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4. Electronics

This page discusses 
  • mirror reversal 
  • video camera
  • video mixing
  • the presenter's monitor
  • microphones
  • control desk
(For part names shown in red, see the Parts page.

Mirror reversal

Visitors often ask how I learned to write backwards.  It's fun to explain that the board is photographed in reflection, using a mirror.  My studio uses a mirror, but there are actually 4 good solutions:

1. Mirror: The instructor and the glass board are videotaped in reflection (via a mirror) to correct the left-right reversal which results from viewing from the “wrong side” of the glass. Instead of a tripod, I used an adjustable ball head to mount my video camera (Canon XF105) on a shelf, together with a 4x6 inch front-surface mirror. It's about 20 feet from these to the Lightboard, but closer would be OK.

2. Scan-reverse cameras: Some cameras have a scan-reverse option built in, so that they electronically produce a mirrored video stream. Cameras that I know of that have this feature are the Canon Vixia HF10, Canon XA10, and Canon XF105, and the higher models of each of these lines. Although I enjoy the "aha" factor when visitors understand the mirror, I think the electronic alternative is more convenient. It's easier to aim your camera and frame your image accurately if you don't have a mirror involved.

3. Scan-reverse appliance: The Decimator MD-HX Converter has a setting to left-right reverse a video stream on the fly, so you can use any DSLR or video camera. (Thanks, Justin Melick, for finding this.)

4. Reverse in video editor software:  Most video editors give you the option to left-right reverse the whole video file in post-production. I value being able to upload my video immediately, without post-production, so I use a mirror.


Video camera

You won't get good results if your camera is hunting for focus, changing exposure levels, and raising/lowering audio. The black backdrop makes focus and exposure hunting worse than usual.

I used the semi-pro Canon XF-105 videocamera, which makes it easy to set aperture, gain, focus, zoom, and audio levels manually, and keep them. Unlike consumer cameras, it won't forget its settings when it's powered down.

The much less expensive Canon Vixia HF10 can also be used as the main camera, but it's more difficult to set and keep your manual exposure, manual focus, manual audio level, and zoom. To set these, tape some stuff to the glass so that the camera can auto-focus and set its exposure on something, and then switch the camera to manual and adjust from there. Focus, exposure, and audio level all need to be turned to Manual. There is a setting to tell it not to go to sleep.

Video mixing

The signal from the video camera is combined with other video sources by a Video Switcher (Blackmagic Design ATEM Television Studio Production Switcher). Video Switchers are intended for use in live broadcasts. They allow a certain amount of editing and combining of signals, on the fly. This is in contrast to Video Editors (e.g. iMovie) that you use between recording and later presentation. The Switcher selects sources, superimposes one image on another, does "wipe" transitions, green-screen effects (chroma-keying), etc. Using the Switcher and a PC-based recorder, little or no post-production editing is needed. Even live streaming is possible.

If you don't need to live-merge other video sources (such as powerpoint), you can get away without the Switcher. You can instead record directly to a computer using any video capture device such as a Blackmagic Design UltraStudio Mini-recorder (via Thunderbolt) or H.264 ProRecorder (via USB). You could also skip the computer, and just record to your camera's built in SD card. If your camera doesn't have a built-in scan-reverse option, MPEG Streamclip or other video editors will do the mirror reversal for you.


One of the things the Switcher can do is superimpose graphics from a computer onto the live image from the videocamera. The graphics can be from a powerpoint presentation, and can even include other videos. The Switcher sums the signals from the videocamera and the powerpoint presentation. My powerpoint slides are therefore done in 16x9 aspect ratio, and with a black background, so that the background doesn't show up in the summed video stream (see example slide below). I've provided a sample powerpoint deck.


Presenter's monitor

I can't see the powerpoint graphics on the glass because it's not projected there. Instead it's added-in digitally by the Switcher. If I want to interact with the graphics, or even draw on the graphics, it's necessary to have a monitor that I can see while at the Lightboard, so that I know where my marker tip is relative to the graphics. Even so it's a little tricky to draw in the right place.

The monitor (a 50-Inch 1080p HDTV) is near the videocamera, so that I can see it while looking in the proper direction toward the camera. This causes a problem: the reflection of the monitor, in the glass panel, is very visible to the videocamera, as you see below:


To block this reflection I take advantage of the fact that all the light from an LCD TV is polarized; a polarizer is inherent to LCD technology. I chose a TV that has a linear (specifically, vertical) polarizing film as its frontmost layer. Then I used a 58mm linear polarizer on the videocamera to reject that polarized reflection. (Many LCD TVs are circularly polarized, and a CPL filter on the videocamera should work well for those. In fact a CPL filter will even work with the linearly polarized LCD TV. ) The polarizer (linear or circular) can be adjusted to a very good extinction of the reflection.

The Canon XF105 camera is connected to the Switcher via a 50 foot SD/HDI cable. SD/HDI tolerates much longer cable runs than HDMI. I also use two Canon VIXIA HF M50 camcorders for recording from other angles, or macro. These were connected via HDMI cables.

Microphones

It's impossible to overstate the importance of good audio. The HVAC in my studio was loud and I had it revised. I put sound absorbing foam on the wall behind the black cloth that the presenter stands in front of. Sound is captured by a Sennheiser wireless microphone system with a lapel microphone on the presenter's collar. Sound is fed into the left or right channel XLR microphone input of the Canon XF105 camera. When I have two presenters, I use two wireless microphone transmitters and two receivers. The Switcher includes an audio mixer. Earlier I used an Audio Technica wireless microphone, but I found that it occasionally would create a loud click or pop. The Sennheiser does not.

Control desk

Video feeds from all three cameras, as well as HDMI output of one or more laptop computers (e.g. for powerpoint), are delivered to the six available inputs of the Switcher. A 22" LCD monitor is attached to the Switcher for the use of its operator, to display all the video feeds and the program video output.  The ATEM switcher has an output that displays all these at once on a single monitor.


The switcher needs a computer to display its array of switches and controls, and also to record the video stream to disk. We initially used a PC but found that the software runs more stably on a Mac mini.   It's helpful to have plenty of flash disk space in the computer that ATEM's "capture" writes to.  We have 256GB but I wish we had more.

The control software for the Switcher, running on the Mac mini, communicates with the switcher over a TCP/IP link. We established a LAN for just these two devices. I used a Netgear WNDR4000 N750 Dual Band Gigabit Wireless Router to establish the LAN. The router also provides an uplink to the university network.
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