405 to 625 conversion

[ppppenguin]

The BBC had 2 different versions, the CO6/501 and CO6/501A. I'm not au fait with the differences. I'm not sure if Pye built the 601 or the 601A. Pye also built a version of the analogue field store converter. Apart from the unique field store parts, it also had the equivalent of a pair of CO6/501 in it. One for luminance, the other for chrominance.

I wasn't aware that the BBC had built field store synchronisers. At least I assume they were field store. CO6/512 info: http://www.bbceng.info/EDI%20Sheets/10505.pdf By 1985, the date on this document, Quantel and others could supply commercially available alternatives.

I remember seeing a demonstration of a BBC prototype of the ACE converter. The interpolation could be switched to any desired aperture. The effects of a badly chosen aperture were very obvious. In the 1990s I designed a universal TBC/synchroniser/standards converter for one of my clients. The field rate conversion was crude in the base model but I also did an add-on card that was a rough equivalent of the ACE. It used some field memory chips, all the interpolation (4 line, 4 field) and control was in a couple of FPGAs. I was then able to play with the interpolation coeffts and optimise them for what my client wanted. ACE gave a very good conversion, only bettered when motion vectors became feasible.

Another factor that made standards conversion more difficult was the rise of CCD cameras with shutters that could give a very short effective exposure. A Plumbicon camera inevitably had a limited temporal frequency response which made life easier for conversion.

Whay wasn't the CO6/509 licensed? Some speculation...

Pye had possibly got fed up with difficult BBC designs. The /501 and /508 can't have been easy to reproduce. I know from direct experience at Micahel Cox Electronics that BBC designs were not always fully production engineered. That's being polite about it. I think McMichael wanted to break into the broadcast market and so thought the ACE licence was worthwhile I don't know any ex McMichael engineers but I have little doubt there would have been some less than polite language about the BBC designers. Yes, BBC Research and Design engineers were good, in some cases superb. Richard Russell is name that comes to mind in that category. What they didn't have was much experience of putting complex designs into commercial series production. Having made my own share of design and tolerancing errors I'm all too aware of the problems.

The IBA decided not to invest in new 405 converters. The BBC took the view that the 405 service would outlive their /501s. Perhaps the IBA took the opposite view.

[colly0410]

Wondering how the final 405 picture would be degraded after a 405 to 625 conversion at the studio's, & then 625 back to 405 at the main TX sites? I suppose worse would be the 1'st moon shots, 525 to 405 then to tape, then 405 to 625 & back to 405. Mind you would anyone have noticed with the band 1 ignition & co-channel interference?

[ppppenguin]

I doubt anyone cared to much about the cumulative effect of 405>625>405. Actually I don't think it would have been too bad, as 625 has more lines.

The Apollo 11 pictures from the moon's surface were relayed to earth at very non-standard line and field rate. They were converted optically to 525 at the ground stations. I don't know if later missions used electronic conversion. The colour cameras on later Apollos were field sequential, with a colour wheel in front of the tube. I think the greatest loss of picture quality was on the moon to earth link. Everything else was fairly minor. 405 wouldn't have been a factor here. Initial conversion was to 525, then to 625 for Europe. The only conversion to 405 in 1969 onwards would have been at 405 TX sites.

https://www.tvtechnology.com/news/equipp...television

[Daviddean1]

Thanks for the interesting insight.
As the original poster of the question I am aware of 2 electronic converters from 405 to 625 these are used with stable video sources, and whilst I can get the pictures visible through a Sony TBC, I have no way of framing the VTR to the TBC. I wonder if it would be practical to replace the memory in the TBC with a frame store which would nullify the need to frame the VTR and would also allow for the addition of interpolation. Any ideas? Thanks

[ppppenguin]

David, thanks for coming over to GVR and contributing to this thread.

I doubt it would be practible to modify a complex device like a TBC unless you had some very comprehensive documentation and significant logic design skills.

The question of stable sources is not very important. From expereince we know that the BBC's CO6/509, designed for stable sources, was entirely tolerant of VHS replay. All 625 to 405 converters I've seen have been tolerant of VHS replay. Even a linestore converter imposes a measure of timebase correction, simply because the output clock will be smoothed by a PLL compared to the input clock. A framestore converter is by its nature a timebase corrector as the output clock is independent of the input. A 405 to 625 converter woul dbe no different.

