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Television Standards Converters, Modulators etc Standards converters, modulators anything else for providing signals to vintage televisions.

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Old 8th Feb 2011, 1:53 pm   #1
llama
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Default Crystal-down to 405-ish

Here's something I've been pondering a few years but never got round to checking out.

One (at least) of my monochrome cameras has strappings for 625/50 and 525/60. My idea is to use the 525/60 straps but fit a crystal suitable for 50Hz field rate. By my reckoning that gives a line rate corresponding to 437.5 lines per frame - surely close enough to 405 lines for most sets to resolve without problems.

Now I develop the idea a little further. Are there any (early so less integrated) FreeView/OnDigital boxes that can be abused this way to read out from their frame-store at a rate suitable for 405 (or maybe 437) lines? What about satellite receivers which can resolve "digital-NTSC" such as AFN? Could one of these be fudged?

Anyone else consider this a fascinating prospect? Anyone got the time to try it?!?

Graham
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Old 8th Feb 2011, 3:34 pm   #2
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Default Re: Crystal-down to 405-ish

The camera mod would work but you would still get 525 lines, best to get the line frequency about right as the frame has more range of adjustment. However the set top box fudge may not as it would probably use only one crystal, the change in frequency would totaly stuff the input decoding. But you may be able to freeze a frame and then switch frequency.
 
Old 9th Feb 2011, 2:27 pm   #3
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Default Re: Crystal-down to 405-ish

Even if it was to work 437.5 lines does not have a proper mathematical relationship to allow interlaced scan with 50hz.
Also most sets would work but there will be a little cramping on the left with a slight reduction in width.

Altering the clock speed on a digi box actually slows everything down line & frame rate and also will effect picture timing and sound, I have tried it, its just a waste of time.
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Old 10th Feb 2011, 9:03 am   #4
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Default Re: Crystal-down to 405-ish

I thought about doing something like this a while ago for some other reason than 405 but cant remember now.

Unfortunately digiboxes are pretty much entirely digital right up to the output stage where the video feed gets turned into composite video.

If you slow down the (usually the main) crystal then you slow down other things too, meaning the sound will begin to 'oversample' and things like buttons and remote control's may stop responding. Being a digital system, each frame of video is held in a RAM chip, just slowing the crystal could cause timing and access violations.

If someone had the knowledge then it wouldn't be out of the question to hack the firmware of a box to drop 220 lines from the decoder feed, thus giving 405. Beyond that it should be more than possible to alter the counters for frame rate and line frequency.

I did have a box which would output NTSC, my TV was able to decode this perfectly fine, but it looked rubbish in comparison. Being able to output in NTSC shows that the decoders are 'flexible' at least.

Dave.
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Old 10th Feb 2011, 2:43 pm   #5
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Default Re: Crystal-down to 405-ish

This has given me an idea - I've only worked it out roughly but I hope I haven't made any mistakes ...

Set a digibox to display 16:9 on a 4:3 screen. This reduces the vertical display area to about 430 lines - or 215 lines per field, which is 36.5 more lines than in a 405 line field.

Feed composite video to an ADC and clock it into a field store starting at the 50th active line on each frame, continuing to the 239th line. Only the centre of each line is needed to match the 4:3 ratio required, the actual pixel count being determined by the clock frequency.

Then clock the info out as 405 line video.

Apart from the obvious horizontal cropping, there will be slight vertical cropping - about 6% top and bottom - but that probably won't notice most of the time.

Provided the input is 16:9 material, it should be possible to get a good 405 line result ...
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