Date: Mon, 9 Jun 1997 14:19:28 -0400 (EDT) From: Chuck Robey <chuckr@glue.umd.edu> To: Wm Brian McCane <root@bmccane.uit.net> Cc: multimedia@FreeBSD.ORG Subject: Re: Old standards Message-ID: <Pine.BSF.3.96.970609141552.323c-100000@Journey2.mat.net> In-Reply-To: <199706091651.LAA12694@bmccane.uit.net>
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On Mon, 9 Jun 1997, Wm Brian McCane wrote: > Howdy, > > I used to work for a company which built equipment for the railroads. > We had a box called a Hot Box Detector (HBD) for detecting bad bearings on a > train and telling it to stop. Anyway, it used a little dingus which did > ADPCM, at 32K samples/second, and `spoke' the alarm to the train over the > radio. If anyone knows about ADPCM, it only requires about 4K bytes to do a > 32K/second audio stream. I was thinking about putting together a program for > audio over the internet using a software version of ADPCM for real-time > chatting. An 11K/second stream would need about 1375 bytes per second to > tranfer, so it should work on a 14.4 modem with bytes to spare for the > TCP/IP overhead, and ADPCM is fairly compressible by modems which might allow > even better performance or quality. Anyway, I was curious if this is old hat > (Real Audio maybe), or if noone has considered using this 10+ year old > technology. ADPCM (at least when I last looked at it) wasn't all that good as you say it is. It's one of the non-lossy algorithms, meaning that it keeps all the information (or nearly all) in the original stream. It was used for taking a 64 kbps PCM stream and condensing it down to 32 kbps. It maybe could do a little better, but only at a much great loss of fidelity. One of the other, lossy algorithms might be more suited to what you want, because their compression ratio is much higher. That's the reason that ADPCM hasn't really gone anywhere. > > brian > > > > ----------------------------+----------------------------------------------- Chuck Robey | Interests include any kind of voice or data chuckr@eng.umd.edu | communications topic, C programming, and Unix. 213 Lakeside Drive Apt T-1 | Greenbelt, MD 20770 | I run Journey2 and picnic, both FreeBSD (301) 220-2114 | version 3.0 current -- and great FUN! ----------------------------+-----------------------------------------------
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