>I am seeking a simple expedient path to a "VoicePort" audio digital
>sampler to RS-232 port driver.   Parameters for this device and

John, when dealing with telephone audio a simple method of data compression
is frequently used: CVSD (Continuous Variable Slope Delta). I've seen CVSD
chips (called CODECs) used in PBXs and other telephony equipment - the
resulting audio quality is comparable to what Ma Bell delivers.

CVSD works as is follows:

A continuous single 8-16Khz digital bit stream is used to represent the
audio data. The Encoder keeps track of previous audio samples and only
indicates if the new sample is higher or lower than the last (0 vs 1) by a
step size (delta threshold). If the audio samples moves faster than the bit
rate can keep up with, a series of 3 zeros or ones will be sent and the
Encoder will increase the step size for the next sample (the Decoder will do
likewise).

Hence, CVSD is a Continuous, Variably Sloped, Delta system.

This method increases the digitization noise for high slew rate signals
(upper frequencies) but lowers it for the lower frequencies. Since the
frequency response of your typical phone system is 300-3300hz, but voices
contain most of their energy below 1.5Khz, the tradeoff works well.

CVSD enCOers/DECoders (CODECs) used to be readily available. However, you
can easily implement such an approach in PIC firmware! Think of it:
Acceptable telephone audio quality for 8Kbps instad of 8Ksps. This
translates into a data rate compression ratio of (roughly) 8:1. Even if you
have to go to 16Kbps to give good results you still save plenty of bandwidth.

I tried to find CVSD CODECs in my IC Master and library, but PCM now seems
to dominate. There was one from Plessy (MA367) but I don't have a datasheet
for it. I wanted to confirm that the data rate was in the 8-16Kbps range as
its been over 10 years that I dealt with this technology (in the Prodigy PBX
from Ericsson).

Hope this helps.

Regards, Dana Frank Raymond
dfr@icom.ca