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S-Video and Audio over Cat5


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8 replies to this topic

#1 of 9 OFFLINE   Mike*Williams

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Posted January 06 2004 - 04:36 AM

Hello all,

I'm just settled into my new house and got my HT all setup. I like =)

When building, I ran a lot of extra Cat5 cable to each room for various purposes. One of which was so that I could send audio and video over the Cat5 using a device like seen here:

http://www.muxlab.co...s-av_balun.html
and
http://shop.store.ya....iobalun17.html

Now that I'm in, I'm thinking that I could build these devices. It seems to me that you'd simply connect the appropriate connectors' (RCA or S-Video) pins to the wires in the Cat5 (or RJ-45 connector, whichever way you chose to). I know that these commercial devices use a single pair for each audio channel (which make sense) and 4 wires for S-Video (which also makes sense).

I guess my question is, wouldn't a straight connection work for this stuff? I mean, you shouldn't have to put in any resistors or anything to change the signal as you're using the Cat5 essentially as a long piece of wire with much lower loss.

Any input is appreciated.

Thanks,
Mike W.

#2 of 9 OFFLINE   Bob McElfresh

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Posted January 06 2004 - 04:11 PM

No you cannot just attach connectors and expect it to work. Twisted pair Cat 5 signals are good for DIGITAL, not analog. The devices shown work in pairs to convert the analog signal to digital for the run down the wire. The separate signals are decoded by the matching box at the other end.

#3 of 9 OFFLINE   Mike*Williams

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Posted January 06 2004 - 04:53 PM

Talk to me more about this. I'm believing what you're saying, but I'm the type that needs to understand (the electronics/physics) behind things like this.

I realize most uses for Cat5 is for networking and digital data, but it's also used for phone lines (0-4Khz analog)..... seems to me the twisted pair would be excellent for transporting analog signals, especially audio at 20-20K Hz (as long as it were line level/low voltage signals and not powered signals that need thicker cable to keep from burning out or something). (actually now that I'm typing this, I remembered that I play in a band where we use these devices that carry up to 4 channels of audio across Cat5 and allow each musician to mix their own output..... would this use a similar D/A A/D system? or be analog only?)

These devices, where do they get the power to run the chips for the A/D and D/A conversion? From the sales info, it appears that you simply need to buy 2 of the same devices (one for each end) so would this D/A and A/D circuitry be connected to the "input/outputs" at the same time? (i.e. from what I've read, it seems this device is oblivious to which way the signal is going (in or out), so if there's D/A and A/D circuits they'd both have to be connected to the intput/outputs).

As you can see I'm full of ideas and questions and not really sure about any of it Posted Image

Reason I'm asking is because I know A/D and D/A chips aren't expensive and wouldn't mind seeing a diagram to make a small breadboard design to accomplish this, not so much for the cost savings but for the sake of doing it....

Mike W.

#4 of 9 OFFLINE   KurtBJC

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Posted January 06 2004 - 05:50 PM

These are baluns; it's not just a matter of connecting one pin to another. Each balun is an impedance transformer, converting the 75 ohm unbalanced video circuit to 100 ohm balanced so it can be run through the CAT5 without a really nasty level of return loss. The problem, however, is that the impedance tolerance of twisted pair is very poor compared to coax and so this will still underperform coax over distance; how badly it underperforms, and whether that's acceptable, will depend on the particular application. You can, indeed, build them yourself; but you'd need to start with raw transformer cores (I think these are often done on small toroidal cores), and you'd need to figure out what you need to do in the way of windings--how many turns on each side, basically.

#5 of 9 OFFLINE   Mike*Williams

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Posted January 07 2004 - 03:01 AM

Good info. Part of the reason I was assuming you could just go pin-to-pin across Cat5 is because a friend of mine showed me this:

http://www.smarthome.com/865166.html

Now this piece may indeed have a small transformer to do said conversion. I might just pick up a couple of these (would be alot easier than computing the winds on each side of a transformer Posted Image )

Having said all of that, and now seeing what you mean about 75 Ohm unbalanced to 100 Ohm balanced for video, I'm still thinking the audio (line-level, not powered) could go pin-to-pin as the Cat5 should act like a pretty good "extension"...... Is there anything you can think of that would prevent this from working?

Thanks!
Mike W.

#6 of 9 OFFLINE   KurtBJC

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Posted January 07 2004 - 03:48 AM

It'll work. However, because it's an unshielded line, you'll have no protection against noise introduction. There is, by the way, a common misconception that twisted pair cable, like CAT5, will reduce noise without shielding; that's true only in a balanced circuit, which this is not. If you use a balun for the audio, the advantage is that the line becomes balanced for the CAT5 run and therefore should provide decent noise rejection. How important that is will depend on how much exposure to noise there is--I would worry, when feeding line level signals over a distance on unshielded cable, that every time the refrigerator compressor goes on, you may get a nice "pop" in your audio, or that fluorescent lights may give you some hum. There's really no way to know whether you'll have problems until you try it. I think the Leviton is just pin-to-pin. How well that works will depend on distance, and how important signal quality really is to you--I would avoid it for video.

#7 of 9 OFFLINE   Mike*Williams

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Posted January 07 2004 - 07:00 AM

Thanks for your input. I'm going to dig around more and see what I could find out about this. I might try the audio just straight thru the cable just to see how it works (I figure the total run length will be about 100ft or so).

I've give an update when I figure something out Posted Image

Mike W.

#8 of 9 OFFLINE   Clifford Manuel

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Posted January 07 2004 - 10:26 AM

Mike, This is the sort of thing that I was looking to do. I found the A/V baluns for sale ($120 for a pair) and I thought that would be the way to go. If I can just do audio through the CAT5, I have a run of coax I can use for the video. I'm looking to send the DVD audio/video signal from the HT in the basement up to the "wiring closet" and then use a channel modulator to make it possible to watch the DVD in the theater on all TV's in the house on a channel not used by the cable company. I can send the video through the coax and I'll just send mono audio through the 85570BW RCA quick port jack. Those jacks are only $5 a piece. That would be much more cost efficient. Let me know how they turn out. I would be very interested.

#9 of 9 OFFLINE   Bob McElfresh

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Posted January 08 2004 - 05:54 AM

Looks like I was wrong. I thought it was a Modulator type of device. (That'll teach me to download the product sheet) Posted Image




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