Video Recording Gotchas

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Camera features

The typical video camera starts in auto-focus and auto-exposure mode. This works well for wide-angle to normal shots, but when we do closeups of the speaker (which we do most of the time), this may lead to issues. Exact focus is more critical the longer the focal length (tele). The auto-focus may produce visual blur from time to time attempting to find better focus.

Same applies for auto-exposure. If the speaker moves in and out of the projector beam, auto-exposure is great to compensate the difference in brightness. But it also may happen that the background changes its brightness periodically as the speaker is moving left and right.

A MiniDV camera may switch off after a few minutes, if there is a tape in the drive and you are *not* recording to tape. Remove the tape or do record to tape.

Best is to be aware of these effects, and know where the buttons for manual focus and manual exposure are.

The recommended Camera for dvswitch is a Canon Legria HV40. This is a HDV camcorder, and we need to enter the setup menu, find the menu entry 'HD Standard' and make sure this is set to 'DV(Normal)' -- all other settings, especially HDV are not recognized by dvswitch.

Twinpact 100 gamma/brightness

Problem: A pure white is captured as medium gray. Medium and light gray values are undistinguishable. Several things need to be checked for normal white:

  • DIP Switches A6,A7,A8 control VGA termination. They need to be set to OFF, if RGB out is connected, and need to be set to ON otherwise. If the connected device has a very low termination, the captured signal is too dark, if nothing is connected, the the signal is too bright. These DIP switches have immediate effect, no reboot needed.
  • A button on the remote can toggle between two levels of RGB gain. Select the brighter one.
  • Press Contrast or Brightness on the remote. Then try the up/down arrows. The buttons Reset and All Reset do *not* reset these settings to sane defaults.
  • DIP Switch B5 'NTSC Setup Level' must be set to ON. The '7.5IRE' setup value sometimes (upon first power up, not reproducable later) also influences RGB capture from VGA. (All our 2012 Linuxtag videos were captured with this DIP switch in the off position. Sigh)

Check your cables. Is there a splitter that can adjust its output level? Try Twinpact in loop through mode.

Twinpact 100 overscan

Sometimes we see letterbox (black stripes left & right). Sometimes we se overscan (black border all around). Toggling the input selector or powercycle may help in both cases. But overscan normally needs the twinpact ir-remote control, there we have a button to control overscan or even zoom pan.

Firewire cables

Firewire cable connectors do not lock in place. Especially the mini 4pin connectors easily slide in and out. For real onstage use, those connectors need to be taped. Use gaffa tape on the bottom of your laptop or video camera.

We use a short firewire cable between twinpact and its laptop, as they are close together. We use 5m cables between camera and mixer laptop. 10m also works, but is already beyond the official specs.

Audio sync

We route Audio through firewire. Either twinpackt100 for a simpler setup or camera line-in if we have an audio mixer ourselves. this routing mixes audio into the DV signal and dvswitch takes care of keeping the sync. We avoid normal (alsa) audio devices, they have a different delay than the incoming video. Audio through camera is prefered, the sync through twinpact, if sent via ethernet may have a delay relative to a camera that is not tunneled through ethernet.

Audio hummmm

Avoid ground loops. If you have long audio cables running across the room, get all your power outlets from on central place, (either end of the long audio cables). Run power cables to distribute to all needed places from there. But keep audio cables and power cables separated by half a meter.


Have a sufficient quantity of 9V block batteries and Mignon-AAA-cells available. Wireless microphones are often available but unused, due to empty batteries.

Headset. Always provide speakers with a head mounted microphone, connecting to a wireless pocket transmitter.

Microphone tripod. This is inexpensive and can be used when the speaker needs both hands free for e.g. typing on his laptop.

Laser Pointers

Most speakers prefer to shoot a small green or red laser spot on the presentation wall to show important items. These pointers are usually very small and fast moving. While they work well for the audience in smaller rooms, they are hardly visible in larger rooms where a much stronger video projector is used.

With our setup, we digitize the slide presentation as it comes out of the speaker laptop, not as it is visible on the wall. Thus the laser-pointer is not shown in the slides signal. Even with a video-camera, it takes a brave camera-person to make a laser-pointer on the wall clearly visible. Brightness, focus, zoom, and movement all need to be correct.

Discourage speakers to use a laser pointer. Ask them to walk into the projector beam and use their hands or a long stick if needed.

Network issues

All laptops need to be configured correctly. Static IP. We normally use for the mixer. for the twinpact machine. Hostnames should be mixer and twinpact respectively. Those hostnames are important, as soon as you ssh from mixer to twinpact, so that you can remember which is which.

Gigabit saturation. Connecting a twinpact via dvsource-firewire and a webcam via dvsource-webcam to the same machine and sending both over ethernet, may cause network congestion with long buffer delays. One single DV feed takes ca 3.7 Mbytes/sec; a 20m cat5e cable appears unable to handle two of them. dvsource-webcam at the mixer machine works fine, but is not practical due to the short USB cables. Webcam would be great to look into the audience from the speakers perspective.

Never rely on a wireless network at a conference venue. It will work well on setup day, but will certainly fail you when hundreds of laptops connect later.

Smile, don't panic

Don't worry if things go wrong. Often issues go unnoticed. Sometimes a missing recording is okay-ish. We have never claimed, we are professionals, have we? Just ask for more volunteers, if your own errors are apparent.