Subwoofer cable vs other types, I'm confused

[Jack Hammer]

I made a post in another thread about there being differences between types of rca cables one should use for a subwoofer and got told I was giving bad information and even even earned myself a very manly unsigned red chicklet from another member who's obviously too afraid to speak openly.

Anyways, my understanding is that audio and video cables are different. And that subwoofer cables are the same, or close enough, to video cables. That if one were to use rca cables to connect the sub, they should use a video cable, not an audio cable. Has to do with shielding and impedance. What issues am I confusing? Please help to better understand.

Thanks

Jack

 

[bandphan]

I think your just getting confused between audio and video. Most every descent cable made is going to 75ohm what ever you lable it(aside from optical and industry formated). Wether its rg6/59/mini. Cheap cables will have less sheilding, and as stated in the other thread sometimes made up in two wire(not 75ohm).

Im not sure who red chickleted you, I dont feel that it was warranted since you were also trying to assertain information.

 

[markw]

To hopefully clarify what I alluded to in the aforementioned thread, I'll just make a few statements:

0) I'm talking coaxial cables here. A few oddballs may be twisted pair, but they don't figure into the big picture.

1) Video cables are speced to be 75 ohms. That's a standard. This goes for composite or component video cables.

2) Audio cables have, at various times in the past, been anywhere between 50 and 110 ohms. All work well.

3) Since video cables are 75 ohms, and this number falls conveniently between 50 and 110, they work just dandy for audio cables as well.

4) Since 75 ohm cables work for durn near anything, it makes sense that manufacturer's would simply buy boatloads of 75 ohm cables and simply change the color of the RCA plugs on the ends.

Granted,there may be differences in the manufacture as far as quality, gauge, insulators, RCA plug construction, and a host of other things, but essentially, aside from the color of the RCA plugs on the end, they are the same.

Simply put: Video cables MUST be 75 ohms. Audio cables can be almost anything, but odds are any modern one will be are 75 ohms.

P.S.. you don't deserve a red chicklet, particularly an anonymous one.

 

[OttoMatic]

Here's a question to which I should know the answer: What is a 75 ohm cable? I mean, where is the 75 ohms come into play? I'm very familiar with EE concepts, but for some reason I'm not up on this issue.

Thanks.

 

[Jack Hammer]

When I was first learning this stuff a few years back, there were a few threads here were it was common to recommend using video cables for your subwoofer as a cheap alternative. I know video cables are 75 ohms, but I recall that not all audio cables are, I know I've seen several that are very thin and cheaply made. Most of what I'm visuallizing is the cheap red/white audio interconnects that are included with most audio equipment. I have to leave for work, but I'll post some pics later if needed.

Jack

 

[bandphan]

http://forums.audioholics.com/forums/showthread.php?t=1980

quick audioholics search

 

[OttoMatic]

http://forums.audioholics.com/forums/showthread.php?t=1980

quick audioholics search

Thanks. I had actually found that through Google just a few minutes earlier. A very clear discussion.

 

[mtrycrafts]

When I was first learning this stuff a few years back, there were a few threads here were it was common to recommend using video cables for your subwoofer as a cheap alternative. I know video cables are 75 ohms, but I recall that not all audio cables are, I know I've seen several that are very thin and cheaply made. Most of what I'm visuallizing is the cheap red/white audio interconnects that are included with most audio equipment. I have to leave for work, but I'll post some pics later if needed.

Jack

Sorry about that red, not me and certainly as was posted, not deserved, not for that.

The video cable suggestion is just a practical suggestion we give out as it can be had for not much $, especially when the need is for a longer cable.

 

[Jack Hammer]

Just for clarification, the interconnects on the bottom are what I believed to be not so great for sub use, especially for a long run. These were what I was trying to make sure the OP of that thread wasn't using for his 25' sub cable. It's not a great pic, I put the ones that came in the box with one of my components, above an ARII one. I'm under the impression that the thinner (bottom) ones wouldn't be good for sub/video usage and that they are not shielded, coax or 75ohm, yet are still an rca cable. The upper one I've understood to be fine for all usage.


Jack

 

[TLS Guy]

Just for clarification, the interconnects on the bottom are what I believed to be not so great for sub use, especially for a long run. These were what I was trying to make sure the OP of that thread wasn't using for his 25' sub cable. It's not a great pic, I put the ones that came in the box with one of my components, above an ARII one. I'm under the impression that the thinner (bottom) ones wouldn't be good for sub/video usage and that they are not shielded, coax or 75ohm, yet are still an rca cable. The upper one I've understood to be fine for all usage.


Jack

Both cables are shielded. The smaller cable has a smaller diameter insulator. The small cable will not be a 75 ohm cable and should not be used for SDIF digital signals. Either cable should be fine for audio, although the cheaper cable may have inferior shielding and pick up hum. However if hum is not a factor, for audio either cable should work.

