(monster) subwoofer cable as spdif coaxial?, (monster) subwoofer cable as spdif coaxial? Options

[alexbarcenas]

i Have a Monster subwoofer cable, it is 12 feet and Im using it as digital coaxial from my digital pc out to my DAC. any quality difference with a factory digital coaxial?
are these subwwoofer cables 75 ohms?

[Light-Fire]

No difference at all.

[Iain]

You can do some RMAA test to prove it is the same if you like.

http://audio.rightmark.org/

[insane_alien]

its digital. digital either gets through fine or not. once the cable is capable of transmitting an on-off signal then there can be no further improvement made signal wise.

any wire that can be used to transmit electricity can be used to transmit digital data.

[alexbarcenas]

so is not important if the cable is 75 ohms or not and what about the lenght?

[rpp3po]

It is important for greater lengths and the subwoofer cable is probably not 75 ohm, but 12ft is not much, that should work perfectly.

[kornchild2002]

I wouldn't read too much into Monster labels. They like to rate their products so that people feel a need to upgrade to the next cable in line. Case in point; their $300 HDMI cable will perform the same as a $2 HDMI cable. Digital is digital and everything is fine so long as the 0's and 1's are being sent and received. A digital coaxial cable is just that, a digital coaxial cable. When it comes to digital cables, length really doesn't make a difference. I have been using a 150 ft HDMI cable (that I purchased for $20) without problems at all. I also have a 20 ft digital coaxial cable that I have been using for 5 years without issues.

[alexbarcenas]

so what is all the thing about digital coax, and the fact that it should be rated at 75 ohm for a perfect fit?

[GregDunn]

All that really means is that an infinite length of coax would measure an impedance of 75 ohms if you put a meter on it. For very long runs it might change the SWR a little on the cable if it's not close to 75. The idea is that the source and destination electronics are usually designed with an impedance of 75 ohms, so a cable of that rating will deliver the maximum power and degrade the signal the least, compared to a mismatched cable.

[rpp3po]

A matching impedance is important for greater cable lengths, also for digital transmissions. All lengths mentioned here aren't great in that sense, though. See this insightful post by John Siau for further detail.

[Sycraft]

Most audio cables are 75ohm anyhow. Most audio cables are, in fact, video cables. Video cables do a fine job carrying audio, so why not use the same cable for both? One of the only companies I'm aware of that actually has purpose-built audio cables is Blue Jeans Cable. Their LC-1 cables are made for audio and aren't 75ohm since they don't need to be. However other cable manufacturers I've seen use the same cable for audio and video (and Blue Jeans did too until recently) and all (good) video cable is 75ohm.

If you are really worried, you can go buy a cheap 75ohm cable from Monoprice.

[Speedskater]

Digital audio cable systems are rather robust. Steve Lampen of Belden Cable Co. wrote this some time ago. I know it's not SPDIF but it's close.

Over on the Ampex list, we were having a discussion about how long a crummy cable one can get away with using to carry AES/EBU audio date. Thanks to Larry Miller for reminding us that the critical number is the rise time and gave an empirical rule of thumb (1/6 wavelength) for when the cable goes into the transmission line mode and needs to be the correct impedance to avoid getting into trouble.

So I'll bet the AES/EBU (IEC-60958) standard specifies the maximum and minimum rise time for the data waveform. Does anyone have a copy of that standard or know what the rise time is? I'm looking for a simple answer based on real facts to the ever popular question: "Can I use a plain old (not made from special 110 ohm cable) for AES/EBU connections?" - the answer being "Yes, if it's shorter than X feet."

Friends-

Only just saw this ancient email before it fell off my list and I had to comment.

I normally tell people that the critical distance is 1/4 wavelength which then varies depending on the sampling rate (and resultant bandwidth) of a cable. I also have some eye-opening graphs (no pun intended) of 48 kHz audio running on 110 ohm cable and Belden 8451 (both 22 AWG, so we're comparing apples to apples). The result is 50 ft. is probably do-able for 8451. At 100 ft. literally half of the signal is reflected because of the impedance mismatch (8451 =~38 ohms) and it screws up the risetime (clock) because the capacitance is about 3 times as much as the 110 ohm cable (Belden 1696A).

If you go by the 1/4-wave numbers the critical distance (after which the impedance is important to match) is:

44.1 kHz = 5.6448 MHz= 44 ft.
48 kHz = 6.144 MHz = 40 ft.
96 kHz = 12.288 MHz = 20 ft.
192 kHz = 24.576 MHz = 10 ft.

Hope this helps set a limit!

Steve Lampen
Belden
04/06/2007