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Old 10th Nov 2014, 10:32 pm   #1
avocollector
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Default How did they sync the scan head to the tape track

Could anyone explain or send me to a thread that shows how the VCR's managed to get the helical scan heads to 'line up' with the actual scans on the tape? I've wondered exactly how it was done since the players don't seem to run to a predetermined point to start? I've only 1960's 6th form Physics/basic radio understanding so please not too technical if possible. Thanks.
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Old 10th Nov 2014, 11:08 pm   #2
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Default Re: How did they sync the scan head to the tape track

Does this help?

http://repairfaq.cis.upenn.edu/sam/icets/vcr.htm

Scroll down to the "Synchronization Track"
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Old 20th Nov 2014, 3:55 am   #3
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Default Re: How did they sync the scan head to the tape track

Spot on! many thanks for that ref.
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Old 20th Nov 2014, 6:36 am   #4
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Default Re: How did they sync the scan head to the tape track

I understand that this was a significant problem in the early days of professional VT before everything became automated, and one of the reasons that VT editors were so well paid.
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Old 20th Nov 2014, 11:20 am   #5
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Default Re: How did they sync the scan head to the tape track

That site gives a very good explanation of the fundamentals of most VCRs; a few additional points,
1) Philips (as ever) did it a bit differently - their first VCR attempt used a cassette where the 2 reels were on top of each other instead of co-planar but the priciples of the recording were broadly similar to the others. However in their V2000 system the control track was abandoned and a pilot tone was added to the signal on the scanning video heads. The pilot tone was varied in a field sequence; therefore if the video heads slightly mistracked it would pick up 2 pilot tones. Depending on which way the head mis-tracked the 'beat' frequency of the 2 pilot tones would vary, the machine then deduced which way to re-track to get a pure pilot tone. Clever - yes, complex - yes, successful ......

2) In the original Beta and VHS US/Japan NTSC specs Beta had both the highest tape and video head to tape speeds, by the time the formats were modified and launched in Europe and the other 50Hz countries, Beta linear tape speed had dropped to less than the standard play VHS speed, although the video head to tape speed (governed by head diameter) remained higher on Beta.

3) As Paul mentioned above, in the early years of video recording the interchangeability of video recordings could be a bit of a trial and, in the very early days, the video head block was shipped between stations with the tape!. However please note that the formative years of professional video recording did not use Helical scan machines but a Transverse scan format call Quadruplex - invented in the US by Ampex (Ray Dolby was a young engineer on the project), it was an ad hoc world standard into the 1970s.
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Old 20th Nov 2014, 5:22 pm   #6
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Default Re: How did they sync the scan head to the tape track

With regards to point 3. The reason for sending the Quad (transverse scan) headblock with the tape was because of the limited mechanical ability to get precisely 90 degrees between each recording head which would result in objectionable timing errors between each 'block' of the recorded tracks - so better to fit the headblock that recorded the tape to the machine that was playing it back. As electronic timebase correctors became better so the need was obviated. The mechanical spacing between the transverse heads and the control track record/replay head was mechanically tightly controlled but could never be 'perfect', so, the VT machine had a control track adjustment control to vary the phase (mechanical physical positioning if you like) of the position of the video tracks on the tape relative to the scanning heads which you adjusted when playing back any pre-recorded tape.

If you changed a headwheel panel (the panel which contained the rotating heads and the control track record/replay head) then one of the standard re-installation procedures was to electronically adjust the timing between the recorded video tracks and control track signal to be correct. In effect you were electronically correcting for minor mechanical errors between the control track head and the rotating video heads when the machine was in record mode.

Ampex manufactured the AVR1 quad machine which could play back a tape with no control track at all! It would subtly mechanically phase shift the tape over the rotating heads to maximise the RF output and take it from there! Marvellous machine, expensive, the BBC only had one machine and it can be seen in action on youtube in a sequence where Noel Edmonds gives viewers a guided tour behind the scenes on 'Swap Shop'in BBC VT. This was cutting edge technology at the time.
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Old 21st Nov 2014, 5:35 pm   #7
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Default Re: How did they sync the scan head to the tape track

The AVR1 was a marvelous machine. The tape was held in vacuum buffer chambers either side of the headwheel and could spool at enormous speeds and took about (IIRC) 230mS to produce a stable locked picture from standby (the headwheel running). The earlier machines took anything from 5 to 10 seconds - sometimes more!
When red16v says the AVR1 was expensive, he was right. A fully loaded machine was upwards of £80k in the mid 1970s but it was a beautiful piece of engineering which was wonderful to look at. In the same family, there was an ACR25 which was a cassette machine which could play back 10 second clips of video, rewinding, replacing and picking up a new cassette, threading and cueing before the clip playing had finished. It was meant for playing out commercials!
During that time, I was very proud to work for Ampex!
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Old 21st Nov 2014, 6:14 pm   #8
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Default Re: How did they sync the scan head to the tape track

If I may continue on from Brian's post? I occasionally worked on the ACR25, but spent many years working regularly on it's RCA 'older' brother (by a few months?)- the RCA TCR100. The TCR came first, and some might say suffered because of it maybe, in the sense that it was very much a mechanically engineered solution to a problem that Ampex engineered in a more elegant way using vacuum threading tape lacing (or was it compressed air I can't remember after all these years?). Still loved the TCR though.

