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Data Transmission and Retrieval

The term data is applied to a representation of facts, concepts, or instructions suitable for communication, interpretation, or processing by human beings or by automatic means. Data in most cases consist of pulse type of signals.
The PCM signal is a series of 1s and 0s. Following three modulation techniques are used to transmit a PCM signal:
Fig. 17

Amplitude shift keying (ASK)

Two different amplitudes of the carrier represent the two binary values of the PCM signal. This method is also known as on-off keying (OOK):
1: Presence of carrier of same constant amplitude.
0: Carrier of zero amplitude.

Frequency shift keying (FSK)

The binary values of the PCM signal are represented by two frequencies.
1: Increase in frequency.
0: Frequency unaffected.

Phase shift keying (PSK)

The phase of the carrier wave is changed in accordance with modulating data signal.
1: Phase changed by π.
0: Phase remains unchanged.
The analog signal is sampled by the sampler. The sampled pulses are then quantized. The encoder codes the quantized pulses according to the binary codes. After modulating the PCM signal (by ASK, FSK, or PSK method), the modulated signal is then transmitted into free space in the form of bits (Fig. 18).
Fig. 18

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