Last edited on 4. Aug. 95 by gs/
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Design and Implementation of Data Analysis Systems
Heidelberg, March 23-26, 1995
For the workshop on "Design and Implementation of Data Analysis Systems", we had established a video transmission by internet. For the background of the workshop setup, and some of the difficulties we encountered, see the report "An experiment in Virtual Conferencing" by Mike Meyer in Statistical Computing & Graphics, Vol. 6, No.1. Although Mike has given several useful lessons, he did not tell you what actually can be done. So here is the side of the story Mike did not tell: how to set up a video link over the internet.
The software we used for video and audio was CU-SeeMe from Cornell. Audio code for CU-SeeMe is based on Maven by Charlie Kline, UIUC. After the workshop, in a more quiet time, we have performed several experiments to get more experience. This report is collecting information from both the workshop experiment, and from our additional laboratory experiments.
For video conferencing, you need several media channels. Two channels are obvious: video and audio. Data should be considered an additonal medium. Of course in many situations you can transmit them using an audio or video channel, but in most cases this is an abuse. If a slide is shown, this information is better transported using a separate data channel than using the common video channel. A talk channel, similar to the usual UNIX talk, is helpful to coordinate the conference presentation. CU-SeeMe
gives simultaneous support for all of these channels.
How to set up a Video Link Over the Internet
The information we give here applies to the Macintosh. For setup on Windows, and for using video and audio on Unix machines, see the information from Cornell.
Ingredients for the sender:
- a CPU
a digitizer. A simple AV card will do.
- a camera. Taking a good camera side pays.
- a microphone. An additional amplifier helps, but is not necessary.
- a connection to the internet.
Ingredients for the receiver:
Plug, and play. No special configuration is needed.
- a CPU
- a connection to the internet.
The video equipment was set up initially to Paul Velleman in Cornell to participate in the special opening session. This session was dedicated to "Teaching in Statistics" with a talk of Piet Groeneboom on the TWIStat project, and we were eager to get Paul' comments on this approach.
We got a video connection both ways and could see Paul Velleman in his office. Actually when the talk started the first response came from Paul: after the first few minutes, he was waving a note "no sound !" - we had forgotten to switch the microphone. We corrected it, but still did not get a satisfactory audio connection. The next morning we could see the problems.
A screen shot taken from the Internet transmission: Piet Groeneboom, TU Delft.
1. Using two way video and audio hogs bandwidth, and we were on rather limited bandwidth.
If you need two way audio and are on limited bandwidth, you should not try for a simultaneous audio transmission in both directions. Use CU-SeeMe in Push-to-Talk mode. To send audio, press the mouse button. To be safe from unwanted mouse actions, take the habit to press the "microphone" button.
The historical first: Paul Velleman in his office in Cornell (left), Antony Unwin joining the discussion in Heidelberg (right). Audio-settings set for Push-to-Talk mode. Audio is only sent if you push the mouse button.
2. If you use CU-SeeMe in free running mode, CU-SeeMe filters off low-level noise. The filter level is controlled by a small pointer left to the left of the audio level bar - you will find this information in the documentation of course. But only if you look carefully for it.
To reduce the load on our site, we looked for a site to reflect or mirror our transmission. Lund in Sweden turned out to have the right software for us, in combination with a sufficient reachability from Heidelberg. So we set up our site as sending-only, transmitted our information to Lund, from where it was broadcasted.
Audio settings for Send-Only in free running mode. Audio is sent if the signal level is above the filter level marked by the small slider left to the indicator bar.
Bandwidth considerations: Audio
To get some information about the audio bandwidth requirement, look at the documentation which comes with Maven. CU-SeeMe provides a choice of several compression schemes. It is good using 32Kbps Intel-DVI ADPCM. It is acceptable using the 16Kbps delta-mode by Charlie Kline, author of Maven at UIUC. 12.8Kbps GSM encoding, as used in European cellular phones, is expected in one of the next versions, but need it will need PPC horsepower to run.
Audio code is tighly packed. So if you are on a modem line, as far as audio is concerned you will have little advantage if you are using a compressing modem in between. Using 16Kbps delta-mode or 12.8Kbps GSM, you may be able to use transmission over a 28.8Kb modem.
Bandwidth considerations: VideoMost of the video bandwidth is saved by frame differencing, which is done on an 8 pixel by 8 pixel basis. You can help saving bandwidth with a careful setup:
- use a few colours and grey levels as necessary. For many applications, a bit depth of 4 will be sufficient.
- Do not use big windows or frames unless you must. For most applications, 160 by 120, the default size, is sufficient.
- choose a background which is stable.
- get a trained person on the camera who avoids unneccessary camera movements and is able to focus on the spot of interest .
- use a high quality camera to avoid camera noise.
- try for stable light conditions.
Bandwidth considerations: General
CU-SeeMe allows to limit bandwidth. Use these possibilities. Think with whom you are competing for bandwidth, and try to be friendly.
With good audio compression and a total bandwidth limited to about 32Kbps you can arrange a video and audio transmission which can be received over a telephone line using a compressing 28.8Kbps modem.
With good audio compression and a total bandwidth limited to about 60Kbps you can arrange a video and audio conferencing.
If you have limited bandwidth on the sending side, you can use reflector software to take away network load from your site.
Using a reflector for one is a means to control the location where bandwidth is required. Moreover, the reflector software is able to use multicasts and can make use of MBONE connections where available.
We mantioned the need for other channels besides video and audio.
CU-SeeMe allows to pass small messages along with the video. While CU-SeeMe is in the front, simply type. Your message will appear in the bottom of your picture.
If you move the CU-SeeMe addition TalkFM to your CU-SeeMe folder before starting, an additional menu item will appear which allows to open a "talk" window.
You can pass additional data along with your video information. CU-SeeMe comes with a SlideWindow facility built in, which allows to pass still images using JPEG compression.