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Manipulating Digital AudioManipulating Digital Audio
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Pulse Code Modulation (PCM)
Pulse Code Modulation (PCM)
This is a means of encoding the digital signal for transmission or storage
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PCMPCM
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
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Types of CodesTypes of Codes
There are several possible codes that can be used with PCM
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Error CorrectionError Correction
With analogue audio there is no opportunity for error correctionIf a signal is distorted or disrupted then it is
irrevocably damagedDigital 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.
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ISBN- Error DetectionISBN- Error Detection
International Standard Book NumberISBN 1-55105-083-8. 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
•55105 : This is the identification number of the publisher
•083 : Identifies the specific title and edition
•8 : is the check digit
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ISBN- Error DetectionISBN- Error Detection
ISBN is a ten digit numberMultiply the first number by 10Multiply the second number by 9Etc..Sum the resultsThis number should be divisible by 11
0-7167-8321-50-89678-354-10-333-77339-X
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Error Correction MethodsError Correction Methods
Cyclic redundancy check (CRCC)Cyclic block code that generates a parity
check word
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Digital to Analogue Digital to Analogue
DemultiplexerProcessing
(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)
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D to A ConverterD to A Converter
Requires precisionWith a +/- 10v scale a 16-bit converter will
output steps of 0.000305vMain problems are:
Absolute linearity error – deviation from ideal quantization staircase
Differential linearity error – relative deviation from the ideal staircase by any individual step
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Absolute Linearity ErrorAbsolute Linearity Error
Straightness of converter output
Digital input
Analogue output
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Differential Linearity ErrorDifferential Linearity Error
Deviation of individual steps
Digital input
Analogue output
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Zero-cross DistortionZero-cross Distortion
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High-bit D/A ConversionHigh-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
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Sample and HoldSample and Hold
D/A converters may require time to stabilise to the correct output levels
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Low-Pass FilterLow-Pass Filter
Anti-imaging filterRemoves 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
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OversamplingOversampling
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 rateFilter through interpolation
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OversamplingOversampling
The original signal is sampled at fs and so has images centred around multiples of fs
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OversamplingOversampling
Re-sampling involves inserting zero valued samples between the original samples
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OversamplingOversampling
The digital filter performs interpolation to form new sample values
The output filtered signal has images around multiples of the oversampling frequency fa
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OversamplingOversampling
As the distance between baseband and sidebands is larger, a gentler analogue filter can remove the images without causing phase shift or other artefacts
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FinFin
Fin