Does anyone know how a VCR auto-tracks the information on a tape?
I understand mechanically dithering the read heads, but as I understand it, it's only for TV stations and higher quality VCR's. But almost every VCR has auto-tracking now.
To auto-track the information, I'd assumed that there would be two ways. The first being dithering, where by the head takes a sweep of the available side to side track space and then moves to the point with the highest signal amplitude. And the second being an unchanging written signal strength somewhere on the tape that the head would be able to reference it's self to as being at the maximum signal strength.
But the information on the tape isn't unchanging, and apprently cheap VCR's don't feature mechanical dithering of the heads.
So how can the VCR know when it's found the highest signal strength for that strip of information when it has nothing to reference it to?
My last guess would be that it's sweeping the speed of the head and just looking for a wide average of peak output. Which might explain why cheap VCRs take quite a while to auto-track the signal.
I'm thinking of it like this...
Imagine the cylinder is spinning and on track with the written information. Now give the tape a pull or push before or after the cylinder. The tracks could easily be misaligned with the read heads.
So how does the auto-tracking put it back on?
I understand mechanically dithering the read heads, but as I understand it, it's only for TV stations and higher quality VCR's. But almost every VCR has auto-tracking now.
To auto-track the information, I'd assumed that there would be two ways. The first being dithering, where by the head takes a sweep of the available side to side track space and then moves to the point with the highest signal amplitude. And the second being an unchanging written signal strength somewhere on the tape that the head would be able to reference it's self to as being at the maximum signal strength.
But the information on the tape isn't unchanging, and apprently cheap VCR's don't feature mechanical dithering of the heads.
So how can the VCR know when it's found the highest signal strength for that strip of information when it has nothing to reference it to?
My last guess would be that it's sweeping the speed of the head and just looking for a wide average of peak output. Which might explain why cheap VCRs take quite a while to auto-track the signal.
I'm thinking of it like this...
Imagine the cylinder is spinning and on track with the written information. Now give the tape a pull or push before or after the cylinder. The tracks could easily be misaligned with the read heads.
So how does the auto-tracking put it back on?
Thanks Frank,
Do you have any good references for information on how the VCR does this and other things, such as erasing the tape.
I understand how the erase is laid out, but I am looking for information on general voltages, frequencies and methods of how VCR's operate.
Ever since I started reading about how VCR's actually work I've had a lot more respect for them!
Do you have any good references for information on how the VCR does this and other things, such as erasing the tape.
I understand how the erase is laid out, but I am looking for information on general voltages, frequencies and methods of how VCR's operate.
Ever since I started reading about how VCR's actually work I've had a lot more respect for them!
You may want to visit this site for some basic information.
http://electronics.howstuffworks.com/vcr.htm
http://electronics.howstuffworks.com/vcr.htm
You can bulk erase tapes for better S/N, too. The erase heads/circuits are pretty lame on machines from the last 5-8 years. Some can barely erase the signal on the tape. This is compounded by the dual depth recording of hi-fi units.
The drum & capstan motors are both servos & lock very tight. As mentioned above, the machines can alter speed to test for best output (tracking). Pro decks (Sony 5800) have tracking VUs on the front & manual controls, too. The heads (video & audio) on the drum are fixed.
VCRs still amaze me. After what I have seen folks do to the units that came through my shop, I am always impressed that they even work!
The drum & capstan motors are both servos & lock very tight. As mentioned above, the machines can alter speed to test for best output (tracking). Pro decks (Sony 5800) have tracking VUs on the front & manual controls, too. The heads (video & audio) on the drum are fixed.
VCRs still amaze me. After what I have seen folks do to the units that came through my shop, I am always impressed that they even work!
Thanks for your help Warren!
I started taking two VCR's to bits to see how they work in detail.
If you could make any comments where I'm making mistakes that'd be excellent!
I noticed four RFI shielding enclosurers in the first. I'm not sure about the other as I've only just opened and forgot to check. In both, one of the enclosures is right beside the read write helical head, so I would guess these to be the RF demodulators.
The another, in both, is right at the very input of the RF and I would guess this may be a tuner.
I haven't had a chance to check yet, so I'm not sure if TV broadcasts are sent out on FM. If they aren't the others may be modulators. I'd say modulators for recording Hi-Fi audio through the helical head but these are old two head VCR's.
Do you have an idea of the carrier frequency for helically recording Hi-Fi on VHS? I know the video is recorded somewhere around 4.5 - 5.5Mhz and has a bandwith of about 4.2Mhz.
