View Full Version : Re: Video Analysis

Paul Guy
12-09-1994, 01:20 AM
Dear Dr. Ariel,
In your letter below, I think you are being unduly pessimistic about
the time accuracy that can be attained. We are satisfied with no worse
than about 1/4 to 1/2 frame!

> Dear Biomch-L Subscriber:
> After corresponding with few of the Subscribers I can clear my
> statement about the video fields/second problem.
> 1. The crystal frequency of the video cameras is extremely stable.
> 2. In order to digitize manually or automatically 60 Hz video signal
> there is a shift due to the Interlace signal . This can result in an
> error of 1 field.

.....if you use a VCR that will examine fields (such as a time lapse
VCR, ie Panasonic 6750), the frame that the computer sees is really a
composite of the same field, so that what you digitize as a frame is
really a field. We used to use a Sony Motion Analyzer (long since
obsolete) that did the same thing. If you use a shuttered camera at
1/1000 sec., and GEN-LOCK them all together, and derive your analog sync
signals from the vertical TV synch pulses, you can achieve very close
synchronization. We can use video or Northern Digital Optotrak 3D camera
systems together ALL gen-locked, and achieve 1/4 frame or better
synchronization between video, 3D coordinates, and analog data.
Although such tight synchronization may seem a bit crazy, all the
collection systems can effectively 'shutter' their processing in full
synch to get this performance. We had some temporary problems at one
point when we were doing an assessment of efficiency of a prosthesis
that replaces the lower leg and foot. At 1/4 frame increments from true
synchronization, the efficiencies went from 70%,120%,150% and going the
other direction of synch, they similiarily decreased. All this was done
at a sampling rate of 60 Hz. Needless to say we were highly motivated to
fix the synch up!

> 3. If data is collected with 2 video cameras, as long as gen lack
> is not used, another discrepancy of one field can be occurred.
> 4. If you rely on the VCR counter, it seems that the counter is not
> consistent with the Panasonic 7350 and lack on the even or add fields at
> random. This can result in additional one field discrepancy.
> 5. If the system grab in RAM multi frames, and then after the
> digitizing this frames, lets say 16 frames, needs to find the position
> on the VCR again, there is another discrepancy of one to two fields.

....hmmmm what system are using for this? It sounds like the PEAK
measuring system.

> 6. If the user intended to collect analog data such as Fore Plate,
> EMG etc. with the video, synchronization may be another source of error
> up to 2 fields.

...absolutely not.... you have severe problems with the way you are
collecting analog data if this is the case. If you are really stuck,
sample your analog data at least 4 times the video frame rate, and use
the vertical synch signal as one of the analog inputs. With the added
start/stop from the rest of the system, you can achieve very close

> All the above potential problems can be resolved by grabbing the
> fields digitally to the hard disk and then process digitally the frames
> to correct for potential errors in synchronization. Then synchronization
> of external signals such as force plate, EMG, and other transducers, has
> to be carefully evaluated and tested.
....true, but what a painfull and expensive way to go! You CAN do it
on the cheap, but you would need some minor electronic addons.

> Enclosed is two of the correspondences I received from Dr. De Leva:
...[deleted, they were mostly queries].....

I can offer some help in these areas, as we have been fighting the
synch problem for some 15-20 years, with a very wide range of new and
ancient equipment. It's not that difficult, but when you don't have a
good grasp of the electronics and video stuff, it can be a really alien
mind game.