Explain the basic architecture of digital switching systems. Explain in detail companding.

Explain the basic architecture of digital switching systems. Explain in detail companding.
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Answer :

A simple N X N time division space switch is shown in Fig. The switch can be represented in an equivalence form as a two-stage network with N X 1 and 1 X N switching matrices for the first and second stages respectively as shown in Fig. The network has one link interconnecting the two stages. Each inlet/outlet is a single speech circuit corresponding to a subscriber line. The speech is carried as PAM analogue samples or PCM digital samples, occurring at 125-µs intervals. When PAM samples are switched in a time division manner, the switching is known as analogue time division switching. If PCM binary samples switched, then the switching is known as digital time division switching. In Fig, the interconnected by a suitable control mechanism and the speech sample transferred from the inlet to the outlet.

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Companding: Companding is the process of compressing and expanding. It consists of compressing the signal at the transmitter and expanding it at the receiver. The modulating signal to be transmitted is passed through an amplifier which has correctly adjusted non linear transfer characteristic, favoring small amplitude signals. These are the artificially large when they are quantized and so the effect of quantizing noise upon them is reduced. The correct amplifier relations are restored at the receiver by expander. It is desirable to agree to some standard for companding in order to realize hardware efficiencies and to permit sending and receiving by a variety of users. The most common application of companding is in voice transmission. North America and Japan have adopted a standard compression curve known as µ law companding. The law µ companding formula is given by 

Vout = ( Vmax X In (1+ µ Vin / Vmax) / In (1+ µ)

 Where

 Vmax = Maximum uncompressed modulating input amplitude.

 Vin = Amplitude of the input signal at particular instant at time 


µ = Parameter used to define the amount of compression

 Vout = Compressed output voltage. 


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