Re: Audio/video question?

Tim_Dorcey@cornell.edu
Fri, 3 Mar 1995 11:17:29 -0500


At 8:04 AM 3/3/95 -0600, Kahan, Joseph M. wrote:
>How accurate is the Data window for the Macintosh CU-SeeMe version 8b
>application. Is the Data rate is 95% accurate, frame rate is ???, and so on.

The Kbps and fps that is displayed in the rates bar should be very accurate
for Macintosh version 0.80b1 and above. It includes all data sent (in the
local window) and received (in remote windows). However, the outgoing
"cap" applies only to video and aux data (slides + talk window). The pull
down statistics panel is more selective in its display, including only
video and conference control packets, and tabulated only while video is
being received. [This is because the pull down panel tries to provide an
accounting of lost packets, which can only be determined for packets which
share a common sequence number space which is not true for audio and aux
data since they can be turned on and off independently of video].

>I understand audio is not counted, but at the 16K selection what does it
>truly cost in bandwidth?

As described above, audio is counted in the Kbps display, but it is not
controlled by the "cap" setting. Maximum output should be: cap + audio
rate. I would guess that 16K audio translates to around 17K in practice.
You can see for yourself by stopping video, and then observing the Kbps
display while talking.

>If I exceed the selected maximum video rate does CU-SeeMe truly cap
>the video output rate even for buffered video data?

CU-SeeMe uses something like a token bucket scheme to meter outgoing data,
except that an arbitrarily large burst is allowed in order to complete
transmission of a frame. The burst would then be followed by a
sufficiently long interframe delay to get under the desired rate cap. In
actuality, the size of the burst is limited by the maximum amount of data
that would ever be required to update a frame, which, with every square in
the 120 x 160 image changed and worst case (reverse) spatial compression,
would amount to 89 Kbits.

>
>If I exceed the maximum data rate selection for a long period of time will
>>CU-SeeMe crash?

It is not possible to exceed the selected data rate for a long period of
time, unless you mean exceed it by the amount of the audio encoding. In
any case, that should not cause CU-SeeMe to crash, but then again, nothing
should cause CU-SeeMe to crash, but evidentally some things do.

>And in that same note, if I exceed the video capability of my
>machine to process two way audio plus video, will this cause CU-SeeMe to
>crash or will the extra incoming data just get dropped?

Incoming data will be dropped if you can't keep up with it.
__________________________________________________________________
Tim Dorcey T.Dorcey@cornell.edu
Sr. Programmer/Analyst (607) 255-5715
Advanced Technologies & Planning
CIT Network Resources
Cornell University
Ithaca, NY 14850
__________________________________________________________________