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Digital Cable Magic Length - Plausible or Busted?


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Posted (edited)

Well its 4:45am and I've woken up, laying in bed I started thinking about length... cable length¬†ūüėÖ

 

I've read on various forums, websites and digital cable manufactures sites that digital cable lengths are important to help minimise jitter & or data loss. Some sites have mentioned SWR - Standing Wave Ratio. Some sites state a digital cables optimal length should be 1.5m. I have never read an explanation where it convinced me 1.5m is optimal and as I'm hard to convince without sound practical theory behind a claim, I figure its time we put this to bed.

 

While I do know from my AOCP days that an impedance miss match between two devices will result in a portion of the signal being reflected, ie. a signal loss. For this to occur, the cable between the two points must either be the wrong impedance or have an electrical length equal too the transmitting frequency or an octave of it. Octave being multiples of the fundamental frequency, same as harmonic frequencies in audio. So to ensure the cable length is correct allowing maximum signal transfer and minimal signal loss, the operating frequency must be known to determine the correct cable length.

 

Q1;   I understand a DACs output frequency can vary, can someone shed some light around this please?

 

Q2:   If a DACs output frequency does not vary, then whats its fundemental?

 

Q3:   Where or how might these claims of 1.5m being optimal coming from?

 

 

Thanks

 

Edited by Allan
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This has been discussed many times and possible explanations already given on various sites, you just need to look. 
If you after another debate then why not? 
Neo

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Posted (edited)

debate

noun

a formal discussion on a particular matter in a public meeting or legislative assembly, in which opposing arguments are put forward and which usually ends with a vote.

 

There is no opposing argument here when I'm asking to understand a DACs output frequency, unless you wish to argue a DACs output frequency is not what it is. Once known, we can apply the SWR calculation to see if 1.5m has any factual relationship.

 

Think this is more along the lines of reverse engineering to dispell snake oil or validate fact

 

If you understood 468/MHz you wouldnt label this a debate. If you would like to understand some fact, read up on matching  transmission lines which in a nutshell transfer signals between 2 impedence points with minimal losses and maximum efficiency ie. minimal signal degration

 

 

 

 

Edited by Allan
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Posted (edited)
46 minutes ago, Neo said:

If you after another debate then why not? 

 

17 minutes ago, Allan said:

If you understood 468/MHz you wouldnt label this a debate. If you would like to understand some fact, read up on matching  transmission lines which in a nutshell transfer signals between 2 impedence points with minimal losses and maximum efficiency

This has been discussed many times and possible explanations already given on various sites, you just need to look. 
Case closedūü§£

Edited by Neo
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Posted (edited)
5 minutes ago, Allan said:

Easy to say case closed, care to explain why?

No, it’s been discussed for years and possible explanations already given. Do you want me to site links or you ok typing into google search yourself. 
Neo

Edited by Neo
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Not interested in know all arogance, I asked a simple question around a DACs output, if you cant answer this take a walk or sh*t up and sit down, you may learn something for a change

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Just now, Allan said:

Not interested in know all arogance, I asked a simple question around a DACs output, if you cant answer this take a walk or sh*t up and sit down, you may learn something for a change

I already asked that question and researched it myself to great satisfaction. 
Absolutely no need to get personal like you just did. 
What you asking is absolutely not new and have been discussed before even on this site.

Neo

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I've made digital RCA cables significantly shorter than 1.5m and found no audible problems, subjectively.

 

Not even shielded, maybe that was a benefit in avoiding reflections, which they claim to be the issue.

 

IMG_20230303_205114_420.jpg.232390a1a7a87cf24df66410b48cc047.jpg

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4 hours ago, Allan said:

Q1;   I understand a DACs output frequency can vary, can someone shed some light around this please?

Q2:   If a DACs output frequency does not vary, then whats its fundemental?

Q3:   Where or how might these claims of 1.5m being optimal coming from?

