A/V + Audio Switcher

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krypton_john

Senior Member
Hi,

My home theatre reciever will only switch between two component video sources - but I need to switch between 3 or more.

Generally this means not only switching one of n sets of Y/Cb/Cr signals and associated L/R Audio (or possibly digital audio) but possibly also being smart enough to turn off via IR or RS232 command all the sources except the selected one and turning that one on. My A/V reciever can accept RS232 commands to turn on, and select the source that is the output of the AV switch.

This is all conceptually simple, but I have absolutely no idea about a good way to switch A/V signals. What's a good device to do this? I guess the big issue is to not introduce noise, interference or other signal degredation into the system.

There are off the shelf A/V switches around that do component video, but they are either expensive and/or don't have the smarts to be controled by I/R or RS232 nor control other devices by IR and RS232.

Any pointers or references?

Thanks
JohnO
 
hippy

hippy

Technical Support
Staff member
That looks to be the sort of thing you're after, and they are not that expensive either. As you say, an easy project in concept, the hard part being to get the signals as free from interference and crosstalk as possible.

The chip itelf interleaves signals and 0V's so should be easy to wire up. Put the LT's close to the rear panel / connectors, shortest leads, expose as little unscreened cable as possible at the join to board. Put smoothing and decoupling caps on the board and run the control lines to the PICAXE on a separate board. Should be able to build it on veroboard / stripboard.

I don't know what load component signals expect to drive into but would guess it's 75R like most other video signals. When putting the R's vertically as an inverted "U" ( best option for pin-to-next-pin link ) I'd go for the exposed wire being on the 0V side, body of R up against the board on the signal side.

The only inconvenience seems to be that the chip needs a negative supply.
 
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inglewoodpete

Senior Member
John, Careful design of signal paths is required for video. Particularly when you have several parallel signal paths (RGB/YPbPr). If not physically designed properly, phase shift and propogation times can have ugly effects when viewed on a TV screen.

It depends on your level of knowledge of RF design (modern video can have bandwiths up to 15 MHz). I would look closely at commercially available switch units unless I had a lot of development time available.

-Peter
 
K

krypton_john

Senior Member
Thanks Hippy,

There's also the MAX454 chip as referenced here:
http://www.glolab.com/freeinfo/images/videosw.pdf

It needs +/- supply as well. Simple enough but it I hate it when the component count goes up - my projects always end up looking messy.

But anyway, the good news is component video cables are also 75ohm impedance so these a/v switch ics should still be suitable - just three times as many needed.

Now to the audio switching - can audio be switched in the same way? I'm not sure if impedance matching applies?
 
K

krypton_john

Senior Member
inglewoodpete said:
John, Careful design of signal paths is required for video. Particularly when you have several parallel signal paths (RGB/YPbPr). If not physically designed properly, phase shift and propogation times can have ugly effects when viewed on a TV screen.

It depends on your level of knowledge of RF design (modern video can have bandwiths up to 15 MHz). I would look closely at commercially available switch units unless I had a lot of development time available.

-Peter
Click to expand...
Yuck! I was kind of hoping that if I made a PCB with earth planes betwen all the signal tracks and had short, shielded connecting wires that I wouldn't have to worry about all that stuff.
 
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inglewoodpete

Senior Member
I don't want to stifle your project but there are pitfalls when dealing with video. You will notice that most of the designs you find for video switchers are for AV, surveilance and occasionally S-video: all on the lower end of the quality scale.

If you take care with the signal paths etc, it will work. However, the quality of the picture when compared with the original may be disappointing.

As hippy mentions, good quality audio can be switched using this method much more easily.
 
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andrew_qld

Senior Member
I have bought and used various analogue switching IC's, but video needs high bandwidth chips (200MHz) to be done properly. You can get them, but they are expensive (I recently bought a 16 x 4 matrix switching IC just for audio at work and they were something like $20 each).

To be perfectly honest, cheap reed relays may be the simplest option. Farnel here in Aus had some 5volt SIL (single in line) relays for $1.72 each!

Just try to keep the Y/Cb/Cr signals away from eachother, play with it on a piece of circuit board first and use coax cable up to the relays.

