Audio vs Video cables

[Andrew Pratt]

I know 75 ohm is the standard for Video cables and digital coax cables but what about audio? I was led to believe that audio cables should be at 110 ohms but I've heard otherwise...whats the deal?

[migliore]

I want to say unbalanced cables (RCA) have an impedance of 50ohm, while the balanced (XLR) connections have an impedance of 110ohm. Digital unbalanced are 75ohm, while digial balanced are 110ohm, I think..

Rob




Reviews and System

[Lexman]

Well, I have heard that a 75 ohm cable is good for audio or video technically speaking. But not vice versa. But I wonder if the difference in the ohm rating can make a difference on the high OR low end range perhaps by using a 75 ohm cable on audio? I really would like to know the correct answer to this.

I do know that an audio cable isn't capable of delivering the required 75 ohms for quality video transmission. So, there could indeed be a like reason on the audio end. I just don't know!

Lex





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[EMT]

You guys might check out the articles by Steve Lampen at the Belden and Wired for Sound web sites. Or check you local library for his book Wire, Cable and Fiber Optics for Video and Audio Engineers ISBN 0070381348.

[Lexman]

Thanks for the tip Ellen! Looks to be good info, I printed the Belden out, as it was just to much to read online, lol.

Lex

[JonMarsh]

Hey guys,

Hope everyone is having a great weekend.

Re cable impedance, in the case of RF and video, cable is selected for a characteristic impedance which is a function of the distributed inductance and capacitance. Furthermore, in RF and video systems, the transmitting generator must source at the characteristic impedance (say, 75 ohms) and the load must be terminated at the same impedance. This kind of transmission system has a voltage loss of 6 dB, compared with conventional audio interconnection, which is low source impedance, high load impedance. Between the 75 ohm source and load there is a 2:1 voltage divider effect. BUT, with a cable with a true 75 ohm impedance in that path, the net group delay and amplitude response is flat over an extremely wide frequnecy range- as wide a range as the cable manintains it's true characterisitic impedance, minus the effects of secondary parasitics, including mis-termination in connectors.
I have actually seen an audio system sold by a Japanese company in the seventies which used this approach for audio, and I've done that myself, but you have to have some pretty hellacious circuits to drive 150 ohm loads with low THD.

Audio cables *typically* are in the 110 ohm or so characteristic impedance, but there is no spec, and they don't run with source and load impedances matched, so the wideband benefit isn't there. It *could* be, if you wanted to take the trouble. And yes, I though the gear I built with this interface sounded very good, but it was probably because of the ludicrously low distortion it had due to circuit design and high class A current in the preamp buffers (100 mA). Each complete driver module sank 175 mA- that's the output stage bias current of a lot of class A/B power amps!

With audio cables, quality of copper, quality of plating, ensuring gas tight metal to metal connections, using solid machined connectors, and well damped cable free of microphonics seems most important to me. Best "cheapie" cables I've put together have been with DH Labs Silver Sonic cable and DH Labs premium RCA's. I also like CARDAS cables, but we're not talking cheapie in that case.

Regards,

Jon




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