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I need to build a SCART switcher (like this one here: Lotharek's Hydra Revised, but with fewer inputs, simpler - no fancy features, just switching between a few inputs, and cheaper). I have some experience with building purely digital circuits, but I'm very inexperienced in the analogue domain, hence I need some advice.

The part that is tricky to me is switching all the 20 lines of each SCART input without a noticeable signal quality loss. The lines that seem critical are the video signals (I need both RGB and composite) and the audio signals.

I've come to a conclusion that I could take 2 possible paths.

Option 1. Use integrated analogue switches like this one: Vishay DG9426EDQ-T1-GE3

Pros: small form factor of the components, solid state (no moving parts), relatively easy to drive.

What worries me here:

  • ON resistance (RON) seems to be important (especially in case of audio signals?), and the lower it is, the more expensive the component; but what values would be acceptable for which signals? Components with values around 200 ohm are cheaper by an order of magnitude compared to those < 10 ohm;
  • what other parameters should I take into consideration (apart from the obvious like max signal voltage)?

Option 2. Use relays (like this one TE 9-1415899-4?)

Pros: simple fool-proof solution (?), negligible ON resistance.

What worries me here:

  • large form factor of the relays (are there any small ones that are cheap?); I also can't seem to find relays with more than 4 poles (more poles per relay could possibly result in less space used);
  • relays seem simple, but are there any specific parameters I should still consider? (apart from allowed voltage & current);
  • generally it doesn't seem cheaper than the solid-state switch solution despite being seemingly simpler - am I missing something here?

General questions:

  • did I miss some other approaches that would make sense?
  • where can I find the values of max currents that may occur on each SCART line?
  • do you have any other generic advice for an analogue noob like me?

Thank you for your help!

Marcus Müller
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KotBehemot
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    For ON resistance and everything else, you definitely should worry about video signals instead of audio. Basically you might want to use a specific IC meant for switching video signals. SCART just carries standard video and audio so you can follow standard video and audio signal parameters. Relays may not be good for video. And the without signal quality loss is virtually impossible. There will always be *some* signal quality loss, you just need to determine that insignificant amount of loss is acceptable. – Justme Apr 16 '23 at 11:45
  • @Justme thank you for commenting. I'm aware that some quality loss is unaviodable, my problem is that I don't know even a rough ballpark of ON resistance values that would be ok. Also, why would relays be bad for video? – KotBehemot Apr 16 '23 at 12:09
  • How many inputs? – Bruce Abbott Apr 16 '23 at 12:43
  • @BruceAbbott I'm aiming for 4 inputs (most probably), but I suppose that if I have a solution for 1 input, I can replicate it as many times as I want? – KotBehemot Apr 16 '23 at 12:54
  • @Justme btw, thanks for directing me towards dedicated video switching ICs - looks like they do exist and aren't very expensive. – KotBehemot Apr 16 '23 at 12:55
  • Have a look at LMH6574, the gain of 2 is very useful to drive 75 ohm outputs with matching impedance. – Jens Apr 18 '23 at 00:15

1 Answers1

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There's signal relays that might have reliably enough bandwidth for the RGB signals and composite, and sufficiently low crosstalk, but their cost is really not justified in this application.

On the other hand, the analog switch IC approach is cheap, scales well without too much thinking.

Regarding resistances, you're on the wrong track. Reading the relevant standard, EN 50049-1:1997¹, the audio sink impedance has to be between 300 and 1000 Ω, so that adding 10 Ω in series really makes no difference. On the other audio direction, we're talking 47 kΩ, so even less of a problem.

Video, on the other hand, is a high-bandwidth signal and hence specified with an RF-typical termination (and also inherently characteristic waveguide-) impedance of 75 Ω. That's a bit harder. But, even for that 10Ω are absolutely tolerable (assuming cabling doesn't get too long).

Luckily, there's special purpose analog video switch ICs; on digikey and other distributors, they're a bit sprinkled throughout different categories (is an audio+video switch IC an audio IC? Is a video switch an "analog switch – special purpose"? Or are all these analog multiplexers?)

Anyways, something like the TI TS5V330C should do well for 2:1 plexing the RGB + Composite lines. At the typically low rON of these, you could also cascade them at least at a depth of 2, allowing you to use three ICs to switch four inputs to one.

For audio, you could actually use relays or use higher-supply-voltage switch ICs, as the maximum levels for audio can exceed 5.5 V (the maximum supply of the video switch IC).

Note that you would need to AC-couple the video and audio, or use a dual supply centered on the shared ground of all attached devices.

For the function switching pin, you'd need to be 12V-tolerant, so if you do that with a switch IC, you'll need one that can do 12V (and a +12V supply, usually).

For the blanking pin, standard TTL logic or an analog switch would do.

I'd probably ignore the bidirectional control pin, but if you don't, analog switch with high supply voltage to be safe.

I'd recommend adding TVS diodes to the video lines at both inputs and output – thanks to USB, low-capacity TVS diodes are very cheap these days, and you will have to worry much less about what happens when you forget to power your device first.

¹ archive.org link

Marcus Müller
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