[asterisk-users] meetme conference using g729?

[Michael Graves]

On Wed, 3 Oct 2007 08:35:06 -0500, Tilghman Lesher wrote:

>I invite you to try it.  You could make a lot of really smart people look like
>fools if you're able to mix compressed audio together without decompressing,
>or you might make yourself look like a fool, because you get back garbage for
>attempting to mix compressed data.

I wholly understand the problem here. You can't, at present, mix
compressed audio stream, in compressed domain. You must decode them to
baseband, do the manipulation, then re-encode. OK, we get that. That's
today.

Such things have parallels in my day job, which is television
production & transmission. At least in the US the signal that a TV
station delivers to its DTV transmitter (ie the new digital one, not
the old analog one that the feds will make us turn off in 2008) that is
a compressed stream. Typically MPEG2 @ 19.2 MPBS. There was a time when
that was a signal stream that could not be manipulated. It was just the
transport mechanism from the last leg before the transmitter. 

Many companies wanted to be able to perform what seemed simple
manipulations on the stream, for example to add a station logo, without
taking the very significant quality hit of decompression and
recompression. Such hardware systems have become available over time.
Manipulation of the transmission streams in the compressed domain is
possible, but its very compute intensive...and so expensive. It's done
in massively parallel hardware architecture. There are a few vendors in
the broadcast business who provide such systems.

And that's for high bandwidth broadcast video. It would also be
possible for voice streams, but the math is very complex. Hardware
acceleration of encoding is already very common, witness Digium's own
encode/decode board. 

Given the right motivation to spur the development this could be
possible. In truth I suspect that there's little economic reason to do
it.

Michael


--
Michael Graves                           mgraves at pixelpower.com
Sr. Product Specialist                          www.pixelpower.com
Pixel Power Inc.                                 mgraves at mstvp.com

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