The combination of passion towards DIY and inability to find a comparable quality solution, leads me to this question. I want to add additional component inputs to my front projector. Active switches introduce noise and are unable to handle the bandwidth. Off-the-shelf passive ones lack quality. The slightest artifacts are quite apparent on a 124" screen.
My solution is to take a three-pole rotary switch. Tie RGB ground together at one point and use the switch to switch the signal leads individually. The passive composite switch I have seems to have some grounding through resistors. I'm aiming to achieve the same result as if physically swapping cables. I also plan to incorporate an analog audio switch in the same enclosure. My concerns are:
Generally all analogue video is 75 ohm impedance, you should use good quality 75R coax to and from the switch, you could use coaxial or RF relays for switching, they go up to GHz , which would be better than any crap rotary switch from RS eg
Professional studios use active switches all the time, neither you nor i am seeing (or hearing) artifacts from their systems. DIY is fine, just don't expect quality results, from junk parts.
If they are quite apparent on 124 inches they will also be clrearly visible on 60 inches.
Are you trying to ruin your projector?
The video grounds maybe, tying the audio grounds to the video grounds is pure foolhardiness.
Do you like your equipment to survive?
terminations, buffers and associated ancilliary equipment will help prolong the life of the other equipment.
same answer
As previously stated, good parts / equipment does not have the problems you are bothered by. Expect to research, learn, pay, accordingly.
Check out (google) video matrix switches, both relay and electronic.
I am adding to and reinforcing Martin's information.
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JosephKK
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Cheap passive crap comes from China. Absolutely no attempt is made to control the mandatory 75 ohm impedance from one end of the product, to the other. They use cheap connectors, poor layout, both mechanical and electrical. Professional video uses a 75 Ohm BNC connector. There is no stated impedance for the RCA (Phono) connectors, because it was designed as a cheap and easy way to connect a crystal or other type of phono cartridge to a table radio where the impedance was 50K ohms or higher. Variations in the impedance had very little effect in that application. Any change in impedance shows up as lines on the screen where the brightness or color changes.
Wrong. The coax shield should reach within 1/4" of the terminal to maintain impedance. "Tightly bundling" affects the impedance and causes impedance bumps. You've NEVER designed any video switching gear. Even the engineers who designed the early mechanically operated video switches in the '50s knew that. When the Japanese copied them, theirs didn't work as well because they were careless in controlling the impedance in their designs. Six inches of hookup wire to a connector, and buss wire between switches just doesn't cut it.
Video grounds need to be bonded in one set, audio in another.
Wrong. Any video switching is break before make. With composite sync it takes time to settle to the new sync, unless the sources are ALL genlocked. The more crap that hits the sync separator, the longer it takes to lock to the new source.
NO. that will introduce losses. Only use and EXTERNAL Terminator for unused inputs, if you build more than you need at the moment.
Very vague and useless reply. Any switching noise would be from the contacts. If there are "thumps" when switching, proper split supply buffer amps are needed to keep a constant load on the source while removing any DC offset.
Proper construction reduces the need for additional shielding, and if it is required, it is usually in the form of a metal plate separating wafers. A cheap three pole three way rotary switch may all be on one wafer, and there is nothing that can be done to provide any more isolation. Good rotary switches are made ot of Ceramic. Centralab made excellent, commercial grade rotary switches. They were bought out by Electroswitch, and their switches are still available. Once you price a good quality switch, you may change your mind about using a rotary switch.
Maxim makes the ICs used for broadcast video switching, with performance you won't believe. With proper PC board layout you won't even know they are there:
As far as small 75 ohm coax, a cable from a dead SVGA monitor should supply enough 75 Ohm coax, and is rated to 350 MHz use, or higher. The video coax has Red, Blue, and Green jackets.
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