Passive Component Video Switch Design Q

[fourrings]

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:

1. Is tying grounds together acceptable?
2. Does the switch need to be break-before-make?
3. Do I need to introduce any resistors, etc?
4. Does 1-3 apply to audio switching as well?
5. Thoughts on result?

Any input would be greatly appreciated!

 

[martin griffith]

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.

you are standing too close

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:

1. Is tying grounds together acceptable?
2. Does the switch need to be break-before-make?
3. Do I need to introduce any resistors, etc?
4. Does 1-3 apply to audio switching as well?
5. Thoughts on result?

Any input would be greatly appreciated!

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
http://www.pickeringrelays.com/sil.htm#sil109

Read up on video signal termination

I have my doubts about your statement

Active switches
introduce noise and are unable to handle the bandwidth

martin

 

[fourrings]

Thanks for advice. I may look into a solution with relays as you
suggested. At the moment though, I was looking to build a non-powered
switch.

 

[jasen]

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.

that'll probably work ok especially if the RGB wires to the projector are
tightly bundled,

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:

1. Is tying grounds together acceptable?
probably.

2. Does the switch need to be break-before-make?

no need. video equipment can handle low-level signals on its outputs, and/or
shorts, opens etc...

3. Do I need to introduce any resistors, etc?

probably not.

4. Does 1-3 apply to audio switching as well?

a resistor from wiper to ground may redude switching noise.

5. Thoughts on result?

put shielding between audio and video parts. banding during loud noise and
buzz during bright scenes doesn't look good.


Bye.
Jasen

 

[joseph2k]

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.

you are standing too close

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.

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

Check out (google) video matrix switches, both relay and electronic.

eg
http://www.pickeringrelays.com/sil.htm#sil109

Read up on video signal termination

I have my doubts about your statement


martin

I am adding to and reinforcing Martin's information.

 

[Michael A. Terrell]

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.

that'll probably work ok especially if the RGB wires to the projector are
tightly bundled,

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.

probably.

Video grounds need to be bonded in one set, audio in another.

no need. video equipment can handle low-level signals on its outputs, and/or
shorts, opens etc...

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.

probably not.

NO. that will introduce losses. Only use and EXTERNAL Terminator for
unused inputs, if you build more than you need at the moment.

a resistor from wiper to ground may redude switching noise.

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.

put shielding between audio and video parts. banding during loud noise and
buzz during bright scenes doesn't look good.

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. <http://www.electro-nc.com/>


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: <http:www.maximic.com>


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.

Bye.
Jasen

--
Service to my country? Been there, Done that, and I've got my DD214 to
prove it.
Member of DAV #85.

Michael A. Terrell
Central Florida