Sonntag, 24. April 2011

Sync-Test of Kodak ZX1-Rig: Negative Results so far

Yesterday evening I made a few measurements of the resulting sync of the videos. The following test-sequence was made using 720p and 30 pics/sec:
  • Switch on cameras synchronously via USB remote
  • Start video recording on both cameras
  • Fire a flashlight (thanks Matthias for this great tip!)
  • Stop video recording on both cameras
  • Shutdown cameras
This sequence was repeated about 10 times and the resulting video-pairs were uploaded into the StereoMovieMaker. The video-pairs were then adjusted for frame-accuracy (this was still necessary because the start of the recording was done using the buttons on each camera) and the first frame that shows the flash was observed in detail.

Due to the rolling shutter, the flash manifests itself as a white area that starts at a specific line in the frame. In theory, with perfectly synchronized cameras, the lines where the flash starts would be identical in this frame and the resulting sync-error would be less than 46 microseconds (again, thanks Matthias for the explanation!).

What I observe for the ZX1-Pair is, unfortunately, a completely different behaviour: The lines where the flash starts differ quite randomly between about 100 lines and half a frame. This means that the cameras are in fact still completely unsynchronized.

A possible explanation for this behaviour could be that camera performs some additional steps during startup before the actual video-oszillator circuit starts and these additional steps are dependent on each camera and somehow random. For Sony Cameras and a LANC-remote the start of the cameras using LANC immediately starts the oszillator and the cameras (usually) start in perfect sync and then slowly drift apart later.

Maybe somebody has additional suggestions what went wrong here, but based on these results the USB-remote is completely useless and it would be sufficient to switch on each camera separately.

It would be very interesting to repeat this test with camcorders of other brands: If a camcorder starts automatically when connected via USB to a PC and you happen to own two of these cameras and a simple USB-hub then try the following:
  • Connect both camera via USB to the USB-hub
  • Connect the hub to a PC until the cameras boot
  • Start video recording on both cameras
  • Fire a flashlight
  • Stop video recording
  • Check the sync of the resulting video pair in StereoMovieMaker
If we find a cheap HD camera that delivers synched videos in this test, we have the perfect low-cost 3d action cam ;-)

Samstag, 23. April 2011

Poor Man's LANC for Kodak ZX1?

After more than two years (and countless photos using my Ixus 55 Stereo-Rig) I stumbled over a real bargain for Kodak ZX1 camcorders: A german online-shop sold them for 49,- EUR. I decided to order two of them to check if they can be used for HD stereo videos. Apparently this is by no means a new idea: Several people have successfully used these camcorders for 3D-Stereo especially because this model offers a 1280x720 resolution at 60p, so that the maximal sync-error between frames is not larger than 8ms.

For 3d Stereo-Vids having perfect sync is very important to avoid eye-strain in scenes with a lot of movement. For the more expensive Soncy-camcorders a very good stereo solution based on the LANC-protocol exists. Here a suitable remote control can switch on both camcorders at exactly the same time so that their internal oszillators start in perfect sync. Using LANC it is even possible to measure the deviation between the oszillators so that you can see when the cameras begin to drift apart slowly.

For the low-cost Kodak ZX1 there is no such thing as a LANC-port, but the Kodak has a micro-USB port that might serve a similar purpose. When I plugged the USB-cable into the camera, I noticed that it automatically turned on. Apparently the camera is sensitive to the 5v USB voltage that is provided by the USB-host. I still had various electronics parts lying around from my experiments with Canon CHDK/SDM and decided to construct a remote control that would switch on both cameras in sync.

First of all I had to work on the USB-cables. Unfortunately the Kodak uses a non-standard 8-pin micro usb-plug so I had to use the original cables provided with the camera. Using a sharp knife, I removed all the plastic and ended up with the following small plug:

I then used the most simple electric circuit possible to power the micro usb with a 3,6V lithium battery and a push-button:

Pressing the button provides 3.6V to the corresponding USB-contacts. So I ended up with the following Stereo-Rig:

I still need to work on the second plug, but this should be sufficient for testing purposes. So how does this work:
A short press on the yellow push-button powers on both cameras in sync. Bingo. Even better, when I press it a little longer, the camera switches off again. I have no idea why the camera does that but apparently I created a real on/off switch by accident, nice. ;-)

To start and stop recording you still have to use the buttons on the camcorder, but since you have to edit the video clips afterwards anyway this is only a minor inconvenience. As a workaround I could buy a IR-remote for this purpose but the IR-receiver is located on top of the camera so it is not really more convenient to start recordings this way.

The next step will be to test the sync of the resulting videos and in particular to compare the sync with the standard method of switching on both cameras separately. Stay tuned ... ;-)