 

I think you mean the SOURCEs output frequency.

i.e. The connection from the source to the DAC is the digital connection.  The DACs output is the analogue audio.

 

Typically, the source output frequency (bit rate) would vary, depending on the file being played, so, if cable length matters, there is no optimum in this case.

Unless the source has the option to set a fixed output format and upsamples or converts everything to that setting. e.g. 24/192kHz PCM. (~9.2MHz).

 

As to the effect, well, most DACs have the signal all tidied up, clean & synchronized, well before it hits the actual D to A chip.

To make the DACs job easier, it is best to use cable that is the correct characteristic impedance, but at the wavelengths involved, the length is not going to make any practical difference.  Don't lose sleep over it.

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6 hours ago, Allan said:

Q3:   Where or how might these claims of 1.5m being optimal coming from?

Yeah it doesn't gel with transmission line theory. Reflections are caused by impedance mismatch of either device ports, connectors or cable. At the frequencies of digital data, this isn't an issue.

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3 hours ago, surprisetech said:

Typically, the source output frequency (bit rate) would vary, depending on the file being played,

 

and if the data was being sent by something like a USB connection, or over the network, a word or data packet at a time, to the DAC, it becomes meaningless.   

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Posted (edited)
21 hours ago, muon* said:

I've made digital RCA cables significantly shorter than 1.5m and found no audible problems, subjectively.

 

Not even shielded, maybe that was a benefit in avoiding reflections, which they claim to be the issue.

 

 

 

without shielding if there was any present, I imagine EMI could become an issue. Additionally, not having shielding can make a dramatic change for the worse as the cable would now be acting more like a ladder line instead of coax, ladder lines can have characteristic impedances well up around 600 ohm. There is a plus though.. ladder lines generally have lower insertion losses compared to coax and its the reason they're used in RF applications. 

 

Edited by Allan
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Posted (edited)

@surprisetech I screwed the quote feature so my edit workaround...

 

I think you mean the SOURCEs output frequency.

i.e. The connection from the source to the DAC is the digital connection.  The DACs output is the analogue audio.

 

Typically, the source output frequency (bit rate) would vary, depending on the file being played, so, if cable length matters, there is no optimum in this case.

Unless the source has the option to set a fixed output format and upsamples or converts everything to that setting. e.g. 24/192kHz PCM. (~9.2MHz).

================================

my bad.. appreciate the correction

 

Given the bit rate varies, can we consider the highest output frequency the better choice to decide cable length? This way the lower frequencies (being longer) won't come into play??  468/9.2MHz = ~50ft (1/2 wavelength). So anything under 50ft long. Makes a mockery of the ideal cable length being 1.5m

 

 

 

 

Edited by Allan
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15 minutes ago, Allan said:

 

without shielding if there was any present, I imagine EMI could become an issue. Additionally, not having shielding can make a dramatic change for the worse as the cable would now be acting more like a ladder line instead of coax, ladder lines can have characteristic impedances well up around 600 ohm. There is a plus though.. ladder lines generally have lower insertion losses compared to coax and its the reason they're used in RF applications. 

 

They sounded subjectively a lot better than two retail 75ohm shielded cables I had (cheap ones from jaycar and jbhifi)

One was made with Duelund silver, the other with a silver gold wire.

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Posted (edited)
27 minutes ago, muon* said:

They sounded subjectively a lot better than two retail 75ohm shielded cables I had (cheap ones from jaycar and jbhifi)

One was made with Duelund silver, the other with a silver gold wire.

 

if the 2 wires were quite close together there's a likelihood you had the cable characteristic impedance close to 100ő©. The further the wires are apart the higher the impedance¬†

 

300ő© ladder line

image.png.41cf46672becf42771974f1dcca5c7f0.png

¬†450ő© ladder line

image.png.f7045de2614d3d9cd191259bba301f28.png

 

Two above side by side

image.png.205e206950a4d83e22bdb7c308d9c89a.png

 

Edited by Allan
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