companding vs expanding-Difference between companding and expanding

This page on companding vs expanding describes difference between companding and expanding techniques. The other useful difference between terms are also provided here.

The analog voltage Range -> 1000 V to 1V
Lower levels predominate
Most of the conversations take place at normal low level. Most of the signals are at low level, the quantizing error will be larger. This results into distorted or garbled sound.

In addition to incresed quantization error, low level signals are more susceptible to noise.

At the transmit end, voice signal which need to be transmitted is compressed (i.e. dynamic range is reduced). Here low level signals are emphasized and high level signals are de-emphasized.

At the receiving end, recovered signal is fed to an expander circuit which does the reverse of compander circuit counterpart i.e. it de-emphasize the lower level signals anf emphasize the higher level signals. Hence transmitted signal is recovered back at the receiver.
Refer Pre-emphasis VS De-emphasis➤➤.

The most common form to overcome problem of quantization error and effects of noise is known as companding. The companding process involves both compression and expansion.
The companding has following advantages:
• It minimizes quantization error
• It minimizes effects of noise
• It reduces dynamic range so that fewer bits are needed to digitize the audio signal.
• It helps in improving quality of signal before transmission.


compression amplifier

As we know Analog to Digital conversion (ADC) is used in PCM modulation. During A to D conversion analog signal gets distorted, this result in quantization error. The figure-1 depicts simple compression amplifier.


expansion amplifier

At the receiving end, digital signals are translated into analog signals. An analog signal is passed through expander amplifier. It performs opposite function of compression amplifier. The figure-2 depicts expansion amplifier typically used at the receiver.

companding vs expanding curve

The figure-3 depicts variation of output voltage w.r.t. input voltage during expansion. As shown low level signals are amplified less compare to high level signals. If compression and expansion curves are equal and opposite, then it results into highly accurate re-production of original transmitted signal.

There are two basic types of companding in telephone systems across the world viz. a-law and μ-law.
Refer basics of a law and μ-law➤➤.

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