Video Switcher Scripts

by Jamie Zawinski

The machine that implements our Our video webcasts takes a single NTSC baseband video signal as input to its video capture card. We've got a bunch of cameras in the club, so something needs to switch between the cameras in order to feed different signals to the webcast machine.

That something is a Knox RS16x16 video matrix switcher. Basically, it's a big box that has 16 inputs and 16 outputs, and you get to pick which of the inputs each of the outputs is fed from (so each input could go to all of the outputs, none of them, or anywhere in between.) There's a keypad on the front panel that lets you change the routing and program simple loops. But the cool thing is that it also has a serial port on the back, so you can do your switching from a computer.

We used to have a small computer up there next to it, and it talked to the serial port directly. But these days we have a Lantronix UDS-10 serial port to ethernet adapter plugged in to the switcher instead, and the software that controls it is running on a computer that is physically elsewhere.

One of the outputs from the switcher goes directly into the video cards on the webcast computers; the other fifteen outputs go into a set of RF encoders (NetMedia MM73 Triple Play) that put each of those signals onto a different UHF channel on the private cable TV system we've got running through the club. We used to have a bunch of televisions scattered around, and by just plugging them into the cable and picking a channel, we could have them all displaying random images from the various cameras. Since there were 15 channels to choose from, it was easy to set things up so that no adjacent TVs have the same image on them. (We've since gotten rid of all of those TVs, though, so the "cable system" is now vestigial.)

switcher-cmd.pl This is a shell script that talks to the Knox switcher over the serial port (or network, via the serial-to-ethernet adapter). Command line options control what gets switched where. It also has provisions for looping and for randomly switching certain ranges of inputs to certain ranges of outputs.
switcher-initd A simple init.d script that launches switcher-cmd.pl and pancam.pl daemons at boot-time: they runs in a loop and poll the file /etc/switcher.conf for the current command-line to use. To change what the switcher is doing, I can just change a single file, instead of having to kill and restart the program.
switcher.cgi This is a CGI script that controls the switcher remotely: it lets you edit the switcher.conf file graphically. The switcher-cmd daemon will notice and react to the changes.

Here's an example of what it looks like when it is running (no, you can't use this page to control our cameras!) The rows represent inputs (cameras) and the columns represent outputs (TV channels, and the webcam.) A checked box means that the switcher may connect that input to that output. So, each time delay seconds pass, the switcher looks at the set of allowed inputs for each output and picks one at random.

One unfortunate thing I've noticed is that the switcher doesn't seem to like to be told to switch more than about twice a second: any faster than that, and it seems to suffer serial port overruns, or something along those lines.

This makes me sad, because it turns out that our lighting controller can output MIDI, so I had this idea that some of the video switching could be keyed along with lighting changes by simply putting a MIDI card in an adjascent Linux box. But alas, it seems that it can't switch fast enough to make that be realistic...


In addition to the scripts to control the Knox switcher, I also have scripts to control a Panasonic WV-NS324 servo-mounted video camera. This camera is pretty cool in that it has a "real" video output, but also has an ethernet interface and a built-in web server, so you can directly control it over the net.

But, it's very much not cool in that every now and then it gets confused and all of its preset camera positions shift by around 30 degrees, so it will suddenly be pointing at completely different parts of the room than you wanted until you manually re-enter all the presets. Maybe there's a belt or gear slipping inside it or something; I can't figure out what the cause is. This problem started only a few weeks after I bought the camera, and the response from both Panasonic and the store I bought it from was, basically, "sucks to be you."

So I wouldn't recommend this camera at all, and I would have returned it if they hadn't refused to give me my money back.

Due to a comedy of errors where we thought the camera was dead when it actually wasn't, I actually ended up with two of these, so one is mounted up high, and one is under the balcony.

pancam.pl This script that reads the same config file that switcher-cmd does, and based on the commands in that file, tells the camera to point at one of its preset positions. In this way, I have a single config file that controls both the switcher and the robo-camera (and that file is written by the switcher.cgi web page.)
pancam.cgi The WV-NS324 has a built-in web server, with controls that let you manually zoom the camera, save positions as presets, etc. Those web pages have some problems, however. They're overly complicated (many commands are spread out across multiple pages), but most importantly, the "joystick" control on the web pages (by which you actually move the camera) doesn't work in Mozilla or Safari (it was coded for MSIE and Netscape 4.x only.)

So, I wrote my own UI for camera control, that works by posting to the underlying "control" URLs on the camera, bypassing the on-camera user interface. Works pretty good.