A-law method

The A-Law Method, A-Law Method, is a method for the dynamic compression of audio signals described in ITU RecommendationG.711. The purpose of dynamic compression is to improve the signal-to- noise ratio( SNR) under the same transmission conditions.

The method uses a logarithmic dynamic range characteristic, which has a high dynamic range especially at low input levels and a very low dynamic range at high input levels. This reduces the noise at low levels, i.e. for soft sounds. In this case, for example, the increase of the input signal at the lowest levels between 0 and 0.05 causes a tenfold increase of the output signal.

The A-Law method is an ITU standard. The name A-Law is related to the logarithmic function of the compressor curve, and to the constant of A = 87.6 used in it. If the A constant is 1, then no compression takes place, and the increase of the output signal in this case corresponds to the increase of the input signal. The method is used in the conversion between analog and digital signals in PCM technology.

The A-law method is mainly used in Europe, while in the USA a method that differs slightly in the quantization steps, the µ-law method, is used. This method is characterized by a dynamic characteristic curve that is even steeper in the low level range than that of the A-Law method.