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Digital Communication  PAM and Time Division Multiplexing             

Deepti Gupta                    2009EE10387  Nisha Mohariya 

 

2009EE10403 

PAM AND TIME DIVISION MULTIPLEXING  

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The Transmitter Consider the conditions at a transmitter, where two messages are to be sampled and combined into a two-channel PAM/TDM signal. If two such messages were sampled, at the same rate but at slightly different times, then the two trains of samples could be added without mutual interaction. This is illustrated in Figure 1.

The width of these samples is δt, and the time between samples is T. The sampling thus occurs at the rate (1/T) Hz. Figure 1 is illustrative only. To save cluttering of the diagram, there are fewer samples than necessary to meet the requirements of the sampling theorem. This is a two-channel time division multiplexed, or PAM/TDM, signal. One sample from each channel is contained in a frame, and this is of length T seconds. In principle, for a given frame width T, any number of channels could be interleaved into a frame, provided the sample width δt was small enough.

The Receiver Provided the timing information was available - a knowledge of the frame period T and the sampling width δt - then it is conceptually easy to see how the samples from one or the other channel could be separated from the PAM/TDM signal. An arrangement for doing this is called a de-multiplexer. An example is illustrated in Figure 2.

The switching function s (t) has a period T. It is aligned under the samples from the desired channel. The switch is closed during the time the samples from the desired channel are at its input. Consequently, at the switch output appear only the samples of the desired channel. From these the message can be reconstructed.

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This is the TDM output from the TRUNKS module.

Here we multiply the TDM signal, with a pulse train, to separate one of the signals.

Here one period of TDM output is multiplied with a narrow pulse, to recover signal 1.

Here the period is multiplied with a shifted version of a wider pulse to recover signal 2.

 

 

—–‘”‹ƒŽ—‡•–‹‘• Q What is the effect of (a) widening, (b) decreasing the width of the switching pulse in the PAM/TDM receiver? A (a) widening the switching pulse, may result in interference, of the two signals carried in the time division multiplexed signal. (b) decreasing the width of switching pulse, can cause incomplete extraction of the signal.

Q In practice there is often a ‘guard band’ interposed between the channel samples at the transmitter. This means that the maximum number of channels in a frame would be less than (T/δt). Suggest some reasons for the guard band. A If there is no guard band, there may be overlap between the different channels, if δt is large. Also, at the time of demultiplexing, it would be difficult to separate the two channels perfectly.

Q If the sampling width δt of the channels at the PAM/TDM transmitter was reduced, more channels could be fitted into the same frame. Is there an upper limit to the number of channels which could be fitted into a PAM/TDM system made from an infinite supply of TIMS modules? Discuss. A Even if δt is reduced, there should be a guard band between the consecutive signals, to avoid interference, therefore there is an upper limit to the number of channels which could be fitted into a PAM/TDM system.

Q What would you hear in the HEADPHONES if the PAM/TDM was connected direct to the HEADPHONE AMPLIFIER, with the 3 kHz LPF in series? This could be done by placing a TTL high at the TTL CONTROL INPUT of the DUAL ANALOG SWITCH you have used in the DUAL ANALOG SWITCH module. A HEADPHONES part was not done.But the guess is, One would hear, all the different frequency components present in the audible spectrum. A single frequency could not be recognised, it sounds like noise.