JEE Main & Advanced Physics Communication System / संचार तंत्र 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 consists of pulse type of signals.

The pulse code modulated (PCM) signal is a series of 1's and 0's. The following three modulation techniques are used to transmit a PCM signal.

(1) 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.

(2) Frequency shift keying (FSK) : The binary values of the PCM signal are represented by two frequencies.

1 : Increase in frequency

0 : Frequency unaffected

(3) Phase shift keying (PSK) : The phase of the carrier wave is changed in accordance with modulating data signal.

1 : Phase changed by \[\pi \] 

0 : Phase remains unchanged.

The analog signal is sampled by the sampler. The sampled pulses are then quantised. The encoder codes the quantised 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.