Remote AV A/B/C Switch

I teach Television Production at a local High School. The school has built a new media center and moved the video distribution to that location, about 300 yards from my studio.
Here is the problem: We have 3 seperate incoming signals and a manual 3 position AV switch to send for a single school wide distribution. The 3 choices are, regular cable, Video feed produced from the Media Center and video feed produced from my studio. We currently send a student to the media center to change the switch just before we broadcast morning news, but have run into problems when there is some special activity in the media center (such as a meeting of the county school board or VIP's viewing a special presentation in the center, or even one time the media center personel were out and the rooms were locked)
I would like to send some sort of remote signal down my studio cable (75 ohm coax) to an AV 3 position switch so that I can control the input to the main feed from my studio.
I don't mind buying one (but don't quite now what to order, I haven't been able to phrase my searches that produce any meaningful results), but I also teach the Computer Systems Technology class and those students would like to build one if it seems like that could be done.
The students and I are enjoying learning from this forum, Keep up the great work and thanks for taking the time to help others understand the world of electronics.

Kenny

I used to design video routing switchers for a living. If no one else knows of a canned solution, I can probably design something for you.

My first thought would be to use a DC signal on the line, similar to the DC supply voltage many cable TV companies use to power line-amps. You could have say 0 volts, 5 volts and 12 volts, representing three states or signal selections. You would want to keep the supply voltage pretty clean, and have good separation/isolation at each end to prevent ground loop hum.

Thanks Ron H, I may have to take you up on the very generous offer.

That sounds so easy zevon8, that I am sure I must not be getting the full effect of your suggestion. If I understand you correctly, all I would need to do would be to input a power supply into the line (say from a surplus computer power supply) from my studio, detect it from the other end (maybe 2 relays, one for 5v and one for 12v with 0 as default) and just make some way to 'terminate' or load the remaining current so it doesn't flow through to the components? How would I "clean it up" and what would I use for isolation?

Have I mentioned that this board is great. Two of my CST students have completed simple projects that they found reference to from here and are looking for more to do.

Thanks for the help so far.

TVTeach said:

Thanks Ron H, I may have to take you up on the very generous offer.

That sounds so easy zevon8, that I am sure I must not be getting the full effect of your suggestion. If I understand you correctly, all I would need to do would be to input a power supply into the line (say from a surplus computer power supply) from my studio, detect it from the other end (maybe 2 relays, one for 5v and one for 12v with 0 as default) and just make some way to 'terminate' or load the remaining current so it doesn't flow through to the components? How would I "clean it up" and what would I use for isolation?

Have I mentioned that this board is great. Two of my CST students have completed simple projects that they found reference to from here and are looking for more to do.

Thanks for the help so far.

Click to expand...

You'll obviously need a box on each end. What DC supply voltages do you have, or prefer?
BTW, What kind of coax do you have between sites? 300 yards is a long run for video. Is it all NTSC?

We just went and traced it down. We have a thick cable about twcie the diameter of RG-56u, that runs to the old media center from my studio (50 yards) then RG-56u from the old media center to the new one (about 240 yards) but I found a booster/splitter box that says it has a 30 db gain that looks like it may be between my raw feed and the final send to the new media center (a real rat's nest of cables with not much in the way of labels). We just use a VCR for our transmitter, NTSC and I have a 25 db booster that we added this year at the back of the VCR (because it became available when some of the rest of the school was rewired, not because it was necessary, but it did improve the signal some).
We have several old computer power supplies, so we have 5v and 12v, but we can build/get anything that we need.

TVTeach said:

We just went and traced it down. We have a thick cable about twcie the diameter of RG-56u, that runs to the old media center from my studio (50 yards) then RG-56u from the old media center to the new one (about 240 yards) but I found a booster/splitter box that says it has a 30 db gain that looks like it may be between my raw feed and the final send to the new media center (a real rat's nest of cables with not much in the way of labels). We just use a VCR for our transmitter, NTSC and I have a 25 db booster that we added this year at the back of the VCR (because it became available when some of the rest of the school was rewired, not because it was necessary, but it did improve the signal some).
We have several old computer power supplies, so we have 5v and 12v, but we can build/get anything that we need.

