Alex Balashov wrote:
Like many other compression schemes, it also shrinks the size of the data by referring via shorthand to elements of a waveform table/model that approximate the quantised value of a sample, but do not EQUAL it.
Side note: Actually, G.711 - and indeed, standard DS1 digital PCM bearer channels - do some "rounding" too. It's called logarithmic companding. The type of logarithmic companding is indicated by the "mu" (uLaw) or A suffix. The basic idea there is that the more median portion of the spectrum of human vocal capabilities is expressed with closer fit, but the more outlying portions more approximately, with deltas in the "steps" that grow as a logarithmic progression. This is why hold music sounds very lopsided on a phone - even without the fancy variable bit-rate and adaptive codecs used by mobile equipment. Some parts of it are very obviously clearer than others. The other - and perhaps even more significant - reason is the 3.1 KHz total bearer spectrum. Good human hearing tops out at about 20 KHz. Most music relies a lot on the higher range - certainly, well above ~3 KHz. However, T.30 terminals (fax machines) were designed to work with this logarithmic companding in mind. Companding is not the same as "vicious compression." It's roughly the same difference as between a raw PCM WAV rip of a song from an audio CD, and the MP3 format. The latter gets you upwards of 10:1 compression ratio without significant loss from the point of view of human subjectivity, but if you're trying to encode any data into the analog signal, forget it. -- Alex Balashov - Principal Evariste Systems Web : http://www.evaristesys.com/ Tel : (+1) (678) 954-0670 Direct : (+1) (678) 954-0671