When did the extra sync pulses appear? TV question

[jamesinnewcastl]

Hi All

I'm currently concentrating on old TV cameras. Most cameras, even the studio ones, seem to have a simple synchronisation regieme, That is: Frame pulse then a number of line pulses with video between, repeat.

However the 405 standard (for example) has more than one frame pulse and several line pulses - necessary to perform the interlacing but only on alternate frames.

So, when did those extra pulses get added and who/what added them? Am I wrong and did all studio cameras output the 405 standard or was there a complex piece of equipment that 'stuffed' the extra sync pulses in?

Cheers
James

[Oldmadham]

Normally, the syncs & blanking were generated in the sync pulse generator and fed to the camera, which was basically a "dumb" device, which relied upon that information to produce a signal with the correct timing.

Yes, the differing styles of field sync pulse were to allow for the half line offset needed for interlacing.

The 405-line system field sync pulses did not have pre, or post, equalising pulses, which were added in the US 525-line system to ensure that the early type horizontal & vertical scans were correctly initiated.

This carried over into all the 625-line systems as well, but with later designs of sync circuitry, became more of a "nicety" than anything else.

[jamesinnewcastl]

Hi Oldmanham

Thanks for your reply. That ties in perfectly with a manual that I have for an Image Orthicon studio camera which shows the syncs coming in from a 'sync pulse generator' and interestingly the same signal is passed out again. Might this mean that all the cameras in the studio were all synced together? It would also explain how it would be possible to seamlessly fade/switch between cameras without a 'jump' in the image as the new camera took over.

It was quite difficult to Google this topic!

I'm planning to recreate a Iconoscope based camera and this sync issue was the first stumble. My plan now is to have the camera switchable so it can be true 405 (or 625) or the free running version found in most small CCTV cameras. Having to generate the complex sync pulses - possibly using valves - would have been quite a large amount of hardware. Now I can simulate a studio 'Sync Generator' using a small microcontroller, very simple!

(Moderators this thread can be closed - thank you)

Cheers
James

[Andrew2]

Yes, all the video equipment was synced to the same master generator, as you say this enabled switching between cameras, pattern generators etc without any frame roll etc.

[jamesinnewcastl]

Thanks Andy!


Cheers
James

[Oldmadham]

I remember back in the day, (1965) my then employer was a new TV station which had just started up, competing for viewers with two existing stations which already had OB vans, etc, which we did not.

They both, consequently, had a substantial presence at the Perth Royal Agricultural Show, both as Broadcasters & showing off their technology to the public.

Not to be left out, my employer sent a random "minion" (me) off to the Show with a (very) small "industrial camera" & a Picture Monitor so we could show visitors "what they looked like" on TV.
Strangely, it seems the very basic setup was less intimidating to the public than the full sized OB cameras, & we did draw quite a lot of interest.

Back on topic, the little camera was described as having "random interlace".
I never worked out quite how that worked.

Obviously, in the 1960s, fitting a full SPG into such a small package was not practical, but how free running oscillators could produce any kind of "interlace" was, & is, quite mystifying.

Has anybody else heard the term?

[cmjones01]

"Random interlace" just means that the line sync and frame sync aren't synchronised with each other. They're just free-running oscillators. That means that the exact timing of the lines relative to the frame, and so their position on the screen, varies from each frame to the next, hence the random "interlace". It was simpler and cheaper than using counters to derive the frame sync from the line sync. The infamous Amstrad VMC100 camcorder, which was really stripped down to the bare minimum, used this technique.

Chris

[jamesinnewcastl]

Hi

I too came across the term when looking into the whole standards issue. It was mentioned in the text for the simple vidicon cameras in the various Home Electronics magazines.

I find it a little difficult to imagine the effect but it seems that what you might see is a single raster where the start of the line sweep could slowly change position as the two oscillators drift apart.

The history of TV 'pick-up' tubes is fascinating, it seems that the earliest vidicon was simply a 2in CRT with the phosphor replaced by a photo sensitive surface and an electrode attached!

Cheers
James