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I'm looking for a device to switch video signals; a video switch. I want to use a dual/triple switch for two purposes, first to switch between two video signals, and secondly to switch the signal between open and +3.3V (which drives the pixel on the frame high.) (The third channel may be used to switch between on and off for the pixel.) It's for an on screen display application.

However, most chips I've seen have an on resistance of nearly 60-80 ohms. This is way too much surely for video which is terminated with 75 ohms? If I'm switching with 75 ohms in series with the required 75 ohm termination, I've effectively created a voltage divider and halved the video signal. It might work, but the video would probably lose considerable quality; at worst, I'd probably lose sync.

As an additional question, related to this one, would there be any bandwidth issues with video switches? I'm using composite (CVBS) NTSC or PAL colour video.

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  • \$\begingroup\$ I'm starting a bounty on this question. \$\endgroup\$ – Thomas O Dec 22 '10 at 18:55
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All the video processing I've seen happens at high impedance, then the video is re-amplified for transmission.

For example, in my sketch below, the resistance of the switch doesn't matter because it is feeding the high-impedance video buffer amplifier.

If you search your favorite distributor for "video buffer amplifier" you will find this is a relatively common part. You can find them with fixed gain of two for exactly this application.

alt text

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  • \$\begingroup\$ This looks to be the best solution. I'd probably use this on the Pro (more expensive) version of my OSD, but since space is critical on the Lite (cheapest) version I'll stick with low resistance switches, as W5VO suggests. \$\endgroup\$ – Thomas O Dec 22 '10 at 21:22
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In my breif examination, it appears that there are two basic approaches. The first is to use an analog mux such as the one you have identified, and then buffer the resulting signal with a high input impedance amplifier. Maxim has a pretty good write-up on how to do this using a separate mux and amplifier. They use the switch on-resistance as part of the 75 ohm matching network.

(95 ohms in parallel with (300 ohms + mux resistance))

Maxim example video switching circuit

I found a single chip (NJM2244) that combines the analog switches with a matched 75 ohm driver. This would allow you to have only one very small chip with very simple controls.

NJM2244 Block Diagram

Another method uses much lower resistance switches. This particular writeup uses switches intended for high-speed logic, and they have an on-resistance of about 5-8 ohms. That small of a resistance could probably be tolerated fairly easily.

Bus wiring diagramBus switching chip CBT3244A

Each bus switch chip has two banks of SP4T switches, so you could switch stereo audio in addition to the video signal. If you're switching only two sources, this could be a single-chip solution as well.

Regarding the bandwidth of composite video, the highest figure I've seen is 6.5MHz, which isn't really that high frequency. You won't have difficulty finding amplifiers for these frequencies, and the wiring shouldn't be very sensitive (aside from adjacent noise sources).

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  • \$\begingroup\$ Thst NJM2244 looks like a perfect solution to Thomas's original question. And for a good price too. \$\endgroup\$ – markrages Dec 23 '10 at 5:29
  • \$\begingroup\$ No kidding. I found it interesting that the low resistance switches were found in the digital logic families. \$\endgroup\$ – W5VO Dec 23 '10 at 5:32

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