What I read was a recommended minimum of 2m. I had two cables of the same construction, one was 2m and the other was shorter, maybe 0.5m, and I couldn't hear much difference between them.
Coaxial digital connections
- Thread starter DEK
- Start date
What I read was a recommended minimum of 2m. I had two cables of the same construction, one was 2m and the other was shorter, maybe 0.5m, and I couldn't hear much difference between them.
Jim Audiomisc
pfm Member
Erm, I'm familiar with ideas like 'signal reflection' in both practice and theory. I've developed and worked with systems that operate at frequencies from FM radio up to over 300 GHz. Some of it for the old NPL.
The cables used in most domestic cases are very short in terms of the wavelengths of the signals used for spdif. And also the cables *don't* have a uniform impedance at lower frequencies. Nor do the connectors on the boxes usually have the 'correct' wideband impedance. Nor, indeed, will the links inside the boxes.
If the 'reflections' are worse at lower frequencies they're better modelled and understood as added *lumped* impedances. Probably easier to tweak that way as well. But you'd first need a way to measure them accurately.
That said, if you think coax for audio is bad, you should check out the insane way HDMI is said to 'work'. Its a scunner.
narabdela
who?
tuga
Legal Alien
Erm, I'm familiar with ideas like 'signal reflection' in both practice and theory. I've developed and worked with systems that operate at frequencies from FM radio up to over 300 GHz. Some of it for the old NPL.
The cables used in most domestic cases are very short in terms of the wavelengths of the signals used for spdif. And also the cables *don't* have a uniform impedance at lower frequencies. Nor do the connectors on the boxes usually have the 'correct' wideband impedance. Nor, indeed, will the links inside the boxes.
If the 'reflections' are worse at lower frequencies they're better modelled and understood as added *lumped* impedances. Probably easier to tweak that way as well. But you'd first need a way to measure them accurately.
That said, if you think coax for audio is bad, you should check out the insane way HDMI is said to 'work'. Its a scunner.
Benchmark's John Siau has participated in a discussion titled "Digital Audio Cable Reflections and DACs"
"The reflected signal can be either polarity. If the termination impedance is higher than the cable impedance, the reflection will be non-inverted. If the termination impedance is lower than the cable impedance, the reflection will be inverted.
Damage is done to the signal at the receiver after the signal has made 3 transits through the cable. The magnitude and polarity of the first reflection is determined by the impedance matching of the termination at the receiver. This first reflection will be reflected again if the drive impedance is mismatched to the cable. The magnitude and polarity of this reflection is determined by the matching of the source impedance to that of the cable. Each successive reflection is smaller than the previous reflection.
A perfect termination at the transmitter or the receiver, will eliminate 3rd transit reflections."
continues here -> https://tinyurl.com/ybv22u4d
tomek
pfm Member
IMHO 2m Belden 1694 with crimped Canare plugs from Blue Jeans cables is perfect:
https://www.bluejeanscable.com/store/digital-audio/index.htm
https://www.bluejeanscable.com/store/digital-audio/index.htm
davidsrsb
pfm Member
SPDIF requires 75 Ohm matching over a significant bandwidth. I have never seen a decent attempt at receiver matching and rarely at the transmitter. The lower frequency end is the hard part, you often see a tiny pulse transformer
Big John
pfm Member
Having read about the 1.5m coax length being optimum with regard to reflections must be at least 30 years ago, with my DIY digital cables I use 1.5m of Webro WF100 coax with BNC fitting on the CD player then to an "F" type screw fitting to a screw fitting 2 way switch box, ( there is only one coax socket on my DAC) then 1.5m Webro with "F" type phono fittings DVD player to "F" type screw to the 2 way switch box then 9" of coax from switch box to an "F" type phono to DAC.
I use "f" type fittings as quality ones are very close to 75 Ohm even with screw on RCA phono fitting. the coax "F" types are screwed on the 75 Ohm switch box with 2 in & 1 out has 2 heavily sprung buttons.
Jim Audiomisc
pfm Member
The absolute assertion about damage iswhat learned academics term "twaddle".
In practice a 'perfect' termination and source match is impossible. No cable has a frequency independent impedance from DC up. Nor will any normal input or output when we're considering wide bandwidths.
Over-claimed problem. i.e. situation normal for audio.
Engineering is about what works, not perfection beyond need.
When I first got a separate DAC I had some fun. I plugged one cable into the DAC, another into the transport and used the cables as test leads to send the signal trough random metal objects. Cutlery, a screwdriver, the ironing board. It didn't matter how big the object was, it worked. Did I hear a change in sound quality? To be honest, not really...
Jim Audiomisc
pfm Member
Agree about the low end. I wonder how many people realise that 'jitter' is often caused by *near DC* effects due to ac coupling... and may not matter anyway. 8-]
Jim Audiomisc
pfm Member
Haven't heard that phrase in ages.
I've stayed in Fife for decades. But still need an Auld Dundonian phrasebook to read "Oor Wullie" or "The Broons". 8-] Perhaps I need a bucket...
I used to enjoy explaining to 1st Year undergrads how Dundee was built on the money made from the Tablet mines under the Law. 8-]
tuga
Legal Alien
The absolute assertion about damage iswhat learned academics term "twaddle".
In practice a 'perfect' termination and source match is impossible. No cable has a frequency independent impedance from DC up. Nor will any normal input or output when we're considering wide bandwidths.
Over-claimed problem. i.e. situation normal for audio.
Engineering is about what works, not perfection beyond need.
That’s why I quoted the engineer, and one with a trail of high performance no-nonsense equipment to his credit.
I sure that he’s not just pulling a leg
Big John
pfm Member
The absolute assertion about damage iswhat learned academics term "twaddle".
In practice a 'perfect' termination and source match is impossible. No cable has a frequency independent impedance from DC up. Nor will any normal input or output when we're considering wide bandwidths.
Over-claimed problem. i.e. situation normal for audio.
Engineering is about what works, not perfection beyond need.
Jim knows a ‘thing or two’ here…
I believe "damage" refers to & means signal corruption, not physical damage.
adamdea
You are not a sound quality evaluation device
There isn't really any argument here because "Damage" is unquantified. The problem is not what John Siau says, but what people are trying to read into it. All that matters is that the cable doesn't really matter very much, especially if the dac has been designed properly (which is incidentally [pretty much] what JS says as the end of the linked passage.)
adamdea
You are not a sound quality evaluation device
I've got an amazing idea. Rather than transmitting information encoded in and read directly from some physical characteristic of the signal, how about encoding it symbolically so that, provided the signal falls within a particular range, a defined piece of information could be decoded from it. Then the encoded signal could be robustly decoded irrespective of fluctuations or degradation of the signal carrier, and hence equally well with modestly specified equipment, provided that it meets very easily-achieved specs. Has anyone tried this?
Hieroglyphics?
Big John
pfm Member
Damage is done to the signal at the receiver end. Though how it's damaged is not specified. "3 trips through" could indicate anything from increasing jitter to multiplying reflections increasing distortion which is what I have inferred.
Jim Audiomisc
pfm Member
My irony meter just exploded. 8-]