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I'm trying to develop an analog video/audio (composite) switcher.

Can I use a 74HC4066 for that, or is it not the same as a mechanical switch?

I thought that analog switching ICs were just like a mechanical switch, but the more I searched, the more I got confused. The possible solutions are overwhelming.

ocrdu
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Marc
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4 Answers4

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Yes, 74HC4066 has been used for tens of years for composite video switching so you should be able to use it.

No, it is not the same as mechanical switch.

Justme
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Yes, but: Analog video signals (Composite) use 75 ohm coax cable for transport, so you must match this impedance with a 75 ohm resistor to GND at the input side.
There you can connect the analog switches with their ON resistance between 60 and 300 ohm, depending on the chosen chip.

Finally you must again provide a line driver with an output impedance of 75 ohm. This is typically implemented using a video capable high speed OpAmp with a gain of 2 and a series resistor of 75 ohm feeding the output cable.

The gain of 2 is needed because the series resistor and the cable impedance form a voltage divider and so you have the original input voltage again.

The video path is DC coupled, this is important to provide the correct black level and allow correct sync separation later.

Here is a pre 2000 example of a cheap 16 channel AV switch board, the input resistors are not shown here, they are on another board. The audio inputs have 620 ohm resistors to GND and no amplifier.

enter image description here

Jens
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  • The DC path may be important but not mandatory. It can be AC coupled as well to remove any unwanted DC offset, and if so, it's just as important to do the AC coupling properly, preferably with a sync tip clamp. – Justme Aug 17 '22 at 23:35
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    @Justme Around 1996 I made some big 128 x 24 analog AV crosspoint switch monsters and was happy having implement this DC coupled. This avoided discussions about who is responsible for some bad picture quality. It was the video source, easy to verify :) – Jens Aug 18 '22 at 00:06
  • How do you generate the -5V for the op amp from just +5V? – Marc Aug 18 '22 at 07:58
  • @Marc This board was one of many, plugged in a 19" rack. The backplane provided dual supply +/- 5 VDC using two AC/DC power bricks. P.S:. There is a bug in the drawing, R04 is connected to C02, not to the output of the NAND. – Jens Aug 18 '22 at 12:48
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The CD4066 works like a mechanical switch, with several key limitations vs. a mechanical switch:

  • the switched signal cannot exceed the CD4066 rails
  • AC coupling isn’t recommended unless a DC-restoring bias is used
  • The switch has series resistance (about 50 ohms) which will significantly attenuate a video signal

The first two limitations come up mainly with audio signals, some of which can have swings as high as 5.6V peak-peak (2V RMS) and can be centered on ground.

Series resistance mainly affects video. If the attenuation matters to you, consider a more modern part or add a buffer to restore its level.

hacktastical
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A 4066 is nothing like a mechanical switch. Once you understand its characteristics, though, it is possible to use it in many cases a mechanical switch would be used.

Video switching requires genlocked sources, so that the horizontal and vertical timing matches in both sources. Without genlock, switching - be it electronic or mechanical - will cause sync slips.

Even with genlock, a mechanical switch produces visual noise, since it switches at an arbitrary point in the field, and it will also cause tearing since for some time, the switch's NO and NC contacts will be both open. A mechanical switch is unsuitable for the purpose. So, you don't want to be using a mechanical switch.

Video source switching has to be done on the frame boundary. Since composite video is interlaced, field determination will depend on which video standard is being switched. The frame always begins on field 1 and contains two fields:

  • for NTSC, the odd field is field 1, beginning with a full-width horizontal line,

  • for PAL/SECAM, the even field is field 1, beginning with a half-width horizontal line.

Kuba hasn't forgotten Monica
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    Back in the day, you simply switched TV between TV channels or auxiliary inputs which were not genlocked at all. Switching between arbitrary and unrelated video sources does not require genlocking. You only need genlocking if you need to keep synchronization, which does not seem to be the case here. – Justme Aug 17 '22 at 21:43
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    In my experience most (if not all) video gear are happy switching sources if you just leave at least one _blank_ (ground/zero) frame in between. Then you can just break mid-frame. Switching mid-frame between two different sources has caused me some problems with buggy input stages. – pipe Aug 18 '22 at 11:24
  • @pipe I presume, perhaps incorrectly, that OP is looking for studio quality switching, which most definitely requires genlock or a frame buffer. Think of what happens when such switched video output is fed to a VCR. If the only output is a monitor - then sure, blank frame will do the job. For anything more than that - it’s genlock or frame buffering, no way around it. – Kuba hasn't forgotten Monica Aug 26 '22 at 12:24