The audio quality with digital voice communications
depends on a ton of other factors. Straight PCM digital is what
you have on CD's and movie sound tracks. We all know that that
can be as good as it gets. Digital cell phones and two-way
radios including DSTAR start out by digitizing your voice in
straight PCM and then they get it ready for the limited
bandwidth of sending it over the air. There are lots of schemes
for compression and all of them loose sound quality, sometimes,
lots of sound quality. One scheme I do know a little bit about
is called LPC or Linear Predictive Coding. On the transmit end,
your voice is analyzed and turned in to numbers representing
frequency, amplitude and wave form information. At the receive
end, a synthesizer turns those numbers back in to audio at the
cost of sound quality due to the inevitable mathematical issues
of rounding and timing that are part of anything this complex.

	Sure, you could send the straight PCM over the air but
every voice channel would be about 100 KHZ wide or more. LPC
encoding actually uses about 100 bits per second during silence
and may go up to 9600 bits during sound which is much, much
better as far as bandwidth.

	Also, you probably remember that the minimum band width
for the human voice is 300 to 3000 HZ. That is what the
telephone industry designs all their circuits to handle. You can
use 8,000 samples per second for voice and it is pretty decent.
If some system like LPC is built around that frequency range,
the mathematical compromises plus occasional damage to the
signal due to radio dropouts will make the sound pretty bad.
Some of those digital systems fail interestingly in that a voice
will suddenly turn in to weirdly musical sound like R2D2 from
"Star Wars" because the receiving filter only got part of the
information it should have gotten so it sends out the last good
sample it got and sort of hangs. It is the audio equivalent of
the picture freezes we get on digital television systems.

	One other thing to remember. You've also got to keep Mr.
Nyquist happy. I'll let you ponder that last statement.

73

Steve Dresser writes:
> > And don't forget, your ts2000 is all digital as well for the audio 
> output,
> > so don't be too quick to discount digital audio.
> 
> I know that the noise reduction is digital processing in the audio 
> circuit,
> and the high and low cut filters are digital as well, but I'm not sure 
> about
> the rest of the audio circuitry.
> 
> Steve
> 
>