Lightboard Home‎ > ‎

4. Electronics

This page discusses 
  • left-right reversal 
  • camera
  • microphones
  • video mixing
  • the presenter's monitor
  • control desk
Left-right 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 camera on a shelf, together with a 4x6 inch front-surface mirror, 6mm thick. It's about 20 feet from these to the Lightboard, but closer would be OK.  If you use a mirror, make sure it is a front-surface mirror, and that the way you mount it to doesn't cause it to bend, nor allow it to vibrate. If it's not quite flat, your images won't be in sharp focus.  
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. I haven't found any 1080p cameras with scan-reverse.

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.) One disadvantage of electronic mirroring is that it adds yet another signal delay.  Latency hurts when you are drawing on superimposed graphics, making it harder to guide your hand. The camera, the switcher, and the monitor each take their toll. Decimator adds 33mS.

4. Reverse in video editor software:  Most video editors give you the option to left-right reverse the whole video file in post-production. This is a good option if you aren't using a presenter's monitor (which needs to show a live reversed image).

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 XF-105 shoots interlaced video, 1080i30.  

Recently I changed over to 1080p60 (or 1080p59.94 to be more precise) to get better resolution and to avoid motion artifacts that arise from interlaced video frames. After trying several consumer videocameras and finding them unsatisfactory in image quality or in allowing full manual control, I turned instead to mirrorless digital cameras that are intended primarily for still photography.  These can be manually controlled in every way, and have excellent image quality.

I used a Sony Alpha A6000 mirrorless digital camera, with a 55-210mm SEL-55210 zoom lens and a 49mm diameter CPL polarizer.  Accessories were an Sony AC power adapter and a micro-HDMI cable. Even when it is not recording to its SD card, the camera has to be set on AVCHD recording mode (not MP4) or else it won't output clean HDMI at a true 60fps (frames per second). I can't recommend the Sony cameras because they overheat and shut down; I had to add a fan to prevent this.  John Mocko (U Florida) recommends the Panasonic GH4, GH5, GX8, and pointed out a camera comparison chart. 

HDMI gets noisy on a long cable run, so it's important to switch signals over to SDI near the camera.  Good choices for this are Decimator MD-HX Converter (if you are doing electronic mirroring) or Blackmagic Design 3864 HDMI to SDI converter.

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 eggcrate foam on the wall behind the black cloth that the presenter stands in front of. This eliminated a hollow echo effect between the back wall and the glass board.

Sound is captured by a Sennheiser wireless microphone system with a lapel microphone on the presenter's collar. Sound is fed into the left and 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.

When I changed to the Sony Alpha A6000 mirrorless camera instead of a videocamera, I found that the A6000 does not have an audio input jack! (Later models in this line do have an audio input jack.)  So I took the audio from the wireless mic receiver directly into the BlackMagic Design switcher.  Now the audio was arriving promptly, but the video was being delayed in the camera, so the audio and video were slightly out of sync.  I used a Behringer Shark FBQ100 to delay the audio.  The right amount of delay turned out to be 33mSec.

Video mixing

If you don't want to mix in any other video sources (such as powerpoint) while recording, you can skip this.

The signal from the video camera is combined with other video sources by a Video Switcher. 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, little or no post-production editing is needed. Even live streaming is possible.

My first switcher was a  Blackmagic Design ATEM Television Studio Production Switcher which supports up to 1080i30.  In 2017 Blackmagic released the  ATEM Television Studio Production Switcher HD at about the same $1K price point. The new HD model supports 1080p60.  However unlike the earlier model, the HD model does not internally do H.264 encoding and provide USB to a computer, for capture to file.  So I also had to add a $500 Blackmagic Design H.264 Pro Recorder, to recover that essential functionality.

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.

The new HD model switcher has a new feature that we found very useful (in addition to its 1080p60 capability of course).  It has two output streams, while the older non-HD model only has one.  We use one of the output streams to mix the lightboard video with powerpoint graphics, and then send this to the H.264 Pro Recorder and via USB to a computer for capture.  On the other channel we can mix not only the lightboard video and powerpoint graphics, but also a "teleprompter" script from yet another computer source. This output goes to the presenter's monitor, discussed next.  

Presenter's monitor

You can skip this if you don't need to see yourself, or any superimposed powerpoint graphics, while recording.

I can't see the powerpoint graphics on the glass itself, 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. 

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; polarization is inherent to LCD technology. I use a CPL polarizer on the videocamera to reject that polarized reflection. The polarizer  can be adjusted to a very good extinction of the reflection.

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 Switcher has a "multiview" output that displays all these signals at once on a single monitor.

The Switcher needs a computer to display its array of switches and controls, and also to record the captured, compressed video stream to disk. I 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. Our mac mini has 256GB but I wish it had more.

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

Below is a wiring diagram of the studio electronics (click to enlarge; or here's a pdf version)

Michael Peshkin,
Jun 14, 2017, 7:20 AM
Michael Peshkin,
Jun 14, 2017, 7:20 AM
Michael Peshkin,
Jun 18, 2017, 12:16 AM