[bit OT] how to assess quality of incoming signal?

[Jkun]

I own a fair bit of NTSC video equipment. For someone in a PAL region, not that usual. I got it to work with the Japanese laserdiscs that I collected.

Times change, and I no longer need much of my NTSC SVHS decks and LD players, just one of each for backup. So the central switcher I used has also become obsolete.

This is a JVC JX-S700. It's a digital NTSC-only switch, a really neat one, but it does not support PAL, or so I thought.

Along with the other equipment, I put it up for sale, but it was no big surprise that there's very little interest in NTSC gear over here. So when someone asked "Does the JVC also work for PAL?", and I answered an automatic "No", I started thinking.

I always thought it would not work in PAL because it's a digital switch - it digitises the input video allowing e.g. for digital 3D combfiltering on CVBS sources. So no way this would work on the different frequencies of PAL signalling, let alone the alternating line color coding.

But having a few minutes time I tried it and was surprised to get a good looking PAL signal through. It did - as expected - not work on the comb filter, but switching s-video sources to an s-video output worked quite nicely.

So I am thinking to recycle the switch and use it for PAL gear. But there's a nagging doubt in my mind - PAL uses higher frequencies, and I am a bit worried that what I see isn't as good when looking at it on an electrical level. When sync signals aren't very clear on the output, will I be able to get stable copies of tapes (yep, I still use SVHS tapes ) later on? I'm a bit worried there's a few hidden imperfections or flaws that could give me nasty surprises in future. Could of course just be a "too good to be true" doubt.

Lacking things like a vector scope or wave monitor, how would I best proceed to "validate" the JVC for PAL use? Is there a way?

Thx!
J-kun

[jeff b]

Switching apparently doesn't go through any processing on that unit, since what you're seeing would otherwise not happen. Point to point passing of audio and/or video is a common
"default" for many types of units that process or play video when the unit is not actually being used for its intended purpose.

From what you've described, the only "digital" thing happening in the box is when the switch for the comb filter is turned on.

Jeff B

[KuroNeko]

I don't think that's right - the signal could still be digitised, if the chipset that handles that part has bandwidth enough for PAL. You cannot conclude it's not happening just because you see PAL get patched through. The combfilter is dependant on the NTSC color signal to work, so naturally it will not work properly for PAL.

And even if the signal was just passed through, it would still be done in an indirect way (through some chips) since it's an electronic (not mechanical) switch, so this does not mean that the signal path is suitable enough for PAL frequencies.

The only way to make sure is test it. Don't know how though, without scopes.

Neko

[jeff b]

I would have to say that Neko may have a point, since that unit isn't cheap. Most of the circuits I've worked on that digitize an analog video signal would have to be tuned to either NTSC or PAL, but would easily appear to pass one without noticable problems when tuned for the other, until you looked at the waveform. Even then, it could easily look fine.

One thing you might try, J-kun, would be to input as many different PAL signals as you can on the inputs, and see if you notice any problems with crosstalk on any of the outputs. Even with a waveform monitor, you'd want to test it that way anyway.

Loading up all the inputs with active PAL signals should show problems if there's going to be any.

Jeff B

[Jkun]

Thanks, I'll give that a try!

J-kun