Analog Switch Application - Composite Video Signals - Page 1
Forum section: electronics / beginners

author: jdgabbard
author_rank: Contributor
timestamp: October 23, 2017, 03:06:42 am
content: First off, since this is my first post to the forum, I feel I should probably give a little information about myself...My name is Doug, and electronics is my hobby. I have been playing with digital electronics for several years, and wouldn't consider myself quite a newbie. But since I know absolutely nothing about analog signaling I chose to post here. My profession is in the legal field. Which lends itself well to digital electronics (both deal with logic). I have my own electronics / retro-tech website, and upload my own videos to YouTube, where I have showed several of my electronics and software projects.Now, for the meat of the question. I'm working on a project where I need to be able to select between two separate composite video signals, and do so without swapping connectors. I have looked at ICs such as the 74HC4051 and 74HC4052 (more ideal, as I do not need 8 signals), and I do not believe that it will work, since there is a small amount of negative voltage in a composite signal. I suppose it would really depend on the tolerance of the television, since several other composite signals I've played with would work fine on a modern LCD but not at all on an older CRT.However, I then got to thinking about using a Flip-Flop and two MOSFETs as gates. I've posted a schematic below detailing my idea. The theory of operation is that at reset the flip-flop Q is cleared to Logic 0, and the !Q is set to 1. This turns on Q1, and allows the Composite Video #2 to display. When the CS line is toggled, this changes the state of Q to 1, and !Q to 0, effectively turning off Q1 and turning on Q2. Effectively a simple switch allowing you to swap gates through software.The reason I chose a MOSFET is because of the low resistance in the open state. However, I still do not believe that this will work. The reason why I believe it will not work is because my understanding of a MOSFET is that it still acts as a Diode, and as such would not be able to generate the negative voltage level. Again, I know very little about analog signaling.Does anyone have an example circuit they might be able to share, or some reading material to point me towards? My Google Kung Fu has left me without. I do not mind doing the research. But I have to know the direct to get to it.

author: edavid
author_rank: Super Contributor
timestamp: October 23, 2017, 03:27:00 am
content: You can connect a negative supply to the VEEpin of a 74HC4051/74HC4052/74HC4053, and then it will pass both positive and negative voltages. See the datasheet.Or, if you don't have to switch very fast, use a relay.

author: Richard Crowley
author_rank: Super Contributor
timestamp: October 23, 2017, 05:02:21 am
content: The CMOS 4066 is more suitable for switching analog audio and video signals. You can probably find many reference designs and application notes for video switching using 4066.Note that there are switches specifically designed for controlling video signals. Certainly not as many here in 2017 than in years past since analog video is nearly extinct. Maxim/Dallas or Analog Devices are prime suspects in making video switch chips.And there are yet other options, as well. For example, here is a design note on using Texas Instruments OPA3684 (or THS4226) high-speed op-amps with a built-in disable function to accomplish quite high-quality switching.http://www.ti.com/lit/an/slyt098/slyt098.pdf

author: T3sl4co1l
author_rank: Super Contributor
timestamp: October 23, 2017, 06:23:59 am
content: Composite can be AC coupled, so that the signal passing through the switches can be biased up, away from ground.Is composite usually 75 ohms terminated? I forget. If it is, you'll want something that performs better than a CD4066. 74HC4xxx equivalents should be pretty good, or at least marginal. There are even better analog switches out there, for video, radio, USB and so on, just search a distributor for them.Tim

author: vk6zgo
author_rank: Super Contributor
timestamp: October 23, 2017, 06:29:53 am
content: I haven't used analog switches for video, but I have used them for audio with no major traumas.Real video switchers either use special ICs or a combination of IC & discrete components.One important point:-If you just "crash switch", & your sources are totally independent, you will not get a seamless changeover.The Monitor will momentarily lose syncs, usually most obviously the vertical ones.Broadcast standard switchers will switch during the vertical blanking interval to minimise this effect, even with separately synched sources, but even so, if both sets of syncs are not "genlocked" to the same source, there will still be some disturbance.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: October 23, 2017, 06:40:44 am
content: Yeah, that causes the classic appearance of, say, automated security monitors flipping between sources, and the image rolls and zaps for a few frames when switched.Without a sync source for the cameras (or other sources) themselves, the only true solution is a resynchronizer, which buffers up to a whole frame, delaying one source or the other, or skipping frames, as needed to ensure seamless switching (as well as other operations, like compositing, chroma keying, overlays, and with more buffering, picture-in-picture and rescaling and such). It took until the 70s before digital circuits were fast enough, and cost effective (still, very expensive studio grade equipment!), for this kind of hardware to become available. Needless to say, it's a very, very different solution!(Of course, digital being what it is today -- it's practically trivial to drop in a few video digitizers and do the compositing on, say, a Raspberry Pi!)Tim

author: Awesome14
author_rank: Regular Contributor
timestamp: October 23, 2017, 06:45:05 am
content: It's probably less expensive, after shipping charges and everything, to just buy a complete analog switch. You can search on "composite video switch".

author: Benta
author_rank: Super Contributor
timestamp: October 23, 2017, 08:15:51 am
content: Been there, done that. the 4xxx and 74HC4xxx switches work for video, but not well.I recommend you use quality switches, like Siliconix/Vishay DG419B.

author: chris_leyson
author_rank: Super Contributor
timestamp: October 23, 2017, 08:18:12 am
content: I've used Analog Devices ADG719 for switch composite video, low ON resistance and wide bandwidth

author: jdgabbard
author_rank: Contributor
timestamp: October 23, 2017, 10:40:06 pm
content: Quote from: Richard Crowley on October 23, 2017, 05:02:21 amThis was a very helpful article. Thank you very much!I think either the 4066, or the hc4052 is likely going to be the preferred method. I'll do some more reading up on the two options and see where the project takes me. I'd like to use a single IC solution, as it would cut back on board space for my project. But at the end of the day I suppose I'll have to prototype out a few options to see which works best for my needs.