Trying to build an analogue video switch - need help with specifics

Thread Starter

TheDude1982

Joined Oct 1, 2018
3
Hi all.

I am a long time electronics enthusiast, i learned the basics next to my father and not in a school so apologies for not having a graps of a few things that should be taken for granted. I mod consoles and other electronics from time to time and i have a fairly good understanding of components.

I want to make a video (scart) switch for my gameroom, as i want to connect a lot of consoles together in a single output but all current offerings of a scart switch in the market are either overbloated and overpriced or too cheap and bad performing. This is just a hobby project i would like to do for fun mainly.

I came across a solution somebody else made using 8 x CBT324 8bit bus switches 'stacked' together and choosing which one to connect using a manual switch and i have a few questions, some of them laughably 101 electronics and digitals and although we did have some training in uni, i cannot for the life of me remember these bits:

1. the CBT324 is only an 8bit single bus switch. I would love to use something more versatile like 8 x 10bit bus switch or even 2 or 4 x 10bit ones to make the layout/pcb smaller/easier with less components. I know i can use multiplexers or encoders BUT this is addressed in another question. Is there another IC that i could use that you know that could handle 10bit buses better so i can connect more devices in, yet only have 10bits 1 way out?
2. How can i understand if an IC can handle analogue voltages across, rather than just digital in/out? For example in a datasheet about the philips CBT324, it explains that the Vin/Vout max is 7v, so it seems it is fine for using it with analogue sources such as video/audio (as long as the voltage needs are covered) but i do seem to remember that encoders/decoders/multiplexers are purely digital (as an example of course) or other bus switches that could be purely digital on/off only, rather than 'connecting' one side to the other whatever comes in/comes out. Hope i make myself clear, the sea of available options is overwhelming to say the least. (philips pdf here https://4donline.ihs.com/images/VipMasterIC/IC/PHGL/PHGLS03783/PHGLS03783-1.pdf?hkey=EF798316E3902B6ED9A73243A3159BB0)
3. Which software can i use to interactively simulate what i am going to build, before i build it? I used easyEDA, some spice programs, even tina and others but STILL, for the life of me, i cannot simulate something interactively (for example by putting switches that react to a led let's say) that i can debug 'visually'. Most of these software are amazing to build, but simulating just produces a list of values in a text response, not very helpful to say the least. I know this is kind of hard to answer since we need to take libraries into consideration, but not sure which one hits the spot better.
4. Which component do i need to switch between enabling/disabling each bus switch/IC (whichever it is going to be) using discrete buttons or a remote or even an up/down button? In case i have say 8 discrete buttons to choose which input to send to the output, an obvious 8bit switch should be enough, but i would need the switch to be mutually exclusive (so you cannot turn on bus1 and bus4 at the same time) and in case it is up/down it would need something to store current value or maybe a shift register (but output would then be in binary so probably need a decoder on the other hand...) I found a 4/8bit switch but not sure how i can make it mutually exclusive, if even these exist?

So, please don't stone me for being a noob, i studies some circuit design back in uni and i know if i persevere enough i can find all the answers on my own, but i am stuck at a few of these things and this is why i want a nice design software too, so i can drop components and see how they react :)

any help greatly appreciated :D

I can post a 'suggested' design if it helps?
 

MisterBill2

Joined Jan 23, 2018
5,806
Some data sheets reference some switches as analog, while others neglect to mention that part. From your comment about the number of bits, I am guessing that you would be switching HDMI lines and not the simple NTSC or S-video signals. Or possibly VGA lines. Driving multiple switch control lines on different ICs will work quite well if they all have the same power and ground connections. I suggest consulting the Analog Devices catalog because they have a whole lot of options. It may also work to investigate the switching scheme used in some of the TV monitors that have several HDMI connectors. If you can find one of those sets discarded you may even be able to carve out the switching portion. And if you can find a circuit for one of them that may provide a bunch of useful design clues.

One other option is relays, probably a bundle of small reed switches with one coil to engage them all. The advantage is that they would not be sensitive to static discharges or reasonable over-voltage accidents. But they would be much bigger than IC devices. But that is another option.
 

Thread Starter

TheDude1982

Joined Oct 1, 2018
3
Some data sheets reference some switches as analog, while others neglect to mention that part. From your comment about the number of bits, I am guessing that you would be switching HDMI lines and not the simple NTSC or S-video signals. Or possibly VGA lines. Driving multiple switch control lines on different ICs will work quite well if they all have the same power and ground connections. I suggest consulting the Analog Devices catalog because they have a whole lot of options. It may also work to investigate the switching scheme used in some of the TV monitors that have several HDMI connectors. If you can find one of those sets discarded you may even be able to carve out the switching portion. And if you can find a circuit for one of them that may provide a bunch of useful design clues.

One other option is relays, probably a bundle of small reed switches with one coil to engage them all. The advantage is that they would not be sensitive to static discharges or reasonable over-voltage accidents. But they would be much bigger than IC devices. But that is another option.
I want to create an RGB switch so fully analogue, similar to svideo/composite/component, not digital (as in HDMI). Which is why this gets a little more obscure, as like you said, some ICs mention analog stuff and some are left to the imagination.

Thanks for the input, will take a look at it.
 

Thread Starter

TheDude1982

Joined Oct 1, 2018
3
Just to be clear about scart, are you talking about this: https://en.wikipedia.org/wiki/SCART

Also, what is the video format? What country are you in?

ak
Hi, this is the connector but essentially i want to switch between (say) 8 of them carrying an rgb signal which takes anything between 8-10 pins of the connector, meaning i need a 10bit bus or an 8bit bus at least. Video format can be PAL or NTSC (does not matter as i do not want to change the signal in any way, just pass it through). 2 of the 8-10 pins can go as high as 9v-12v so the IC i mentioned in my initial post does not necessarily do the job, but the 9v-12v are mainly for auto switching the TV to that particular source and for dictating the aspect ratio so i might be able to get away with that IC and supplying those independently or via user selection (aka 'force ratio' selection).

This is obviously mainly for older CRT tvs or newer ones that still accept the connector, or i can use it with upscalers that take this and convert it to digital/HDMI etc but that has nothing to do with the project.

Thanks
 

ebp

Joined Feb 8, 2018
2,332
I'd suggest doing a web search for video crosspoint switch. Analog Device, Maxim, TI and others have offerings. You should be able to find some ap notes at those sites that will help you understand requirements.

One of the things that is important with video switching is "off isolation" - how well a signal that not being selected is kept out of the output signal. This is a challenge because of the high frequency components and circuit capacitances. Sometimes the switches are quite elaborate, using a T configuration with 3 switches - one for the input signal, one for the output and a third that shunts the connection between the other two to ground when they are off.
 

MisterBill2

Joined Jan 23, 2018
5,806
First, I see the words "this is the connector" but nothing to see. That makes it difficult. Next comes the concern about frequency response which if it is not adequate will result in poor video resolution.
Check out the CD4051 analog-digital 8 channel analog switch, and also the CD4066, quad analog switch, for starters. These are available from a number of manufacturers, making sourcing easier. But they are by no means the highest performance devices available. If you are switching RGB+Synch video then 4 channels would do unless you want stereo audio as well. So for six lines in and 8 channels to choose from you would need six of these plus a switch to give 3 bits of binary code. The claim in my manual is 80dB isolation, but there is a bit of signal loss, so you would probably need an amplifier IC, or perhaps 4 of them. Your total price would still be less than $10 (USD) but you would need to wire the circuit and I recommend sockets for the IC devices.
 
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