[Flac] Standard encoding rates?

[Mark Rudholm]

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.