I also started thinking about the tracking. As the signal frequency increases, so does it's pickup level for a given speed. When the picture changes, the frequency of the signal changes, and the amplitude of the pickup level in the head rises or falls, why doesn't the autotracking get tricked into thinking it's slipped off the track and start trying to track the head? How does it distinguish between a frequency change on the tape and an actual slip off track synchronisation?
I thought that was a dumb question at first because FM isn't intentionally AM modulated. But the signal at the head would appear with AM modulation due to the FM frequency changes resulting in different pickup levels. Which is why FM is used in the first place.
Then I also thought that perhaps the autotracking might be tuned to only look for the carrier of the FM signal. But the carrier won't be there in a pure form unless there's no logic on the tape.
The only two remaining things I can think of is that perhaps it's tuned to only look for frequencies close to the carrier, or within another tight band, so it doesn't start tracking when the frequency dips or peaks and the pickup level changes.
The other way might be that the circuit has a massive amount of lag on it, so it's only looking for an average playback level over a few seconds. But still, dark and bright scenes can last a lot longer.
Finally, I thought about the control track. In old VCR's with manual only tracking, you'd tune the tracking to the cleanest looking picture then sit back and watch the tape.
The VCR would then use the control track and phase difference between it and a constant frequency source, like a crystal, for the timing of the helical head.
If the tape had shrunk or stretched over time, the control track would have to a somewhat similar degree. All the VCR would need do is watch the control track and follow it for synchronisation to keep the picture as you set it.
But if your VCR has autotuning, theoretically there's no real reason for it to reference the control track right? Since you don't have to get up to tune it, it can change speed continually and you'd not have to move. So the only benefit for it referencing the control track might be if the autotracking isn't very accurate or isn't able to keep up in some way.
I'm only thinking, if a VCR only has autotracking, why go to the expense of adding a control head and control track signal tracking circuitary if you can just have the helical head constantly search for the best output?
My only answer to that would be to stop the picture constantly dropping in and out of synchronisation as the system seeks for the best output. So it'd find the best output and then lock it's self to the control track until the output slipped below a certain level. Only reason for that being the situation would be if it's too hard to track the helical head automatically to within an accuracy of one track. So in the process of trying to autotrack it'd damage the picture quality seeking between tracks. But then how would ever lock on accurately in the first place?
I'm sorry for the kind of long post and large amounts of thinking out loud. This is just something that I'm not totally sure about and very few sites seem to provide much information about it. Try typing autotracking and vcr into google at the same time!
Again, please feel free to just cut this post up into bits and answer where ever you can. It might only take one sentence for the whole thing to click.
I started taking two VCR's to bits to see how they work in detail.
If you could make any comments where I'm making mistakes that'd be excellent!
I noticed four RFI shielding enclosurers in the first. I'm not sure about the other as I've only just opened and forgot to check. In both, one of the enclosures is right beside the read write helical head, so I would guess these to be the RF demodulators.
The another, in both, is right at the very input of the RF and I would guess this may be a tuner.
I haven't had a chance to check yet, so I'm not sure if TV broadcasts are sent out on FM. If they aren't the others may be modulators. I'd say modulators for recording Hi-Fi audio through the helical head but these are old two head VCR's.
Do you have an idea of the carrier frequency for helically recording Hi-Fi on VHS? I know the video is recorded somewhere around 4.5 - 5.5Mhz and has a bandwith of about 4.2Mhz.
I also started thinking about the tracking. As the signal frequency increases, so does it's pickup level for a given speed. When the picture changes, the frequency of the signal changes, and the amplitude of the pickup level in the head rises or falls, why doesn't the autotracking get tricked into thinking it's slipped off the track and start trying to track the head? How does it distinguish between a frequency change on the tape and an actual slip off track synchronisation?
I thought that was a dumb question at first because FM isn't intentionally AM modulated. But the signal at the head would appear with AM modulation due to the FM frequency changes resulting in different pickup levels. Which is why FM is used in the first place.
Then I also thought that perhaps the autotracking might be tuned to only look for the carrier of the FM signal. But the carrier won't be there in a pure form unless there's no logic on the tape.
The only two remaining things I can think of is that perhaps it's tuned to only look for frequencies close to the carrier, or within another tight band, so it doesn't start tracking when the frequency dips or peaks and the pickup level changes.
The other way might be that the circuit has a massive amount of lag on it, so it's only looking for an average playback level over a few seconds. But still, dark and bright scenes can last a lot longer.
Finally, I thought about the control track. In old VCR's with manual only tracking, you'd tune the tracking to the cleanest looking picture then sit back and watch the tape.
