or stops, and at what speed it fades or grows. The earliest devices for creating synthesized sound were analogue, and used varying voltages to create this effect. © De Montfort University, 2001

13 Digital Synthesized Sound
FM (Frequency Modulation) Mix oscillator frequencies to produce waveforms Control parameters to produce ‘natural’ sounds Wavetable A table of waveforms (samples) are held in memory (ROM/RAM) Play back these at various speeds to alter pitch Typical examples of FM sound cards use the Yamaha OPL3 chip Wavetable cards are the norm now – most PCs which are new have them. For the whole range of pitch for a particular sound, there is likely to be a number of samples at certain pitch intervals – one consequence of this is that the timbre of the sound may change noticeably at certain points in the scale © De Montfort University, 2001

14 © De Montfort University, 2001
Editing and mixing Editing: modification of waveform to remove recording artefacts (e.g. noise), cutting required parts of waveform, adding effects such as reverb, changing transients characteristics Mixing: creating a composite sound from several tracks Directly analogous to concept of layers in image processing © De Montfort University, 2001

15 © De Montfort University, 2001
Audio Compression “The eye integrates, whereas the ear differentiates” The ear is sensitive to a huge frequency range This leads to very high data rates Audio doesn’t lend itself to high compression with standard ‘lossless’ techniques It has been said that "the eye integrates, whereas the ear differentiates". This refers to the huge frequency range that the ear is sensitive to, and the wide range of frequencies present in everyday sound such as speech and music, which the ear has to interpret. This range of frequencies leads to high data rates, which have driven the development of a variety of compression techniques. For various reasons, audio data doesn't easily yield high compression rates when standard textual compression methods are used - there is not the same repitition of data that lends itself to lossless techniques such as LZW © De Montfort University, 2001

16 None/ Lossless techniques
PCM: CD-Audio uses no compression, simply encoding data in a linear manner ADPCM: Adaptively looks at similarities in adjacent samples and encodes the differences giving 2:1/ 4:1 compression Microsoft WAV files can be in these two forms © De Montfort University, 2001

17 MPEG - Psychoacoustics
Interference: receptors interfere with one another Critical Bands – certain frequencies bands cause masking at certain levels Temporal masks – a loud sound ‘blinds’ us for a short while to soft sounds MPEG Audio uses Psychoacoustics to aid in compression Take a look at: © De Montfort University, 2001

18 © De Montfort University, 2001
Psychoacoustic model Throw away samples which will not be perceived, ie those under the curve © De Montfort University, 2001

19 © De Montfort University, 2001
Masking effects Throw samples in region masked by louder tone © De Montfort University, 2001

20 © De Montfort University, 2001
MPEG Audio MPEG-1 has data rate of about 0.3 Mbits/sec Compression factor from 2.7 to 24 At 6 – expert listeners could not distinguish between original and encoded Supports samples at 32/ 44.1 and 48KHz MPEG-3 gets near CD quality audio at about 112 Kbits/sec © De Montfort University, 2001

21 © De Montfort University, 2001
MPEG Audio Algorithm (MP1) Sub-band filter: divide audio to approximate 32 critical bands Psycho-model: determine masking caused by band – if power below threshold then don’t encode Determine # of bits (accounting for quantization causing masking – remove this) MP3 introduces temporal masking and variable critical bands © De Montfort University, 2001

22 MIDI – Musical Instruments
Digital Interface – supported by many instruments/ computers/ manufacturers (1980) Defines set of messages indicating note/ instrument/ pitch/ attack etc Sound card/ Synthesizer takes this symbolic message and ‘creates’ matching sound Sampled sounds can be stored by users on better equipment Compare waveforms to bitmapped images, midi to vector graphics © De Montfort University, 2001


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