Friday, 6 May 2016

Sony Betacam Dynamic Tracking Video Heads

During the teardown of a Sony BVW-75 Betacam SP VCR (which dates from 1988) i was delighted to see it used a system called dynamic tracking on the video heads, this is an interesting technology so i am going to take a quick look at how it might be working in this example.

Tracking in video tape machines is an important factor in image quality, in a helical scan tape format the heads must follow precisely the path of the signal else deterioration of the signal will occur.

Methods for controlling this are not simple as timing is critical to get the head which is rotating on the drum to meet the track just at the right time and place.

Dynamic Tracking takes a real time correction approach to this problem. By including additional heads onto the drum which scan and read the the video track just before the actual video heads arrive allows the system to servo the heads to the correct location in real time.

In the picture below you can see the heads on the base of the video drum of this Betacam machine. There are a total of 10 heads.

The two single heads are the rotary erase heads used when recording onto the tape. There are two pairs of fixed read heads on the far left and right of the image and two pairs of read heads that can be moved vertically. The pair of movable video read heads are the larger ones mounted on a separate subframes.

I propose the two pairs of fixed heads are the dynamic tracking read heads, i would expect these to be slightly offset vertically from each other so they should track just above and below the signal on the video tape.

If the tracking is perfect the signal from these heads should be equal, but if there is an offset between them this can be detected and the movable video heads can be adjusted up or down to compensate for the offset in time for the video read heads to scan the tape.

The movable heads are operated by piezoelectric elements driven by i believe a +/- 250v supply generated away from the drum in a separate power supply module.

The video read heads are located on the end of a sandwich of two piezo elements which is the light gold coloured plane in the picture below. This is mounted on a small aluminium frame for support. The connections to the heads run down a flat flex cable just above the piezo element and a number of connections run to the piezo element. The actual video heads can be seen glued to a metallic element which itself is glued to the piezo element.

Close-up Of One Of The Video Heads.

There are a number of connections to the piezo element as i believe this has two elements, one to move the head up and one to move the head down. In the picture below you can see the video heads at the far left, the piezo element is the dark grey area above the aluminium frame and the head signal wires are contained in the flat-flex cable suspended above it.

Top down view of the Dynamic Tracking Video Heads.