As I've said before in this thread, several of us know full well how to make a 405 to 625 converter but it's a fairly big step from there to being bothered to put in the work to do it. Recovering a few odd tapes wouldn't push me to do it when an optical approach can be improvised to give a pretty good answer. About the only thing that would give me the incentive to do it would be if Paul Marshall said he would really like to be able to run Project Vivat on 405. That would realistically need 2 or 3 converters and they would have to be genuinely real time, not offline software. Paul is an old friend, Vivat is a big and complex project. I would be happy to contribute in that way.
http://projectvivat.co.uk/Vivat/Home.html

[Daviddean1]

Thanks for your comments.
I will admit I have a commercial need for a converter, be it by adapting what exists or starting from scratch. I have a 100+ hours of material to recover. The clients I have are also willing to consider sponsoring the development work required.

[ppppenguin]

David, what tape format is the material on? If it's VHS or Umatic then I doubt that an all electronic converter will do significantly better than a decent monitor and HD camera. If it's 2" quadruplex then your big expense is finding a VR2000 that's still fully maintained.

I might be prepared to do a converter as a commercial job. I have little doubt it would work and work pretty well. My main worry would be how it performed with VHS replay. This is a very variable quantity and difficult to allow for during development. If you want to explore optical conversion i have a high quality 405 monitor and know what to tweak to get best results. We could have an exploratory session. Since this is for a commercial job there would be a fee involved. If you and your clients are happy with the results then they've got a simple solution with low costs.

I assume you're aware that if your source material is VHS then the drop-out compensator won't work. It will actually make matters worse by inserting incorrectly timed information into the picture. There is nothing you can do about this apart posibly from modifying the VHS to remove the DOC and accepting the dropouts. I also assume you're aware that many later VHS machines just won't replay 405 material at all.

If you'd like to discuss commercial possibilites please contact me via the email address on my website.

[Gryphon]

Forgive me for asking what may seem a rather silly question: it is intended, nonetheless, to be helpful.  Are you clear that the requirement is actually to recover the material rather than merely view it?

I was asked to assist in what sounds like a similar exercise about 25 years ago.  The source material was on a number of reels of 1/2" Sony tape, about which nothing was known except that they were believed to be CV-2000 format, and dating from some time prior to 1970.  Fortunately a machine was available, although this turned out to be in need of new heads (low-band U-matic heads can be made to work, just out of interest).  The client requirement was said to be "please dub to DVD".  From memory there was about 20 hours worth of material.

Once replay had been achieved, examination of the vertical interval suggested that the material was 405, which was confirmed by measuring the line period.  Obviously, this gives rise to the issue you were enquiring about.

Discreet enquiries revealed that the ultimate client wanted to examine the material as possible evidence in litigation which was then at the "we're thinking about putting a case together" stage.  They were persuaded that it was actually much easier for them to view the replay of the tapes, which they did over a number of days at the premises of the people I had been asked to advise.  Even this sounds easier than it was, as the tapes had not been sympathetically stored and were starting to shed oxide.  Viewing was therefore often interrupted by the need to clear a video head clog.

That was as far as the project went: having viewed the material, the ultimate client decided that it did not show what they had been led to believe it might, and, indeed, did not support the case they were trying to make.  They went away very happy, and were happier still when given the bill, which, clearly, was much smaller than it might otherwise have been.

[peter scott]

Quote:

"One problem is that the material is on half inch reel to reel and I do have the play back machines. Does anyone have a service manual for a Sony CV2000 I can look at?"

[ppppenguin]

I missed the bit about the CV2000. From what I remember the output of the CV2000 is a bit non-standard, though I don't remember the details. I have a funny feeling the vertical sync pulses don't comply with the System A standard. It may have only recorded one field out of each interlaced pair.

For those who don't know the machine, the CV2000 was a Sony half inch open reel machine. The UK version was a 405 only machine though you could sort of push it to record 625. Tape ran at 7.5" per second. Its close relative, the CV2100ACE, was a 625 machine running at about 11"/sec and widely used in schools. Utterly solid and reliable.

It used low density tapes, unlike the later EIAJ half inch machines which used a higher density tape. I don't know if the tapes suffered from sticky shed.

Optical conversion should be good enough to recover the relatively poor picture quality from CV2000 recordings.