1 Manipulating Digital Audio

2 Pulse Code Modulation (PCM)  This is a means of encoding the digital signal for transmission or storage

3 PCM  Very robust – only the presence or absence of a pulse is looked for  Several signals can sent simultaneously using time division multiplexing  Can contain synchronization and error correction information

4 Types of Codes  There are several possible codes that can be used with PCM

5 Error Correction  With analogue audio there is no opportunity for error correction  If a signal is distorted or disrupted then it is irrevocably damaged  Digital data can contain redundancy  This enables the reproduced data to be checked for errors  Further processing can correct or conceal errors  Strong error correction techniques relax the manufacturing tolerances for CDs, DVDs etc.

6 ISBN- Error Detection  International Standard Book Number  ISBN The hyphens separate the different parts of the number: 1 : A 0 or a 1 in this position indicates that the book was published in an English speaking country : This is the identification number of the publisher 083 : Identifies the specific title and edition 8 : is the check digit

7 ISBN- Error Detection  ISBN is a ten digit number  Multiply the first number by 10  Multiply the second number by 9  Etc..  Sum the results  This number should be divisible by 11    X

8 Error Correction Methods  Cyclic redundancy check (CRCC)  Cyclic block code that generates a parity check word

9 Digital to Analogue Demultiplexer Processing (error correction) Reproduction demodulation Output sample and Hold Output lowpass filter D to A Converter Output sample and Hold Output lowpass filter D to A Converter Analogue output (L) Analogue output (R)

10 D to A Converter  Requires precision  With a +/- 10v scale a 16-bit converter will output steps of v  Main problems are:  Absolute linearity error – deviation from ideal quantization staircase  Differential linearity error – relative deviation from the ideal staircase by any individual step

11 Absolute Linearity Error  Straightness of converter output Digital input Analogue output

12 Differential Linearity Error  Deviation of individual steps Digital input Analogue output

13 Zero-cross Distortion

14 High-bit D/A Conversion  Using an 18-bit converter to perform 16- bit conversion  Each 16-bit LSB is represented by 4  Gives better 16-bit conversion than a 16- bit converter

15 Sample and Hold  D/A converters may require time to stabilise to the correct output levels

16 Low-Pass Filter  Anti-imaging filter  Removes all frequency content above the half- sampling frequency  Converts the D/A converters output pulse amplitude modulation (PAM) output to a smoothly continuous waveform Output of sample and hold Output of anti-imaging filter

17 Oversampling  Analogue brick-wall filters introduce phase shift and distortion  To get around these problems we can use oversampling with a digital filter  The oversampling filter has two tasks:  Re-sample at a higher rate  Filter through interpolation

18 Oversampling  The original signal is sampled at f s and so has images centred around multiples of f s

19 Oversampling  Re-sampling involves inserting zero valued samples between the original samples

20 Oversampling  The digital filter performs interpolation to form new sample values  The output filtered signal has images around multiples of the oversampling frequency fa

21 Oversampling  As the distance between baseband and sidebands is larger, a gentler analogue filter can remove the images without causing phase shift or other artefacts

22 Fin