Re: 28.8 modem connection vs. Audio

John R. Haggis (haggis@cesium.com)
Thu, 25 Jan 1996 02:14:27 -0800


At 10:40 PM 1/22/96 -0600, you wrote:
>>From: "John R. Haggis" <haggis@cesium.com>
>>
>>At 03:02 PM 1/14/96 -0700, you wrote:
>>>Is 28.8 enough bandwidth to receive the audio as well as the video
>>>signal. The sound comming across my speakers is intermitant and broken.
>>
>>What I've found is that the sender must limit his audio in the "transmission
>>caps" control panel. If you have them limit it to < 12Kbps, it should come
>>through fine. You might want to do this yourself for your transmissions so
>>others can hear yours.
>>
>>In the Windoze version, we don't have separate audio and video transmission
>>caps. Just limit the whole transmission cap to about 10-12Kbps in such a
>>case and it should work.
>>
>>- John

>HUH?
>
>Why limit your bandwidth like that? I set mine to 28kbs max on transmit and
>receive and the audio works somewhat. The video stream is automatically
>paused when sending audio.
>
>
>Bill Woodland (wcw@bga.com)

HUH? Bill, it doesn't work for me. Details and analysis follow.

First, you say nothing about your hardware, version, or type of connection.
(Of course, neither did I in my first reply.) Maybe you're running a modem;
there's natural BW limitation there. It's probably not even sending >
15Kbps even when you set it to 28Kbps. You can't get blood from a stone.
I've got a 128K T1; it sends out data at full blast, at 90-100 Kbps.

Even in WP 10a1 when the general BW cap worked, the PC version has no
separate audio cap. At 20+ Kbps, The audio goes through in such volume that
all the receiver hears is snap-crackle-pop of occasional packets that get
through. I found that for "quality" audio, I needed to limit it to about
half my receiver's bandwidth (she has a 28.8 modem) during a point-to-point
connection. Don't forget, she's transmitting at the same time. If we both
limit our total bandwidth to about 12-14, she has symmetrical Tx/Rx capability.

I don't think you can both send and receive simultaneously at 28Kbps on a
28Kbps line. Yes the protocol is bidirectional but not at full bandwidth
simultaneously on both channels. At least not on Pacific Bell POTS. And
what do you do about a high-BW system talking to a modem system? And have
you ever listened at your receiver's end? We wanted audio to work a lot
better than "somewhat", and it does... at 12Kbps.

Now, things seem to be quite a bit different when talking through a
reflector. I haven't studied it but reflectors seem to do their own BW
limitation. However, it's not going to interpolate between audio packets,
and if you want your audio to reach people well on 28.8 lines, you'd do well
to limit it!

- John

--
John R. Haggis            haggis@cesium.com
Millennium Research
(408) 269-1814 vox 
(408) 269-9323 fax