Click to expand...

With that amount of boost, it sounds like you are sending modulated RF over the cable. Is this the case? Are you using the channel 3/channel 4 output from the VCR? That shouldn't be a problem, but we need to know.

Yes. I am sorry that I forgot to make that clear. We broadcast on channel 3.

OK, here it is. I haven't tested it, or even simulated it. If you build it, please let us know if it works or not.

WOW Ron H! It looks great. I will get the kids started on it today while I order up the parts we are missing. Thanks for all of the help. I'll let you know how it turns out (assuming I am not back here getting more help).

Yeah, that does look good Ron. That chip is pretty cheap too, about $4 at Digikey. The only thing I was wondering about is the 1 volt level shift over a 700 foot plus run. If there is no interference, no problem, but if there is, 1 volt may be shaving it a bit thin?

I had a look at some other ideas for an of-the-shelf approach, but they are all pretty much over-kill professional gear. There was a fairly simple unit that was made by Blonder-Tongue ( IIRC ) that did this, it was a bypass switch for inline amps. Basically it was a RF relay in a metal box, when there was 32 VDC on the line one relay pulled in, when there was 60VDC the other realy pulled in circuit. Couldn't find it on their website though, probably been incorparated into the amps now.

zevon8 said:

Yeah, that does look good Ron. That chip is pretty cheap too, about $4 at Digikey. The only thing I was wondering about is the 1 volt level shift over a 700 foot plus run. If there is no interference, no problem, but if there is, 1 volt may be shaving it a bit thin?

I had a look at some other ideas for an of-the-shelf approach, but they are all pretty much over-kill professional gear. There was a fairly simple unit that was made by Blonder-Tongue ( IIRC ) that did this, it was a bypass switch for inline amps. Basically it was a RF relay in a metal box, when there was 32 VDC on the line one relay pulled in, when there was 60VDC the other realy pulled in circuit. Couldn't find it on their website though, probably been incorparated into the amps now.

Click to expand...

Yeah, I was a little concerned about the length of the run, but there should be no DC attenuation, and I filtered both ends. Hopefully that will be enough to handle any noise. If there is a grounding issue, or some source of AC hum (including poor grounding), there could be a problem. If this is the case, we might have to go to multi-pole lowpass filtering, or some sort of tone encoding with bandpass filtering, which would be much more complex.
If anybody can think of another cheap scheme, go for it!
If there is a booster amp in the middle of the coax run, this scheme ain't gonna work without some modifications!

Ron H said:

Yeah, I was a little concerned about the length of the run, but there should be no DC attenuation, and I filtered both ends. Hopefully that will be enough to handle any noise. If there is a grounding issue, or some source of AC hum (including poor grounding), there could be a problem. If this is the case, we might have to go to multi-pole lowpass filtering, or some sort of tone encoding with bandpass filtering, which would be much more complex.
If anybody can think of another cheap scheme, go for it!
If there is a booster amp in the middle of the coax run, this scheme ain't gonna work without some modifications!

Click to expand...

I'm presuming this is an RF cable run, and NOT a video run? - so it's simple to bypass the amplifier in the middle - it's been done for years with inline amps. You simply use an RF choke to bypass the DC around the amp, and blocking capacitors on the input and output of the amp. So the RF goes through the amp, and the DC goes round it.

We even used to modify inline amps in the field, on roofs, up trees etc. using gas powered soldering irons

Nigel Goodwin said:

I'm presuming this is an RF cable run, and NOT a video run? - so it's simple to bypass the amplifier in the middle - it's been done for years with inline amps. You simply use an RF choke to bypass the DC around the amp, and blocking capacitors on the input and output of the amp. So the RF goes through the amp, and the DC goes round it.

We even used to modify inline amps in the field, on roofs, up trees etc. using gas powered soldering irons

Click to expand...

Good point, Nigel.
It is an RF run. And I was thinking about the DC bypass as I was writing it. In fact, you could just use a big resistor and blocking caps, as I did on both ends. As I said,

If there is a booster amp in the middle of the coax run, this scheme ain't gonna work without some modifications!

Click to expand...