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Modulation Question - RF Cafe Forums

The original RF Cafe Forums were shut down in late 2012 due to maintenance issues. Original posts:

Amateur Radio | Antennas | Circuits & Components | Systems | Test & Measurement


JoshGG
Post subject: Modulation Question
Unread postPosted: Thu Jun 17, 2004 9:44 am
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Hi all- I am looking for someone or some company that can build or design a modulator, etc that can overlap computer graphics onto every TV channel. I have stores that we are running in house specials on our TV's, (like a CNN ticker at the bottom) and overlapping on channel 125- the problem is that customers keep changing the channels and then the ads are gone. If I had a device that I could run coax cable into, and also the VGA or composite out from my computer, and then get a combined output regardless of what TV channel is on.. Can anyone help? Thanks!


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KC2SHO
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Unread postPosted: Thu Jun 17, 2004 10:23 am
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I think somebody alreay makes what your looking for. Isn't this is how a cable TV transmission site works? One cable carries all the channels to your set, the only difference here is that you'll have the same thing on every channel. I get the feeling you may be looking for something cheaper though as I'm sure this type of equipment is not inexpensive. As far as inserting graphics wouldn't a video card with a TV out and a video mixer do the job?

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JoshGG
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Unread postPosted: Thu Jun 17, 2004 12:49 pm
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Right now we are using a video card with TV tuner composite video in, and then the mixed PC graphics-composite video out. This then goes into a single channel modulator and to all the TV's. But I want this to overlap on all TV channels, regardless of what each TV is set to, I dont want to use an external TV tuner like I do now.


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KC2SHO
Post subject:
Unread postPosted: Thu Jun 17, 2004 8:07 pm
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Understood. What I'm trying to say is that the "closed circuit" RF system that cable TV uses is exactly what you are describing. A transmitter that puts out a signal on every channel.

Edit, Maybe there's an easier and cheaper answer to your problem. Delete all the channels except the one you are using on the TVs' selective tunning features.

Edit no 2, Campus Cable Systems: http://www.campuscablesystems.com/system.php

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JoshGG
Post subject:
Unread postPosted: Thu Jun 17, 2004 10:52 pm
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Thanks for the link, cool stuff. I tried erasing all channels but the one I am modulating on, but people still seem to get into the menus and reprogram the TV's. This is why I want to "overlap" my graphics on every channel. I don't exactly know how a modulator works, but I understand that it sends the combined feed out to a particular frequency that the tuner in the TV then picks up as a "channel". So then, in theory, if a person had a modulator that would do this on every cable frequency, would it work?
And yes, cost is an issue. I could understand having high development costs initially, but the developed "box" would have to cheap enough I could use it in all our locations.
Do you have a link for the mixer or system(s) cable companies use?


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KC2SHO
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Unread postPosted: Fri Jun 18, 2004 9:06 am
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Ok, So a modulator on single channel is a very simple device as you know. The problem with transmitting on every channel is that you are talking about a complex signal being reproduced many times on a multiple band/broadband device. Consider (For US standard M) Channel 2 has a visual carrier of 55.25MHz and channel 69 has a Vc of 801.25MHz. This is a very broad range of of frequencies. I'd have to pull out another reference to see where the analog cable channels lie. Also, each channel occupies 6MHz of bandwidth. In order to reproduce the modulated signal for every channel you bassically need a modulator for every channel and then filters and cobiners to aviod problems with inserting all of these signals into the same cable. It is not feasible to me to just make a sigle device that puts the modulated signal out across the entire band. Each channel must be seperated somehow. In your case it's easy to modulate a single IF frequency and then convert that single IF to each desired channel, although I cringe at the thought of a 69 way splitter. :shock:

If you look at the system they show on the campus cable site it looks like a very modular device. Many rack units inside a master rack. I'm sure each of those rack units are a single channel device. Each device consisting of modulator and filter, there must be a combiner somewhere in the system as well. Each of those devices may have a reciever as well which you would not need. I wonder what they would think about desinging a single IF system if they don't already have one.

I hope this helps. Please let me know if I can help any further. If my company designed cable equip I would ask them about designing something like this but we are sttrictly a broadcast transmitter company.

Cheers,

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JoshGG
Post subject:
Unread postPosted: Fri Jun 18, 2004 9:32 am
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You are lots of help and full of information, thats for sure! I am seeing that this is much more difficult than previously thought. I understand the complexity of designing such a piece, I just keep thinking that surely this is possible to do, just a bit more involved and expensive. I am still going to pursue this issue. Now I am more interested in the theory of this working I want it even worse!


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KC2SHO
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Unread postPosted: Fri Jun 18, 2004 11:46 am
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Please let me know what you find. I'd be interested in knowing for the furture as we deal with many custom systems.

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Guest
Post subject:
Unread postPosted: Mon Sep 27, 2004 12:51 pm

I don't know if this is helpful, but there was a paper in the ICASSP Proceedings a couple of years ago which discussed an AM radio equivalent of what you're looking for - they wanted to provide emergency data on all 200 AM broadcast channels simultaneously for cars in a tunnel. The basic idea was to use DDS (direct digital synthesis) to generate all 200 carriers at once (counter to ROM to DAC), then use a single modulator and a broadband amplifier. It's easier to generate AM BC channels because they are evenly spaced, whereas TV channels aren't.

Good luck!




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