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VCR head cleaning sensor? How does that work?

Hello all...

I own a relatively late model Panasonic VCR. Last night I went to play a tape (the VCR had been sitting idle for a while) and the picture came up as mostly snow with good sound. I knew immediately that the heads probably needed a good cleaning, so I took the VCR out and put it on the bench to clean it up.

However, before taking it out, I noticed that the VCR was displaying an on screen message stating that the heads probably needed to be cleaned and that a cleaning tape should be used.

Now the video/audio/erase heads are clean and the VCR is working nicely. However, I am wondering how the VCR "knew" that the picture quality was poor? I don't think it was a simple timer-triggered event.

William

Reply to
William R. Walsh
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don't know for sure, I suspect it monitors/compares the fm signals from the 2 video heads ??

-B.

Reply to
b

Hi!

Oh, yeah. I can't easily get at the model number right now, but I probably should mention that it is a 4 head Hi-Fi stereo unit.

William

Reply to
William R. Walsh

Usually that sort of thing just triggers after a set number of hours of play time, it may do something more advanced though.

Reply to
James Sweet

"William R. Walsh" wrote in message news:wArLf.786449$x96.323663@attbi_s72...

A pure guess- maybe it just analyses the signal strength picked up by the heads, and/or detects sync disturbances.

Of course, how it differentiates between dirty heads and bad tapes (if indeed it does) is another matter.

Dave

Reply to
Dave D

there are 2 ways the manufacturers use to achieve blocked head detection.

For / during recording: mostly used on camcorders, not usually on home deck VCRs The unit detects small differences in the current/frequency response as its recording, since a blob of oxide bridging the gap is a bit like a short circuit at the frequencies being written and the effective load is thus higher. Its not meant to be an exact science, more just an indication. Some tape types can give spurious warnings unnecessarily however.

For Playback... thats easier.. it knows what signal level should be output by the 2 video heads as they switch alternately. When 1 or other goes very low output - as is usually the case when 1 gap is blocked, the 'clean heads' mesage is displayed.

AW

on

Reply to
Cliff top

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