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I fully understand the concept of 2X or greater sample rate etc. But when
WMA or MPG music can be 64K bit and have reasonably good fidelity and 96K
or greater and be damn good then why can't we cram a measly 3K audio BW
with lots of wasted space (silence) into MUCH less then the 90Kb they are
spec'ing. Cell technology does not send silent periods but rather
recreates in at the RX end. Silence isn't really silence but rather a
perception that the line is not dead. I would guess that a channel
contains less then 10% of useful audio at any given time.
The concept of needing a 64Kb channel to send a 4K analog signal dates
back probably 40 years well before any of the technology we have today.
Today a single chip would do the compression back then it would have taken
a whole rack per channel, so it would not have even been thought of.
Doug
On Tue, 14 Dec 2004, Jeff McAdams wrote:
> POTS service uses a bandwidth range of about 400hz to 4000hz, giving a
> range of about 3.5khz. Shannon says that you have to sample at a bit
> more than twice the rate to get the data...double 3.5 is 7, so bump that
> up a bit to a nice round (in binary terms) 8khz, 8000 samples a second.
> Each sample is 8 bits (giving 256 quantization levels), so 8 bits, 8000
> times per second is 64kbps, which is, as someone else, pointed out, the
> exact channel size of a T1 channel (preserved in ISDN).
>
> Jeff McAdams
> "They that can give up essential liberty to obtain a
> little temporary safety deserve neither liberty nor safety."
****************************
* Doug Crompton *
* Richboro, PA 18954 *
* 215-431-6307 *
* *
* doug@crompton.com *
* wa3dsp@wa3dsp.ampr.org *
* http://www.crompton.com *
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