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Date Created: 11/18/15
CM3106 Chapter 4 Introduction to Digital Audio Prof David Marshall davemarshall cscardiffacuk and Dr Kirill Sidorov KSidorov cscfacuk wwwfacebookcomkirillsidorov UNIVERSITY PRIFYSGOL School of Computer Science 84 Informatics Cardiff University UK M 1110 10101010 011111001111 I Microphone I Receives sound I Converts to analog signal I Computer like discrete entities Need to convert Analogto Digital Dedicated Hardware eg Soundcard Also known as Digital Sampling CM3106 Chapter 4 Digital Audio Digital Sampling Sample Rates and Bit Size Bit Size Quantisation How do we store each sample value 8 Bit Value 0 255 16 Bit Value Integer 0 65535 Sample Rate How many Samples to take 11025 KHz Speech Telephone 8 KHz 2205 KHz Low Grade Audio WWW Audio AM Radio 441 KHz CD Quality CM3106 Chapter 4 Digital Audio Digital Sampling Digital Sampling 1 Sampling process basically involves I Measuring the analog signal at regular discrete intervals I Recording the value at these points xi llllllnc 39 739 1739 3T 1739 CM3106 Chapter 4 Digital Audio Digital Sampling Digital Sampling 2 9 1011 1213 V CM3106 Chapter 4 Digital Audio Digital Sampling Nyquist s Sampling Theorem The Sampling Frequency is critical to the accurate reproduction of a digital version of an analog waveform Nyquist s Sampling Theorem The Sampling frequency for a signal must be at least twice the highest frequency component in the signal CM3106 Chapter 4 Digital Audio Digital Sampling Sampling at Signal Frequency CM3106 Chapter 4 Digital Audio Digital Sampling Sampling at Twice Nyquist Frequency iliilliI F nunu CM3106 Chapter 4 Digital Audio Digital Sampling Sampling at above Nyquist Frequency CM3106 Chapter 4 Digital Audio Digital Sampling If you get Nyquist Sampling Wrong 1 on so 39 70 N 0 Q so a 0 M 9 30 a 29 10 ID 6 O O I 16 a a 0 I I5 N 33 00 I0 70 70 0 60 so so N 40 50 2390 20 w w l or o h A A A Q n Digital Sampling Artefacts Arise Effect known as Aliasing Affects Audio Imagery and Video CM3106 Chapter 4 Digital Audio Digital Sampling If you get Nyquist Sampling Wrong 2 CM3106 Chapter 4 Digital Audio Digital Sampling Implications of Sample Rate and Bit Size 1 Affects Quality of Audio I Ears do not respond to sound in a linear fashion I Decibel dB a logarithmic measurement of sound I 16 Bit has a signal to noise ratio of 98 dB virtually inaudible I 8 bit has a signal to noise ratio of 50 dB I Therefore 8 bit is roughly 8 times as noisy I 6 dB increment is twice as loud CM3106 Chapter 4 Digital Audio Digital Sampling Implications of Sample Rate and Bit Size 2 Affects Size of Data File Type 441 KHz 2205 KHz 11025 KHz 16 Bit Stereo 101 Mb 505 Mb 252 Mb 16 Bit Mono 505 Mb 252 Mb 126 Mb 8 Bit Mono 252 Mb 126 Mb 630 Kb Memory Required for 1 Minute of Digital Audio CM3106 Chapter 4 Digital Audio Digital Sampling Practical Implications of Nyquist Sampling Theory Filtering of Signal I Must low pass filter signal before sampling Input Signal h Filtered Signal Lew Pass Filter WE339 S ampled MD Converter iDieieli I Otherwise strange artefacts from high frequency above Nyquist Limit signals would appear in the sampled signal CM3106 Chapter 4 Digital Audio Digital Sampling Why are CD Sample Rates 441 KHz Why are CD Sample Rates 441 KHZ CM3106 Chapter 4 Digital Audio Digital Sampling Why are CD Sample Rates 441 KHz Why are CD Sample Rates 441 KHZ Upper range of human hearing is around 2022 KHz Apply Nyquist Theorem CM3106 Chapter 4 Digital Audio Digital Sampling Common Digital Audio Formats I Popular audio file formats include I au Origin Unix Sun aiff MAC SGI I wav PC DEC I Compression can be utilised in some of the above but is not Mandatory I A simple and widely used by above audio compression method is Adaptive Delta Pulse Code Modulation ADPCM I Based on past samples it predicts the next sample and encodes the difference between the actual value and the predicted value I More on this later Audio Compression CM3106 Chapter 4 Digital Audio Digital Audio Formats Common Audio Formats Cont I Many formats linked to audio applications I Most use some compression I Common ones I Sounblaster voc Can use Silence Deletion More on this later Audio Compression ProtoolsSound Designer sd2 Realaudio ra AAC Apple mp4 More Later Flac flac More Later I I I Ogg Vorbis ogg I I I Dolby AC coding More Later I MPEG AUDIO More Later MP3 and MPEG4 CM3106 Chapter 4 Digital Audio Digital Audio Formats Synthesis Pipeline l Synthesise sounds hardware or software more later I 1 Client produces sound only send parameters to control sound later Synthesis Methods More Later I FM Frequency Modulation Synthesis used in low end Sound Blaster cards OPL 4 chip Yamaha DX Synthesiser range popular in Early 198039s I Wavetable synthesis wavetable generated from sampled sound waves of real instruments I Additive synthesis make up signal from smaller simpler waveforms I Subtractive synthesis modify a complex waveform but taking out Filtering elements I Granular Synthesis use small fragments of existing samples to make new sounds I Physical Modelling model how acoustic sound in generated in software I Sample based synthesis record and play back recorded audio often small fragments and audio processed Most modern Synthesisers use a mixture of sample and synthesis CM3106 Chapter 4 Digital Audio Digital Audio Formats 23
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