Brian Willoughby wrote:
[ > Telephone is nominally 8 kHz mono (i.e. really bad) though I
[ > think the use of digital voice codecs in the last 20 years may
[ > have improved on this a bit.
[
[ Telephone lines (POTS) have a frequency range of 300-3400Hz. That
[ means 7kHz mono should be enough, although 8kHz is generous towards
[ the transition bandwidth/roll-off.
7 kHz would require a tighter brick-wall filter than even CD, are you even
sure it's possible to go from flat to silent in 100 Hz (between 3400 Hz and
3500 Hz)?
Almost all telephone connections are digital. Certainly long distance, and
probably local as well. Since that is all sampled at 8 kHz, you'd be reducing
the bandwidth even further by sampling at only 7 kHz. In other words, the
standard sampling rate for telephone voice is certainly 8 kHz.
Yup, it's 8kHz. The channel is theoretically from 0-4kHz (with only
300-3000 being available to the user).
Also, telephone quantization is only 8 bits (rather than the typical
16) and the quantization levels are not linearly mapped.
8,000 samples per second, each 8 bits = 64kbps = one "B" (Bearer) Channel
in the telephone world. MPEG Audio Layer 3 is optimized for 64kbps
because of its ubiquity.
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