Riddle me this, Batman

[I'mListening]

What would be the expected results of using five DIGITAL AUDIO COAXIAL RCA cables to mate a Preamp/Processor's five PREOUTS to a 5 channel amplifier, instead of using ANALOG RCA AUDIO cables? Hummm?




Thanks for reading!

Enter The Cyberdyne Cinema

Darren

[sfdoddsy]

Probably not much at all. I use them interchangeably.

Steve




Steve's DIY Dipoles

[Danbry39]

I wonder if it couldn't affect things like volume and high frequency roll off due to things such as differing impedences and different gauges of core wire, but I'm not sure. Just thought I read something to this effect at some point.




Keith

[David Meek]

It shouldn't be an issue as far as physically transferring the data intact, but with cable usage (like Danbry points out) there's always those intangibles that affect what you hear. I'm not sure about ALL digital sources, but with video, you want to make sure you have a true 75 ohm capable cable to insure all the data comes across.

Did I get that right, Lex?




David - HTGuide flunky
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[brucek]

I'm Listening,

Let me give you my take on your question. It's just my opinion, and others may certainly disagree.

Your "digital audio cables" will normally exhibit a 75 ohm characteristic impedance. We don't really know the impedance of your audio cables -but it doesn't matter.

I have to say that the fixation that some audiophiles seem to have with regard to the impedance of cables for audio interconnects is unfounded. It's a very common engineering practice to ignore characteristic impedance and consider only the DC resistance of a cable (transmission line) when the line is short in comparison with the wavelength of the electrical energy that it conducts.

Now, I would certainly consider a one or two meter cable 'short' in comparison to a 10 kilometer wavelength (using an example of a 20Khz audio frequency). In fact, in very short cables, in relation to the wavelength of the signal, the resistance of the line is considered completely insignificant and the energy transferred is considered lossless. This would definitely be the case for an audio interconnect with regard to impedance matching.

Characteristic impedance is a culmination of the 4 standard distributed electrical constants that a transmission line will possess of (capacitance between conductors), (inductance along its length), (resistance of the wire) and (low level leakage conductance between the conductors). Actually the L and C are so dominant in the infinite line used to calculate the impedance value, that the formula for characteristic impedance derives down to the simple square root of L divided by C......

When a coaxial cable, for example, is specified as 75 ohms, this is not a DC resistance, but an AC impedance that states if I terminate this cable in its characteristic impedance with a 75 ohm load, then that impedance will be reflected at its input. The line will theoretically then appear as infinite, and will exhibit no standing waves or reflections from the load, with its ratio of voltage to current being constant over the entire length of the line. Well, so much for the technical stuff. Kinda boring.

The analog interface between a audio preamp and a power amp is considered a line level, high impedance connection where the input impedance of the power amp will usually be in the order of 10Kohms-100Kohms with the output impedance of the preamp being in the order of 100 ohms or less. This is a voltage bridge type high impedance connection where the preamp acts as a voltage source with a very low source impedance and almost no current is drawn. We are not interested in power transfer in this interface and we sure don't care much about the characteristic impedance of the cable used as explained in the paragraph above.

Certainly, since this is a high impedance connection where we are considering an input impedance to the power amp of usually greater than 10Kohms, you have to be aware of the small shunt effect that capacitive reactance of a long interconnect can have on higher audio frequencies. This reactance will create a small low pass filter effect. Low capacitance per foot is far more important in audio interconnects than any impedance concern. This is especially important when using a passive preamp. With active preamps, when using short interconnects though, it's statistically insignificant, but since it's a player in the equation, and there is anecdotal evidence that various cables sound different, you have to consider it. I'll give you an example of the significance of capacitive reactance. A good typical audio interconnect might realize about 25picofarads of capacitance per foot. For a three foot cable that would be about 75 picofarads. The shunt impedance at 20Khz worst case would be about 106Kohms. Not much of a low pass filter is it?

It's the job of an audio interconnect to degrade the signal as little as possible. In that regard, since you want to present a relatively low voltage to the power amplifier input with as low a noise floor as possible, a properly shielded cable is important. Coaxial cable certainly fits this bill. Wouldn't matter if it was 50 ohms or 100 ohms characteristic impedance. The characteristic impedance of an audio cable is of little concern here. In fact the characteristic impedance specification of a piece of coax is only valid at higher frequencies - audio doesn't qualify...

The answer to your question if you hadn't pulled it out of this long winded crap - the 5 digital audio coaxial cables will be fine......

brucek

[Chris D]

Good question--let's ask the cable expert Lex:

What's the difference in construction or materials between a digital coax line and standard RCA line?




CHRIS
Luke: "Hey, I'm not such a bad pilot myself, you know"

[Lex]

Well, as usual, I can't touch Bruce's techno-bable. I won't even try!

But let me look at this from the cable manufacturer's perspective, and possibly the audiophile's. Video and digital applications do call for 75 ohm cables. Audio is more like 50 ohms truly.

You can certainly use video for audio, but not vice versa unless you want potential for various technical transmission problems. However, there are some caveates here. For example, coax is by definition shielded. Well, some of the finest sounding cables for analog audio are actually non-shielded. Then too, the dielectric thickness can impact analog sound. Thinner is usually perceived to be better. Coax really doesn't get this right either as the dielectric is usually quite a bit thicker. Then there's the material of the dielectric, as well as the shield material. Very few coaxes get this right for audio as well, meaning bare copper shield, and preferably a teflon (FEP) dielectric. Typical lower end coaxes are PVC. Not the best from an audio perspective. Also, many conductors are tinned, as well as the shield for coax.

Analog audio cables typically if shielded are shielded at the source end only. In theory, then any interference picked up by the shield will "roll off" the signal at the shield, and not be picked up on the receiving end. You really can't do that with a standard coaxial type cable, because you only have a signal conductor and a shield. The shield must make contact on both ends to complete the ground. It requries at either a 2 wire shielded audio wire, or a twinaxial coax in order to shield at one end only.

There's a lot of variables at work here, deciding if you have a suitable coax for analog audio. There's the dielectric differences, shield material, as well as conductor type (solid, vs. stranded).

In the end, a listening test is in order. But my preference is to use the right wire for the job. Thus, I don't typically use standard 75 ohm coax for audio.

Lex




Cable Guy DVD Collection

[I'mListening]

Well, that concludes my question. I have thus returned the Digital CoAx Cables and replaced them with High quality Analog Cables. Thanks so much for the indepth explanation.




Thanks for reading!

Enter The Cyberdyne Cinema

Darren