A lot of members do not understand the importance of using 75 ohm cable for SPDIF digital signals. I have been told in posts by a number of members, that this is not critical. It is, as the error rate will increased unless source, wire and termination impedance match.

 

[bongobob]

Actually, very few if any cables are truly 75 Ohm. How close they come is determined by, amongst other things, the length of the cable and the quality of the connector. The reflections of the signal along it's path alter the Ohm value. This comes into play primarily with video in the 1Ghz + range. Additionally, the jacks are generally not 75 Ohm nor is the internal wiring in whatever you're hooking up. In short, there's a lot of leeway with a signal cable until you start looking at long runs of HD video.
Or so I'm told....

 

[TLS Guy]

Actually, very few if any cables are truly 75 Ohm. How close they come is determined by, amongst other things, the length of the cable and the quality of the connector. The reflections of the signal along it's path alter the Ohm value. This comes into play primarily with video in the 1Ghz + range. Additionally, the jacks are generally not 75 Ohm nor is the internal wiring in whatever you're hooking up. In short, there's a lot of leeway with a signal cable until you start looking at long runs of HD video.
Or so I'm told....

I'm sorry but that is wrong information. The 75 ohm rating, has nothing to do with DC resistance or the length of the cable. It has to do with balancing output, line and termination impedance as a correctly matched transmission line.

The determining factors are the inductance and capacitance of the cable, which determines the E/I ratio. This is the relationship the voltage and current in the cable. It is not affected by length of cable.

This is important for transmission of digital signals, as a mismatch between cable and terminations causes reflections. The reason being that the frequency for digital signals puts their wave lengths in the region of the lengths of the cables involved. This matters, as mismatch greatly increases the error rate of the transmitted signal. This drives the interpolation of data by the error correction system through the roof, and can result in a choppy signal with drop outs.

A cable used to connect a 75 ohm SPDIF RCA coax output to a SPDIF RCA coax input should be a good quality cable with an impedance of 75 ohms.

I have noted a lot of confusion and misunderstanding about this on these forums.

 

[Jack Hammer]

...The small cable will not be a 75 ohm cable and should not be used for SDIF digital signals...

I did some searching for SPDIF cables and found all the posts I recalled from a few years back. You were right, they were about using video cables instead of audio cables for SPDIF connections and had nothing whatsoever to do with subwoofers.

Again, guys Thanks for the help. I'm always learning new stuff.

Jack

 

[markw]

I did some searching for SPDIF cables and found all the posts I recalled from a few years back. You were right, they were about using video cables instead of audio cables for SPDIF connections and had nothing whatsoever to do with subwoofers.

Again, guys Thanks for the help. I'm always learning new stuff.

Jack

Actually, a few years ago I tried using, for digital coax from my DVD to my Denon, a yellowed, cracked ancient 3 foot audio interconnect that I've had lying around since my early days in this hobby (early/mid sixties) and it worked quite well. I didn't measure anythng but the sound was fine.

It looked like hell so I eventually replaced it with a spare composite video cable.

So, while the theory is sound, particularly for longer runs, for short distances I believe almost anything will work quite well.

You can easily verify this for yourself.

 

[TLS Guy]

Actually, a few years ago I tried using, for digital coax from my DVD to my Denon, a yellowed, cracked ancient 3 foot audio interconnect that I've had lying around since my early days in this hobby (early/mid sixties) and it worked quite well. I didn't measure anythng but the sound was fine.

It looked like hell so I eventually replaced it with a spare composite video cable.

So, while the theory is sound, particularly for longer runs, for short distances I believe almost anything will work quite well.

You can easily verify this for yourself.

You got away with it, thanks to error correction. However when it comes to engineering, to get the best results, do it right!

 

[markw]

You got away with it, thanks to error correction. However when it comes to engineering, to get the best results, do it right!

Isn't error correction what digital is all about?

 

[TLS Guy]

Isn't error correction what digital is all about?

It is a necessity otherwise there would be a lot of pops and drop outs. However, it is important to minimize data loss, as the error correction interpolates the data, so there is masking of the problem. Best results require minimizing the need for error correction.

 

[markw]

In theory you are 100% correct.

It is a necessity otherwise there would be a lot of pops and drop outs. However, it is important to minimize data loss, as the error correction interpolates the data, so there is masking of the problem. Best results require minimizing the need for error correction.

But, in practice, as long as the error rate is low enough that the error correction can correct it, it's a moot point.

Remember, if our ears can't detect it, it doesn't matter.

 

[mtrycrafts]

A lot of members do not understand the importance of using 75 ohm cable for SPDIF digital signals. I have been told in posts by a number of members, that this is not critical. It is, as the error rate will increased unless source, wire and termination impedance match.

It certainly can be an issue but some tests have been done using coat hangers for digital CD audio, not very long ones, error rates were measured, etc, with no difference in error rates. Unfortunately that web site is no longer available but it has been around.
In standard TV analog video, experiments have beenconducted with very long 120V power extension cords with interesting results
That link can be had for sure, at the ABX place but I don't have it with me as I am away from home