Digital Communication Block Diagram

Table of contents

Introduction

Source Encoder

The source encoder compresses the data into minimum number of bits. This process helps in effective utilization of the bandwidth. It removes the redundant bits unnecessary excess bits, i.e., zeroes.

Channel Encoder

The channel encoder, does the coding for error correction. During the transmission of the signal, due to the noise in the channel, the signal may get altered and hence to avoid this, the channel encoder adds some redundant bits to the transmitted data. These are the error correcting bits.

Digital Modulator

The signal to be transmitted is modulated here by a carrier. The signal is also converted to analog from the digital sequence, in order to make it travel through the channel or medium.

Channel

The channel or a medium, allows the analog signal to transmit from the transmitter end to the receiver end.

Digital Demodulator

This is the first step at the receiver end. The received signal is demodulated as well as converted again from analog to digital. The signal gets reconstructed here.

Channel Decoder

The channel decoder, after detecting the sequence, does some error corrections. The distortions which might occur during the transmission, are corrected by adding some redundant bits. This addition of bits helps in the complete recovery of the original signal.

Source Decoder

The resultant signal is once again digitized by sampling and quantizing so that the pure digital output is obtained without the loss of information. The source decoder recreates the source output.

Output Transducer

This is the last block which converts the signal into the original physical form, which was at the input of the transmitter. It converts the electrical signal into physical output (Example: loud speaker).

Output Signal

This is the output which is produced after the whole process. Example − The sound signal received.

FM — Frequency Modulation

• High frequency = high energy
• Less power required for transmission
• Unaffected by interference/noise

PCM — Pulse Code Modulation

• Commonly used
• Easy to understand
• Antecedent to all other methods
• Lossless encoding

Instead of a pulse train, PCM produces a series of numbers or digits, and hence this process is called as digital.

Each one of these digits, though in binary code, represent the approximate amplitude of the signal sample at that instant.

In Pulse Code Modulation, the message signal is represented by a sequence of coded pulses. This message signal is achieved by representing the signal in discrete form in both time and amplitude.

PCM Encoding

• Companding PCM
• Differential PCM
• Adaptive Differential PCM
• Linear Predictive PCM
• Delta Modulation
• Modified Discrete Cosine Transform

Basic Elements of PCM

Low Pass Filter

This filter eliminates the high frequency components present in the input analog signal which is greater than the highest frequency of the message signal, to avoid aliasing of the message signal.

Sampler

• p.e. 44.1kHz at 1 second

This is the technique which helps to collect the sample data at instantaneous values of message signal, so as to reconstruct the original signal. The sampling rate must be greater than twice the highest frequency component W of the message signal, in accordance with the sampling theorem.

Based on Nyquist sampling theorem, we can derive that the nominal 4 kHz voice channel can be reconstructed by a sampling rate of 8000 times per second (4000 x 2).

This means that a sample is taken every 125 μsec, or 1 second/8000.

Quantizer

• p.e. 16bits (65.536 levels)

Quantizing is a process of reducing the excessive bits and confining the data. The sampled output when given to Quantizer, reduces the redundant bits and compresses the value.

The exact sample value is replaced by the closest value allowed. The infinite number of levels is transformed into a finite number of levels.

• p.e. 44.1k samples x 16bits = 705.6kbps for a single audio channel
• sample rate x bit depth x number of channels = bandwidth of multichannel audio

Encoder

• The digitization of analog signal is done by the encoder.
• It designates each quantized level by a binary code.
• The sampling done here is the sample-and-hold process.
• Encoding minimizes the bandwidth used.

Regenerative Repeater

This section increases the signal strength. The output of the channel also has one regenerative repeater circuit, to compensate the signal loss and reconstruct the signal, and also to increase its strength.

Decoder

The decoder circuit decodes the pulse coded waveform to reproduce the original signal. This circuit acts as the demodulator.

Reconstruction Filter

After the digital-to-analog conversion is done by the regenerative circuit and the decoder, a low-pass filter is employed, called as the reconstruction filter to get back the original signal.

Hence, the Pulse Code Modulator circuit digitizes the given analog signal, codes it and samples it, and then transmits it in an analog form. This whole process is repeated in a reverse pattern to obtain the original signal.

Broadband, wideband and narrowband

• Broadband — Wideband — Narrowband

USRP (Universal Software Radio Peripheral)

References

• Digital Communication - Quick Guide
• Lecture 11, ELG3175: Introduction to Communication Systems
• Stanford Lecture 13: Pulse Code Modulation
• WHAT IS PULSE CODE MODULATION (PCM)?
• Video 10. Pulse Code Modulation - Digital Audio Fundamentals
• Video Digital Show & Tell
• Video A Digital Media Primer for Geeks