The VCR would then use the control track and phase difference between it and a constant frequency source, like a crystal, for the timing of the helical head.
If the tape had shrunk or stretched over time, the control track would have to a somewhat similar degree. All the VCR would need do is watch the control track and follow it for synchronisation to keep the picture as you set it.
But if your VCR has autotuning, theoretically there's no real reason for it to reference the control track right? Since you don't have to get up to tune it, it can change speed continually and you'd not have to move. So the only benefit for it referencing the control track might be if the autotracking isn't very accurate or isn't able to keep up in some way.
I'm only thinking, if a VCR only has autotracking, why go to the expense of adding a control head and control track signal tracking circuitary if you can just have the helical head constantly search for the best output?
My only answer to that would be to stop the picture constantly dropping in and out of synchronisation as the system seeks for the best output. So it'd find the best output and then lock it's self to the control track until the output slipped below a certain level. Only reason for that being the situation would be if it's too hard to track the helical head automatically to within an accuracy of one track. So in the process of trying to autotrack it'd damage the picture quality seeking between tracks. But then how would ever lock on accurately in the first place?
I'm sorry for the kind of long post and large amounts of thinking out loud. This is just something that I'm not totally sure about and very few sites seem to provide much information about it. Try typing autotracking and vcr into google at the same time!
Again, please feel free to just cut this post up into bits and answer where ever you can. It might only take one sentence for the whole thing to click.
Wow. You have some free time, eh?
VHS:
The control track is linear & that is what the bottom linear head (on the fixed linear head, by the capstan) reads. This is the main syncing signal that times the capstan servo (which precisely pulls the tape through the tape path) tensioned by the back tension arm, and the helical “flying” head(s). There are 2 helical heads, if you look at the small angle of the head drum platform; you can see how long a piece of tape is recorded per half frame (each head). That is why at slower speeds, picture is worse (less tape to record on). The helical heads are precisely aligned & sized to do their job. Many decks have 4 heads (2 pairs), one pair for the faster better quality recording, one pair optimized for slower recording. You may find yet another set of helical heads. Those are the hi-fi audio heads. That track is also helical, but the geometry & depth is different, so as not to interfere with the video tracks. You may find yet another(!) head: that is the flying erase head (or “trick” head for search effects on older units); only on the later high end models, for clean edits. It can erase a single half frame. Not as clumsy as a linear erase head & it does not require précising transport control as the early high end machines that actually moved the tape to position to record over the exact half track. It even worked sometimes.
With the quality of the servos, this mess works very well. It deals with stretched tape & crappy recordings well. Any faraday cages that you see are for high frequency. The one in the back of the VCR with the female F connector for RF out is a modulator. The other is the tuner. Some units had shields all over the place, but they are expensive & few machines do that now.
Did that help?
VHS:
The control track is linear & that is what the bottom linear head (on the fixed linear head, by the capstan) reads. This is the main syncing signal that times the capstan servo (which precisely pulls the tape through the tape path) tensioned by the back tension arm, and the helical “flying” head(s). There are 2 helical heads, if you look at the small angle of the head drum platform; you can see how long a piece of tape is recorded per half frame (each head). That is why at slower speeds, picture is worse (less tape to record on). The helical heads are precisely aligned & sized to do their job. Many decks have 4 heads (2 pairs), one pair for the faster better quality recording, one pair optimized for slower recording. You may find yet another set of helical heads. Those are the hi-fi audio heads. That track is also helical, but the geometry & depth is different, so as not to interfere with the video tracks. You may find yet another(!) head: that is the flying erase head (or “trick” head for search effects on older units); only on the later high end models, for clean edits. It can erase a single half frame. Not as clumsy as a linear erase head & it does not require précising transport control as the early high end machines that actually moved the tape to position to record over the exact half track. It even worked sometimes.
With the quality of the servos, this mess works very well. It deals with stretched tape & crappy recordings well. Any faraday cages that you see are for high frequency. The one in the back of the VCR with the female F connector for RF out is a modulator. The other is the tuner. Some units had shields all over the place, but they are expensive & few machines do that now.
Did that help?
Thank you for that frank,a useful page!!Frank Berry said:
I found this comment noteable: "They are certainly the most complex mechanical systems most people own outside of their automobiles, yet VCRs can cost as little as $75!" I dont think one for $75 would be any good really ESPECIALLY NOW WHERE CHEAP MEANS CRAP!!
I absolutely love VHS
(In my opinion ANYTHING NOT DIGITAL IS AWESOME!!!!! (As far as audio/video))- Status
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