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Old 7th Oct 2006, 9:20 am   #6
Kat Manton
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Join Date: Jul 2005
Location: West Yorkshire, UK.
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Default Re: FotH TV System / Converters Discussion

Hi Darryl,
Quote:
Originally Posted by tubesrule View Post
Actually the PC can be used as the source for a conveter still retaining any of it's existing functionality, so a seperate DVD, DVR or tuner are not required.
It could, but would seem strange to me to do this when a simple change to the graphics configuration means the computer outputs directly in the standard required.

It'd also increase the amount of processing going on, possibly compromising picture quality.

At present, when using digital sources already within the PC, the images are scaled once then output via the DAC in the chosen standard.

Without building additional hardware and using the TV output on the graphics card together with a converter, the following takes place.
  • Image is scaled to a 'standard' resolution, more often than not 800 x 600 is used.
  • Image is scaled again by the scaler in the TV encoder to a resolution appropriate for 625-line PAL.
  • Image is output via DAC as S-video or composite.
  • Image passes through an ADC in the converter.
  • Image is scaled a third time.
  • Image is output by the DAC in the converter, finally in the chosen TV standard.
So I think it'd be counter-productive to consider setting up such a system.

As I see it, standards converters and the system I've come up with are two very different things. Both achieve a common goal of "getting pictures on vintage television sets" but there the similarities end.

A standards converter, particularly one with a built-in modulator, offers an easy, true "Plug and Play" solution, and there lies one distinct advantage.

To set up the PC system, you've first got to dedicate a whole PC to it; this isn't really a good system to set up on a machine you ordinarily use as a desktop general-purpose machine.

Then you need the right bits inside it. Only nVidia graphics cards have the control over pixel-clock frequency required. You need enough processing power and RAM. Digital sources work best hence for off-air television a digital cable/satellite/terrestrial card is needed, along with a DVD drive.

Then there's the fun and games installing the software; it's fairly straightforward but still needs a certain amount of knowledge to get it all installed and running correctly. For example, it took me weeks before I got the infra-red remote-control to work properly.

There's still a requirement for additional hardware, you're going to have to build an RGB/sync combiner as the TV output can't be utilised. Then you still need to build or buy a modulator appropriate to the TV system you wish to generate.

The easy bit is changing the graphics card configuration so it natively produces a reasonable approximation of the chosen TV standard. That's only easy, though, if I've already worked out the modeline and published it.

And note that it's only a reasonable approximation - due to the limitations of the graphics card it's not possible to generate exactly the chosen standard, to specification. The total line length and horizontal/vertical timings will be very close - as that's my primary aim when working out the modelines. But sync timing won't be quite right, and the waveform will generally be crude compared to most standards.

This will remain true when using crude and simple methods of combining RGB and syncs, improvements can be made at the cost of increasing the complexity of the circuitry needed.

But, my aim is to get pictures on vintage television sets - if the set locks to the signal and you get a picture then the video waveform is "close enough"

An area where the computer system is weak and where a stand-alone standards converter beats it hands-down is true standards conversion. If I feed analogue video into a capture card and try to use the system as a true standards converter the results are not likely to be too good. There's no synchronisation between the capture card and graphics card. Using the MythTV system, the captured video is encoded to MPEG, buffered to hard disk, read from disk, decoded, scaled... It's not going to work too well as a standards converter hence I claim it isn't one; it could be used like this but I wouldn't recommend it.

I've never seen my solution as competition, and as it's such a different approach there's a place for both types of system side-by side.

I have an extensive library of VHS tapes and a decent enough VHS machine; as I'd like to watch these on vintage sets I'm likely to buy an Aurora as soon as my disposable income allows as it's the best option. Anyone else wishing to watch analogue source material on vintage sets would be advised to do the same.

Regards, Kat
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