Thursday, July 29, 2010

Some Basic - Digital Signal

First some basic stuff. You will see references to 64K (bits) 'channels' all over the place. This is the basic digital voice signal - called Digital Signal 0 or the infamous DS0 for short. The digital voice signal is encoded using PCM (Pulse Code Modulation) and TDM (Time Division Multiplexing). All other classic copper signal hierarchies, known as PDH - the Plesiochronous Digital Hierarchy, such as T3, are defined as multiples of DS0. Why 64K. Well... to digitize narrowband speech (voice) you take a 4KHz spectrum (actually 3.1K). Normal sampling techniques only give reasonable resolution if sampled at twice the frequency (which gives 2 x 4K(ish) = 8K samples per second). Each sample is 8 bits which gives 8K x 8 = 64K bits per second.

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