I have an Ampex AVR/ACR MkXX headwheel panel in my possession, I use it as a door stop. It makes an interesting conversation piece alongside my original 2" EMT quad tape splicer!
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Old 21st Nov 2014, 9:24 pm   #9
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Default Re: How did they sync the scan head to the tape track

Now there's a machine, the Ampex ACR25. Thames Television had 4 of these 2" cartridge machines with vacuum tape guides and vacuum capstans. 400mS lock up time They where in use with Thames up until the 90's. Used to maintain them and operated them on overnights when ITV went 24/7.
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Old 22nd Nov 2014, 5:38 pm   #10
brianc
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Default Re: How did they sync the scan head to the tape track

The RCA TCR100 was a very simple machine. It used a mechanical threading employing mechano-type chains, rather like a seaside funfair grab crane. It was rumoured that in some of the ACT25s, there was a label which read "Eat you heart out, RCA".
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Old 22nd Nov 2014, 10:07 pm   #11
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Default Re: How did they sync the scan head to the tape track

I'm not sure I'd agree it was a 'very simple machine' Brian. For it's day it was ground breaking. Of course it had it's foibles (and faults) but if you looked after it it was generally very reliable. I'm thinking in the 7 or so years I operated one I can only think of one period when it was out of action. All the logic was designed with DTL and it led to a headwheel panel that did not require the female guide to open. The same headwheel panel made it's way into the TR600 which you could lace up, in well under 5 seconds, with one hand behind your back! So, what Ampex achieved with a relatively sophisticated retractable female guide, RCA achieved with a simple bent piece of metal with nothing to go wrong.

And with my tongue firmly in my cheek I'm sure for every mis-thread on a TCR there was the equivalent ACR going into unwanted 'test mode' just before TX. But it was long time ago and I know we'd both agree both machines were marvels of their day and neither of us would argue with that I'm sure!

If you're at all interested here's a link to the chap at RCA that led their VTR design team and it includes the history of the introduction to the TCR100 - the book the link leads to does miss out some pages (its a google book preview) but you get gist.

http://books.google.co.uk/books?id=F...tcr100&f=false
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Old 23rd Nov 2014, 12:21 am   #12
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Default Re: How did they sync the scan head to the tape track

It was possible to find the synchronising pulses on "Quad" video tape by dipping the tape in a special developing fluid, which contained magnetic particles which stuck to the tape where the magnetism was strongest. You could then splice the tape with a cut at exactly the right angle, and have the next field after the cut line up exactly after the last one before it.

It sounds as though it would be a very messy process -- I would not mind getting hold of some of that developing fluid to experiment with, though .....
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Old 23rd Nov 2014, 11:31 am   #13
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Default Re: How did they sync the scan head to the tape track

'Edivue' is the stuff you're referring to. Have a look at this link to see it in action:

http://www.vtoldboys.com/editm01.htm

The pulse the editor is looking for on the tape is the 'edit pulse' on the control track signal - by knowing where this is (and aligning the tape mechanically correctly in the cutting block) you would cut the tape exactly at the point where the vertical sync period of the TV signal was recorded on the tape.

The editor is shown using a Smith's editing block, I have in my possession an EMT editing block which looks very similar BUT it does not require the magnetic pattern to be viewed. 'My' editing block has a rotating replay head which is aligned to scan/read the control track on the quad tape. So, as the tape is moved back and forth by hand in the editing block to find the edit point, the output of the rotating head (which is scanning the control track) is displayed on a small scope built into the editing block - so you can find the 'edit pulse' electronically rather than by developing the tape. Cut tape editing had been surpassed by the time I joined VT and my initiation to tape editing was using cue-tone editors, I have never switched on 'my' machine as I don't think it would work after all these years, I have no linear tape to try on it, and if it burst into flames it would destroy a very rare machine.

Have a look around that Vtoldboys editing website, there used to be pictures of the Smiths editors and the EMT editor I have. Apparently the people who used to run the site were looking for a picture of the EMT editing block for some time to put on their website, if only they had asked .... and the editing block I have did not come from the BBC, it came from the 'other side'!
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Old 26th Nov 2014, 12:56 am   #14
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Default Re: How did they sync the scan head to the tape track

The bible (which I still have) concerning tape formats and how things work is one by Joseph F Robinson called 'Videotape Recording. So if you can source a copy of that book, you will not be dissapointed. It helped me through a number of interviews.

Going slightly off topic here:
Quote:
Originally Posted by brianc View Post
The AVR1 was a marvelous machine. During that time, I was very proud to work for Ampex!
I got to run those beasts a number of times. I had to stand on a box to lace it up and adjust the headwheel controls. Not made for short people, so the Japanese probably didn't like it.

In the 70's I worked for a while at Rank Video in Wardour Street. We had three AVR1's, two RCA TR70's and two IVC 9000 two inch helical machines that were fantastic. Ohh.. And for a while the ACR25 which was named 'The Beast' as it sucked in more ways than one. Luckily I saw the light and moved to the Telecine area working on the Cintel machines both operationally and technically.
Quote:
Originally Posted by brianc View Post
During that time, I was very proud to work for Ampex!
Ohh.. That must have been in the Acre Road time when Probel and Ampex were there. Sadly both buldings are gone and now it is a storage company.. I do recall (before Rank Video) heading off to Ampex with sick 7000 series machines to be sorted. The guy there was good. He had a beard but that is all I can recall. Last time I saw him was ten years or so ago working as a tape machine fixer at VTR PLC in Dean Street. I suspect that he may have retired now.
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Old 27th Nov 2014, 12:31 pm   #15
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Default Re: How did they sync the scan head to the tape track

Quote:
Originally Posted by Trish View Post
Ohh.. That must have been in the Acre Road time when Probel and Ampex were there.
I worked in the Berkley Avenue building which was Ampex International. I remember visiting Rank Video several times and marvelled at their CMX editing system! Later, I became an editor in Germany using a much later CMX.
Such fun!!
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