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FurrorNos 13th April 2009 05:21 PM

Cat5e Line Level Standards?
 
Hi All -

I found an ancient thread about Cat5 line level audio, and decided I'd start a new thread instead of resurrecting the old one.

I'd like to use an existing cat5 run through my attic to send line level audio through my house. I have a few questions for anyone who has done this already:

1 - Is there any sort of standard about which cat5 wire color goes with what channel?

2 - My plan was to use solid color wire for signal and the striped line for sense... this will let me run 2 channels (left and right) on one cable. Is this what other people have done? I'm worried about sending audio down a single conductor wire...

3 - Does anyone have a Bode plot of their completed signal, from before the balanced driver to after? If/when I get this project done I will grab a few data points, but it would be nice to know what kind of frequency response a single conductor has across ~20m.

Thanks
Jordan

calee4nyaboy 14th April 2009 12:22 AM

From what I know:

1. No, there is not standard for audio. Only for networking.

2. I would not recommend your plan in conductor implementation. The point of the Cat5 is that its twisted pair, so it'll block out interference. Subsquently, you will want to specifically use them in their color coded pairs. Ex: org & org/white = left pos & left neg. Then blue & blue/white = right pos & right neg. Use the pairs *together*.

3. I would think the frequency response would be just fine over that length. Maybe only the signal wouldn't be as strong.

John

infinia 14th April 2009 12:52 AM

Hi
see link http://en.wikipedia.org/wiki/Category_5_cable
For a Cat 5 twisted pair the cutoff frequency is around 50KHz for 100m length, so should be OK for line level audio up to 20m.

Zo is spec'd at 100 ohms. So for single ended drive use 100 ohm source resitance as well as a 100 ohm terminating resistor.
Using differential circuits will give an additional 6 dB of SNR and heaps more CMRR. So now the 2 source resistors will be 1/2 Zo or 50 ohms each and the receive end s/b 2*Zo or 200 ohms.

scott wurcer 14th April 2009 01:10 AM

Quote:

Originally posted by infinia
Hi
see link http://en.wikipedia.org/wiki/Category_5_cable
For a Cat 5 twisted pair the cutoff frequency is around 50KHz for 100m length, so should be OK for line level audio up to 20m.

Zo is spec'd at 100 ohms. So for single ended drive use 100 ohm source resitance as well as a 100 ohm terminating resistor.
Using differential circuits will give an additional 6 dB of SNR and heaps more CMRR. So now the 2 source resistors will be 1/2 Zo or 50 ohms each and the receive end s/b 2*Zo or 200 ohms.

You misread the meaning of "cutoff" frequency. Read the reference [4] for an explanation.

infinia 14th April 2009 01:30 AM

Quote:

Originally posted by infinia
Hi
see link http://en.wikipedia.org/wiki/Category_5_cable
For a Cat 5 twisted pair the cutoff frequency is around 50KHz for 100m length, so should be OK for line level audio up to 20m.



The cuttoff of 50 KHz is using a high impedance load (open termination) so is worst case. Using the proper matching described above, you can use Cat 5 well into video frequencies.

zigzagflux 14th April 2009 02:45 AM

The use of 100 ohm (or 75 ohm) source and terminating resistors is for transmission lines at very high frequencies; it does not hold true for audio. General rule is zero (or small as possible) source resistance, and large (as possible) terminating load resistance.

This idea continues to rear its ugly head in audio circles, but doesn't belong. If you are talking about transmitting a few MHz SPDIF signal, okay, but emphatically not audio signals.

Mr Evil 14th April 2009 03:02 AM

Quote:

Originally posted by zigzagflux
The use of 100 ohm (or 75 ohm) source and terminating resistors is for transmission lines at very high frequencies; it does not hold true for audio. General rule is zero (or small as possible) source resistance, and large (as possible) terminating load resistance.

This idea continues to rear its ugly head in audio circles, but doesn't belong. If you are talking about transmitting a few MHz SPDIF signal, okay, but emphatically not audio signals.

Might it not be helpful on long runs to add a Zobel network to set the impedance to the correct value at RF, if only to help absorb noise?

infinia 14th April 2009 03:05 AM

Quote:

Originally posted by zigzagflux
The use of 100 ohm (or 75 ohm) source and terminating resistors is for transmission lines at very high frequencies; it does not hold true for audio. General rule is zero (or small as possible) source resistance, and large (as possible) terminating load resistance.


Why? I think source resistors are quite useful, esp. using op-amps to drive reactive loads like twisted pair or even coax. You have to be extra careful using zero ohm sources to drive audio unless it's designed to. Granted you have a 3 dB power loss but at line level it's not a huge concern.


I see nothing wrong with using twisted pair for line level audio. In my mind its the right way. If done properly it can be cheaper than, and with performance rivaling coax.

FurrorNos 14th April 2009 03:20 AM

Quote:

Originally posted by calee4nyaboy
From what I know:

2. I would not recommend your plan in conductor implementation. The point of the Cat5 is that its twisted pair, so it'll block out interference. Subsquently, you will want to specifically use them in their color coded pairs. Ex: org & org/white = left pos & left neg. Then blue & blue/white = right pos & right neg. Use the pairs *together*.

John


Good call - so I'd do left-force +/- on the green-solid/green-striped wires, left-sense +/- on the orange-solid/orange-striped wires, etc.

calee4nyaboy 14th April 2009 03:20 AM

Quote:

Originally posted by zigzagflux
The use of 100 ohm (or 75 ohm) source and terminating resistors is for transmission lines at very high frequencies; it does not hold true for audio. General rule is zero (or small as possible) source resistance, and large (as possible) terminating load resistance.

This idea continues to rear its ugly head in audio circles, but doesn't belong. If you are talking about transmitting a few MHz SPDIF signal, okay, but emphatically not audio signals.


Yeah, I was kinda confused by all that talk too. Arent you supposed to just solder RCA ends to each twisted pair and call it a day?


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