L5-pcm - Pulse-code modulation (PCM) A quantised speech...

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1 Pulse-code modulation ( PCM ) A quantised speech signal is seldom directly used for transmission in view of noise immunity consideration. Very often, an L- level quantised sample is encoded into a more desirable equivalent form, representing by a group of n signalling pulses each with M possible amplitude levels, such that n M L = . In this case, M << L for better noise immunity. For example: A 256- level sample can be represented in the form of an 8-bit binary code word (i.e., 2 = M , 8 = n ), or it can be represented by a 4-symbol quaternary code word (i.e., 4 = M , 4 = n , so that 256 4 4 = ). Systems embodying the transmission of digitised and coded signals are called pulse-code modulation ( PCM ) systems. The basic elements of a PCM system Received waveform Signal conditioning circuit Analog input signal Sampler Quantiser Encoder Regenerative repeater Regenerative repeater Equalising amplifier Decision circuit Decoder Lowpass filter Recovered signal Transmission channel Usually, this encoding process is done in two stages: 1. Quantised samples are first encoded into binary words (convenient for digital signal processing). 2. The resultant binary code words are used to control a pulse circuit to generate the desired signalling pulses. DAC Signalling pulses
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2 An M-ary PCM system is one that employs signalling pulses, each with M possible amplitude levels. For example, in a binary PCM system, the number of levels per transmitted pulse is 2, while if a 3 -level pulse is transmitted, M=3 , and the system is termed ternary PCM . The choice of M is a system compromise depending largely on consideration of the following factors: 1. Noise immunity - if M is made too large, then it is easier for noise in the system to obscure the adjacent levels. Therefore, a large M-ary PCM system is used only in a low noise environment. 2. Circuit complexity - Decision circuit, which is used to differentiate the closely spaced individual levels associated with a large value of M , becomes complex. Achievable circuit accuracy will be the limiting factor. 3. Bandwidth - As the number of signalling pulses, n , per code word becomes greater (i.e., smaller M ), the bandwidth required for transmission is increased. A large M (or in the limit n=1, M=L for the uncoded system) will then require a smaller transmission bandwidth. Bandwidth expansion in PCM : Assume the sampled and quantised signal is encoded into an equivalent group of binary pulses. Note:
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This note was uploaded on 05/09/2011 for the course ENGR 601 taught by Professor Kah-chung during the Three '11 term at Curtin.

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L5-pcm - Pulse-code modulation (PCM) A quantised speech...

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