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1. Introduction

The word COMPANDER is a contraction of words COMpressor and ExPANDER. In Telecommunicaton and Signal Processing ,Compander is a non-linear circuit used for improving the signal-to-noise ratio.The data rate is important in telecommunication because it is directly proportional to the cost of transmitting the signal. Saving bits is the same as saving money. Companding is a common technique for reducing the data rate of audio signals by making the quantization levels unequal.

2. How Compander works

A Compander is a non-linear circuit which provides high amplification for low input amplitude signals and low amplification for high input amplitude signals.

A compander operates in a transmitter to compress audio signals before they are transmitted and in a receiver to expand the audio signals after they are received.

An electronic circuit implementing the compander consists of a logarithmic amplifier at the transmitter for compression and an anti-logarithmic amplifier at the receiver to nullify the effect caused by the compression. When an analog signal is applied at input of logarthmic Amplifier,the input signal is compressed and this is sent through channel which comprises of linear variable gain amplifier and at the receiver end,we receive this compressed signal and at end of anti logarthmic amplifier We can retrieve the original signal which is expanded form of compressed signal.

Subcircuit code


.subckt opamp 1 2 6

R1 1 2 1MEG

E1 3 0 2 1 100K

C1 4 0 1.5u

E2 5 0 4 0 1

R2 5 6 100

R3 3 4 1K

.ends opamp

5. Waveforms

Input Waveform-v(1)-green colour Compressed waveform-v(5)-red colour

3. Circuit

4. Main code


.include opsub.cir

x1 2 0 5 OPAMP

x2 8 0 9 OPAMP

v1 1 0 sin(5 10 10k 0 0)

R1 1 2 1K

R2 2 5 10K

R3 2 4 100

R4 2 3 100

D1 5 3

D2 4 5

R5 5 6 100

R6 5 7 100

R7 5 8 10K

D3 8 6

D4 7 8

R8 8 9 1K

.tran 1u 1m




set xbrushwidth=3

set color1=black

set color0=white

plot v(1) v(5)

plot v(5) V(9)



Expanded Waveform-v(9)-red colour

Compressed waveform-v(5)-green colour


Professional wireless microphones use this compander since the dynamic range of the microphone audio signal itself is larger than the dynamic range provided by radio transmission.Compander is used in digital telephony systems, compressing before input to analog to digital converter and then expanding after a digital to analog converter.


Quantization error , effects of noise can be minimized using Compander. Companding is also effective in reducing buzz, hiss and low-level audio tones caused by mild interference. Companding reduces dynamic range so that fewer bits are needed to digitize the audio signal. Companding can improve the useful range of wireless systems as well.

8.Result and conclusion

By narrowing the dynamic range of the signal to be amplified, the noise that is amplified can be reduced.

For better signal to noise ratio (SNR) and to minimize distortion in received signal, compander is used.