On Fri, 15 Jul 2016 13:31:20 -0400, "Jim Wilkins"
wrote:

"Ed Huntress" wrote in message
.. .
Here's one for the digital experts: I do a lot of interviews in
which
the other party is on a cell phone, and I think I've noticed a
difference over time in how they work.

If the call is conducted at a busy time of day, and there are clear
signs of bandwidth becoming limited, it used to be that there would
be
a corresponding number of dropouts in the conversation. Lately (and
I
call mostly the Chicago and Los Angeles areas) I've detected
something
different: it sounds like they're just reducing the audio bitrate.
It
starts to sound like it's coming from a cheap microphone.

Does anyone know?

--
Ed Huntress

I studied voice and data communications in 1970 and took a refresher
class in the phone system in the 1990's when an interest in
infrastructure didn't brand one a potential terrorist. I haven't
looked into it since and don't intend to now.

My cellular internet modem drops out completely when the system hits
voice call capacity. There are a number of ways they could reduce the
audio data rate when that happens such as duplicating frames or
dropping the least significant bit, which they do anyway to some
extent to transmit the on/off hook signal. That's why the dialup
maximum is 56K instead of 64K.

Digital TV drops a lot of data intentionally to stuff 20MB of HD video
into 6MB of TV channel. They send a full "key" frame and then only the
changes to it, until the next full frame. It works fine when the
background is static or changes slowly. It isn't quite fast enough for
football play closeups so the TV may display larger solid blocks
momentarily.

Speech can be similarly trimmed down to taking and sending one sample
every 50 milliseconds or so and just repeating it until the next
sample, because most of us can't change the shape of our vocal tracts
any faster than that. The intelligibility is still tolerable although
JFK demanded and got a higher sound quality digital phone that needed
special dedicated lines.

The underlying tech of digital phones dates from a secure channel
between Roosevelt and Churchill.
https://en.wikipedia.org/wiki/SIGSALY
"The voice encoding used the fact that speech varies fairly slowly as
the components of the throat move. The system extracts information
about the voice signal around 25 times a second."

-- C4I4U


Interesting. If you've played with recording systems, computer or
otherwise, that record digitally and that allow you to select from a
menu of bitrates, you've encountered what I'm talking about. Any
decent digital recorder today has optional record-time settings, which
actually are different bit rates.

--
Ed Huntress