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Pulse Code Modulation PCM

Pulse code modulation is a technique, which is used to convert an analog signal into digital signal. PCM is the preferred method of communication within the public switched telephone network (PSTN).

It is an old but widely used concept. We need to convert our voice into digital form before transmission over long distance. Digital transmission has many advantages over analog transmission.We will discuss it after some time.

PCM Steps

In PCM( Pulse code modulation), an analog signal is sampled and converted to a series of binary bits.It has three basic steps.

• Sampling
• Quantization
• Encoding

Why Digital Transmission

• Less susceptible to interference caused by noise due to a discrete level.
• Easy to detect errors due to a discrete level.
• Easy to encrypt(Higher security).
• Cheaper to implement due to advancement in digital electronics.
• It is simpler to store a digital signal.

Sampling

The function of sampling is to sample the continuous wave signal. It measures the amplitude of an analog signal at regular interval.

Nyquist Theorem

To reproduce an original signal, the minimum sampling rate must be twice the higher frequency component of the signal.

This suggested rate of sampling is necessary for the regeneration of the signal at receiving side.

Human frequency range = 0.3 to 3.4 KHz = 4 KHz

8000 samples per second is telephony standard

Quantization

It is the process to convert infinite value signal into a finite level.After sampling, We get a series of signal samples of multiple amplitudes.Quantization is used to limit the amplitude of these samples.

Quantization Levels

A/D output = n bits per sample (quantization level M=2n)

    8 bits = 256 levels

   16 bits = 65,536 levels

Encoding

It is just a binary conversion for a specified voltage level.

Human Voice 

Now after reading the post and watching the video, we can calculate the bit rate for human voice conversion.

Bit rate = sampling rate x number of bits per sample = 8000 x 8 = 64,000 bps = 64 Kbps