Playback of degraded VHS. Replacement sync insert.

[FERNSEH]

I remember reading an article in the Practical Television magazine about various methods of improving the playback quality of degraded VHS video tapes.
An idea suggested was to employ the head control pulse to insert new frame sync pulses, these would replace the recorded frame syncs. I've never tried this myself but I'd reckon this would be a viable project. The 50Hz CTL pulse is almost coincident with the field sync pulse so no timing problems.

DFWB.

[paulsherwin]

Didn't a lot of higher spec VCRs do this?

[Nickthedentist]

Is this "timebase correction" or am I barking up the wrong tree?

[Restoration73]

You could play the tapes through a DVD recorder set to pause - the sync correction
can provide amazing results. Various devices were made to "sharpen" the picture using
hf boost, but these usually ended up with image edges having an "outline" effect.

[8 Tracker]

Nick, this is indeed timebase correction. I got hold of just such a beast a couple of months back and looking on the scope, it does an excellent job of replacing the sync pulses. It also provides controls for video and colour levels, as well as noise reduction (which is very good) and drop-out compensation (which isn't so hot).
Some professional machines had all this built in.
I would say it would be well worth trying and a very useful project.
Mark.

[Nickthedentist]

Thanks Mark.

[FERNSEH]

Certain ITT video recorders had ASO, not sure what the abbreviation means, but nevertheless it really improved the picture quality.
Was a ASO a Sanyo development?

DFWB.

[Lloyd 1985]

I have a JVC VHS-C camcorder with time base corrector built into it, works quite well!

Regards,
Lloyd

[paulsherwin]

I think JVC were quite keen on this and incorporated it into a lot of their models.

[Nicklyons2]

The drop out compensator will only work when an rf feed from the head amplifier is available. TBCs offering DOC almost always had a BNC or similar input socket into which off-tape rf was fed; the TBC detects the loss of rf due to the dropout and then repeats the video line above which it has in store instead of just showing the noise/shash.

Another 'proto' TBC is known as a 'proc-amp' or processing amplifier (to give it its Sunday name) and the related line clamp amplifier. These did the sync re-insert and reblanking but without the re-timing and storage capabilities.

[cheerfulcharlie]

Certainly in a professional situation such as a studio where you have much more stable sources such as cameras and caption generators a TBC was a must for the VTR and especially helical scan VTRs.

[Nicklyons2]

As 'Charlie' says helical scan VTRs, though amazingly stable for a mechanical device, couldn't possibly maintain electronic timing standards over the length of time of a TV field (1 head transit). The first generation professional VTRs recorded only 16 lines per head transit and could, just about (in the B&W era) be stable enough to sync up with a stable electronic source without a TBC. Helical scan machines which include VHS, Beta, U-matic etc couldn't be used in a fadeto/mix to/wipe to situation until TBCs became available although, in desperation, the most stable could be 'cut to' or 'switch lock' as RCA used to call it.

[8 Tracker]

Thanks for the info. I did find out about feeding in the RF direct from the head and luckily I have a professional JVC machine that has this output on the back. Even so, I don't think it is that brilliant. I can see it working if I pause the machine and move the tape to produce a noise bar. It is adjustable, but if I go too far, it makes things look even worse.

I have noticed that it does not like working with pre-recorded tapes. I presume the dreaded Macrovision is playing havoc with the circuitry.
Mark.

[Top Cap]

One device, no longer made but may be available at some time on an auction site is the ACE.
http://www.gthelectronics.com/featurec.htm