Cheers,
Andrew
 
B

BeanieBots

Moderator
The bit that has been hinted at but not spelled out is to keep the three circuits identical. The slightest variation in one and not the others will be noticed.
I'd actually avoid getting a PCB with ground plane made unless it is designed by somebody with good RF experience. If you want a PCB, then go single sided with no ground plane. Fit a steel screening plate on stand-offs about 1/4" from the PCB. This is actually one of the few times when stripboard is likely to be a better option. Straight lines and equal spacing.
As mentioned by Hippy, keeping all cable lenghts the same is just as important as keeping all track and layouts the same.
 
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papaof2

Senior Member
The latest All Electronics catalog has a shielded coaxial reed relay, rated as 50 ohm impedance (typical for RF; video is usually 75 ohms). Their part # is RLY-475. For $1US each, it might be an option.

Details here:
http://www.allelectronics.com/cgi-bin/item/RLY-475/search/5_VDC_REED_RELAY_.html

Improvements in technology are wonderful - I rember seeing references to capacitor size and the comment that a 1farad capacitor would be the size of a room. All Elect has a 0.5inch by 1.25inch 4.7farad 2.5volt capacitor for $3.85US - how long would that (maybe 2 in series) provide backup for an 08M?
http://www.allelectronics.com/cgi-bin/item/CBC-18/search/4.7_FARAD,_2.5_VOLT_MEMORY_DYNACAP_.html

John
 
M

moxhamj

New Member
I built an 8 to 1 video switcher for a video capture card using relays and a picaxe. A DPDT relay will do audio and video. If you are switching component video just use more relays. There is another thread running at the moment with a link to 40c latching relays - it would be hard to beat that for price/crosstalk etc.
 
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leftyretro

New Member
A great component for building your A/V switcher would be small latching relays. Normally they are too pricey to use in the quanity you might require. HOWEVER, I just received 10 units from an E-bay seller that has a couple of thousand left and in letting them go for 40 cents each + shipping. The DPDT contacts would let you switch the signal and ground without having to common anything together. The relay coil only draws about 18 ma and you only have to pulse it on or off for 10ms, after that there is no coil current needed. Relay remembers it's last setting so PICAXE could be powered off when no changes are required.

http://cgi.ebay.com/ws/eBayISAPI.dll?ViewItem&rd=1&item=120096057860&ssPageName=STRK:MEWN:IT&ih=002

Lefty
 
K

krypton_john

Senior Member
retrolefty said:
A great component for building your A/V switcher would be small latching relays. Normally they are too pricey to use in the quanity you might require. HOWEVER, I just received 10 units from an E-bay seller that has a couple of thousand left and in letting them go for 40 cents each + shipping. The DPDT contacts would let you switch the signal and ground without having to common anything together. The relay coil only draws about 18 ma and you only have to pulse it on or off for 10ms, after that there is no coil current needed. Relay remembers it's last setting so PICAXE could be powered off when no changes are required.

http://cgi.ebay.com/ws/eBayISAPI.dll?ViewItem&rd=1&item=120096057860&ssPageName=STRK:MEWN:IT&ih=002

Lefty
Click to expand...
Sensational!

Thanks Lefty.
 
hippy

hippy

Technical Support
Staff member
I cannot knock the use of relays for video or audio switching ( done it myself ), but I do not see much advantage in that over a solid-state switching solution.

The big advantage I see is that latching relays allows the unit to be powered off and it retains its last set routing, the disadvantages are that swiching is far slower making changeover more noticable visually and the drive electronics become more complex. In terms of RF, propogation delays, cross-talk, tracking and wiring and so on, I don't see much difference.

One thing to bear in mind with any project which relies on second-hand or 'cheap because they came through eBay' components is that any failed component needs replacing. There's an issue with new but single sourced components as well, so I'm not saying "don't", just consider the consequences of any implementation decision.
 
K

krypton_john

Senior Member
Point taken, Hippy,

But to mitigate, a) I can just purchase twice as many as I need at that price in case any are duds or fail and b) as is the case in most of my projects, most of the fun is in the designing, building and testing. The final product is just a bonus.
 
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