Hmm, I always though, and it's backed up in Wikipedia (for what's it's worth...), that it's across the tape at an angle (\\\\\\ rather than lllllllll) but gets read as if it's along the tape due to the angle of the head drum;
It's at an angle, true, - as the tape is moving. If you take 45 degrees as dividing between across and along, the quad is across and the helical scan along. The quad machine needs four heads because the wrap round the heads is limited to just over 90 degrees whereas the helical machine can make do with two because the tape wrap goes round more than 180 degrees. This would be near impossible to do with a head drum at near right angles to the tape.
In message , "Dave Plowman (News)" writes
No I'm not.
Yes, in a helix, imagine cutting a 90 degree section out a nut, then running a piece of studding along it, the contact area and the thread describe the helical path of the heads relative to the tape.
Still not convinced? Let me quote from the "Broadcast Engineer's Reference Book" (p470), quadruplex VTRs section: "The Quadruplex tape format ... Helical tracks were originally 10mils wide"
In message , "Dave Plowman (News)" writes
... Hurrah! and as the drum is cylindrical this makes the scan helical.
DCRSi was a military spec, airborne qualified, ultra high speed data recorder, 240MBytes/second IIRC.
"Don't Count on Recovering Stored Information" tells you all you need to know about it...
Although, having Googled it to remind myself, DCRSi wrote true
*vertical* tracks with a high speed scanning head (wince). I think the herringbone must have been boring old DLT tapes...
That stuff was called "Edi-Vue" and was hellish expensive.
To all intents and purposes in the UK it didn't happen. Except in extremis, for the sake of a hypthetical example to save the last images of a dying king.
I was a broadcast engineer with the BBC in the '60s, and my cousin was a tv actor for 10 years before me, he said they were always told that "You can't edit video tape".
For one thing the tape was very expensive, and once cut for editing the picture disturbances would re-appear not only on the edited master tape but also whenever that tape would be used again.
DG
In the early days of quadrature the head drum and transport were phase locked to the control track read off the tape. I'm guessing but it was probably possible to adjust the VT manually to get the output approximately in frame sync so viewers sets wouldn't frame roll when the studio cut to VT.
It became necessary to lock the replayed video to a remote source to make synchronous cuts to / from VT Ampex introduced another option (Box) called Amtec "Ampex time element correction" which introduced a variable time delay in the output signal which took out all the timebase errors in the output video so that the video could be cut mixed and faded just like the video from a camera.
When colour came along Ampex introduced yet another option (Box) called Colortec "Color Time Element Compensation", this further reduced the timebase errors to limits acceptable for NTSC color.
However the PAL system, because there were even and odd fields and because of Vertical Axis Switching (Pal Ident), it turns out that there was a sequence of 8 fields which had to be preserved during an edit. Eg. A frame 7 always had to joined onto a frame 8 and so on, otherwise 20 million colour TVs around the country would do a colour-kill and display monochrome pictures 'till the local colour carrier had locked back in.
A splice joint could cause the servos in the VT machine to become unlocked, the Amtec unit to become unlocked, the Colortec unit to become unlocked and the time taken for all this lot to get locked back in could be many seconds, not to mention what was happening at 20 million tellies around the country.
Because of this physically cutting video tape was AFAIAA never routinely done in the UK broadcast industry. It could be that in the USA, out in the sticks one horse mom and pop, TV stations did it, but the tape was expensive and the joint would have precluded using the tape in the future to record original programme material.
Errors Excepted. I never worked in Broadcast VT, but got the training.
DG
In the UK I believe it was done routinely for some sports events where they wanted highlights more quickly than dub editing could provide. I worked in studios and can only remember one case - That's Life which was recorded just before the transmission for legal reasons and on this occasion was so late there was no time to re-do the entire prog after a fault.
In message , PC Paul writes
I Have used sellotape many, many times for repairing and recovering snapped VHS and Beta cassettes and have *never* had a problem. I've always copied off the video onto another cassette or media and thrown the spliced tape afterwards though.
Not sure I'd want superglue near video heads, it 'blooms' but if it works for you....
Use a mirror, hard backed razor and a couple of paperweights with felt bases (or something that allows a bit of slip for repositioning) and with a little practice you can achieve very neat splices.
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