The fine art of editing videotape

[ppppenguin]

PS: " Outside broadcast grade kit "wild" subcarrier was commonplace. " doesn't refer to OBs. On re-reading I realise I worded it badly. " In non-broadcast grade kit "wild" subcarrier was commonplace. " is better.

[Yellowtriumph]

Concerning the GVG vertical clamp, I assumed at the time that as the desk was originally NTSC in design the americans were not concerned about such 'exotic' matters as field sequences and it didn't occur to them someone else might be! Hence no reason not to use any line in the vertical interval. Of course you have now identified that as Line 7 - thank you.

The story of subcarrier and line syncs not being locked to one another and causing problems with colour VTR's was told to me first hand by an engineer of the 'correct' vintage. I believe it was somewhat officially referred to as 'Non-phased colour' at the time? Ring any bells? He also designed his own Quad tape cue tone Editor (!) - quite an amazing bit of design I would think so you could add the acronym 'Ghoste' to Ampex's Editec and RCA's TEP.

I'm wondering how the Tek vectorscope was able to 'discover' and accurately relate the Sc/H phase? From distant memory wasn't it defined by the extrapolated subcarrier crossing the leading edge of line sync 1 within +- 45 degrees? Too long ago now for me to beat myself up about it! You are obviously the 'go to' guru!

Definitely used the Cox 350 and 650. I see you say you worked for Cox Electronics. Please don't tell me you worked on the T series of mixers? If the answer is 'yes' I can't relate a story I'm afraid as it would be too painful for you.

The issues arising from Quad and Sc/h phase are both simple and complex at the same time. A 'run of the mill' editing machine will happily edit all day long seeking only to maintain the 4 field sequence over the edit because it will internally correct any incorrect Sc/h errors within its colour timebase corrector by moving the off tape picture by half a cycle of subcarrier as required. It's only when you are seeking to perform an animation edit ( a sequence of edited frames) or inserting pictures within an existing picture sequence that issues and considerations arise. I think this would go beyond the scope of this thread and to be honest is of no interest to anyone so I will desist.

[ppppenguin]

For better or worse I was one of the main designers of the T series mixers. Well the T16 anyway. I left Cox before the T8 but it used a fair bit of my work. Don't worry about telling distressing stories. In retrospect we made several major mistakes:

* The clicky buttons aren't very nice.

* The "incpot" spring loaded pots were not a good idea. We arrived at them after a lot of discussion on how to automate a mixer. The problem was reconciling the logical and mechanical positions of lots of controls. There are various solutions that really don't work. A continously rotating shaft encoder would have been ideal but these were expensive and too large at the time. Of several unsatisfactory options I think we chose the wrong one.

* We should have gone for bigger PCBs (we used 4U high) and 2 part edge connectors.

Before SCH phase was properly controlled any VT edit was a potential nightmare. I've done SCH measurements in various ways. You can digitise enough of the signal and do it in software. I've not done this though Tek probably did in their big all singing digital test sets. You can regenerate subcarrier and sample it. It's hard to get the sampling datum accurate enough but I've done it and it works. I thnk the Tek 1755A etc does it this way. For development work I needed to make absolute measuements, not depending on SC regen etc. I've built special modes into my SPGs which make a non-standard waveform where I can overlay the SC and H datum points on a scope. I claim, with a certain amount of hand waving, to get to within a couple of degrees with confidence. I have also computed SCH accurate waveforms to within 1 degree. The DAC reconstruction filter is often less accurate than that!

This is all very legacy stuff now. Back c1990(?) I wrote an article about SCH measurement for one of the broadcast magazines. This was basically advertorial for Hamlet - I was doing some work for them at the time. Less said about all that the better.

The Cox 350 clock had a special "ABC" keyboard option as well as the common QWERTY. It was compact which was useful on a desk but the main reason was union demarcation rules at Thames. To use a QWERTY keboard you had to be a typist. The ABC version could be used by the production assistant. From what I remember of the 350 design, getting the circle to look reasonable relied far too heavily on the tolerances of a lot of transistors. When I looked at it, my comments about the Thames designers were not printable.

Would love to hear more of your stories. Also about you and where you worked. If you don't want to go public on GVR please contact me by email or private message.

[Yellowtriumph]

More than happy to send you a PM - cannot see how to do that on this forum?

[Amrad]

@ Yellowtriumph,

In the box with the avatar to the left of the message, there are two envelope icons, one black for sending an email and one white for sending a PM.

Regards,

Dave.

[ppppenguin]

Dave, thanks for giving guidance.

You can also see my email address on my personal website which is in the signature line of all my posts.

[Yellowtriumph]

Thank you both, email sent via this site to ppppenguin. Have a cup of tea ready!!

[ppppenguin]

Received OK. Thanks. WIll reply soon.

[DrStrangelove]

Well that's brought back memories of the Ampex VR2000 training course where I met a chap who had a VR1000 in his garage.

The long gone days of Scotch 400 video tape, damn those reels were heavy.

Wish I'd snaffled one of those Scotch viewer thingies that show the magnetisation stripes across the tape.