Six-Decibel Digital Attenuation

In the interest of future studies of digital echo suppressors operating directly on PCM words and of other possible future studies involving digital attenuation, a technique for 6-dB digital attenuation without linearization of the compressed PCM words from an 8-bit 15-segment compandor (μ = 255 law) is presented. The compandor is assumed to have midtread form at the origin, and all adjacent decision levels are assumed to be separated by powers of two. Although only the results for 8-bit words are presented in detail here, the basic form of the algorithm is applicable to any length of PCM word from a 15-segment compandor. Implementation of the technique can be achieved with logic Circuits which interrogate only 3 bits of each PCM word. In addition, the algorithm is precisely equivalent to linearizing, shifting one place to the right, and recompressing each PCM word, For the 8-bit 13-segment compandor (
A
law), realization of 6-dB attenuation digitally and without linearization is an obvious and well-known procedure, but it is included for completeness.