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Does Quality of Coax Input Matter for DACs?

KSTR

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Hmm, it sounds like a lot of additional work is required to recover a signal from coax/toslink as opposed to USB (or maybe I'm misunderstanding).
No, you are quite correct. With USB, all modern desktop DACs are clock masters and just request data from the host at their needed rate. And they can use a local high quality clock circuit for this. As opposed to coax/optical where the clock has to be recovered from the raw signal, with the methods for this being considered mature, though. That is not to say that problematic designs cannot exist, of course. Specific testing and debugging for this is required to see what's really going on under the hood when a Topping DAC fails to connect to a CD player.
 

tonycollinet

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RF theory, impedance matching (VSWR, etc) are the bedrocks of RF transmission; I got that and thank you!
But your reply complicates matters unnecessarily bcuz jitter created within any DAC processing and degradation due to tranmission thereof are related but different topics when it comes to jitter causes from active circuitry (re:cables/coax)...
RF Transmssion doesn't come into it - we are talking a digital data stream albeit at a little over 1mb/s. The topic for a dac is the jitter seen at the input for example, as a result of digital waveform distortion in the interconnect. The measurement above demonstrates that the high level of jitter created by the DAC-SQ3 is completely irrelevant due to the stable clock recovery of the Topping DX3. Same would apply to any jitter created by poor cable quality - right up until the point the waveform is so bad the signal cannot be received at all.
 

Spkrdctr

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Yes, always. Handling jitter at the receiver is the bare minimum we have done to attenuate it since the very early 90s.
In the 80s it was recover the clock from S/PDIF. Then it became recover the clock from S/PDIF and use a PLL to lock a new clock source onto it. Then it became filter the incoming S/PDIF signal, sync only on the pre-ambles which are the most stable transitions in any S/PDIF frame in the time domain and use a PLL to lock a new clock source onto it. Then we did dual stage PLL, using one for clock recovery and a second for more stable reclocking.

When we talk about sample rate conversion jitter is reduced even further by necessity. This is because any data timing errors at the input to the sample rate converter are distributed across the new sample rate as a ratio of the input vs output sample rate, and that mathematically could cause very minor jitter to be massively multiplied. So to compensate this SRCs massively oversample the input, and filters it before resampling and then decimating to get the output. So while there is a lot of people who say ASRC doesn't reduce jitter they are correct about only part of the process.
I am not sure if that is right, but it sure sounds like serious science talk! If I could talk like that I would get a cool job.
 

PeteL

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I bought a Topping D30pro and it is unable to connect to my Cambridge CD player. My RME ADI-2 DAC connects to the same CD player with no problem. The explanation I got from Topping is that this is due to huge jitter from the CD player. I have learnt from others that this is a problem for more owners than me, and with several different brands of CD players, and not just a problem with this specific model from Topping.
If anything, My Sony CDP-D500 syncs well with my Topping E30 DAC, not sure it means anything about the different implementations of spdif in the two models. The Sony is a higher end Pro-targeted player, it's possible that more care has been given to the spdif interface. Over time tough, I am using the Sony analog outs, it sounds a bit different but just as good as the Topping even tough it's a much older dac.
 

Mulder

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If anything, My Sony CDP-D500 syncs well with my Topping E30 DAC, not sure it means anything about the different implementations of spdif in the two models. The Sony is a higher end Pro-targeted player, it's possible that more care has been given to the spdif interface. Over time tough, I am using the Sony analog outs, it sounds a bit different but just as good as the Topping even tough it's a much older dac.
My Cambridge is a pure CD transport. No inbuilt DAC. My Oppo Blue Ray connects to the DAC. This serms to be a problem with SOME player. Not all.
 

deprogrammed

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How are the buffers on Home Theater receivers? I bought a Topping D10 just for this purpose. I know others do as well.
 

rschoss

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Hi to all, I'm not the biggest expert in DACs architecture but maybe modern DACs are recloaking even their coaxial input and old classic DACs ( 44.1/16), are synchronized to the coax input and are influenced by the quality of the coax input ?

It's just a hypothesis and maybe I'm wrong. :)
 
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strummr

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Having been an 'audiophile' sucker for so many years... I love these tests of 'basic' items. (yes I still have a pair of bi-wire MIT speakers cables that cost $3k+) This is real insight into the easily spun gray area of cables.
 

pseudoid

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RF Transmssion doesn't come into it - we are talking a digital data stream albeit at a little over 1mb/s.
What exactly is a digital signal (aka 'data stream') @1Mb/s with sharp edges (tr/tf) but an RF signal in the frequency domain, which is transmitted via a short PCB trace or any length or type of cable/conductor (coax or otherwise)?
 

sam_adams

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Jitter is created by the quality of transmission of S/PDIF over Coax in DAC-SQ3.

@amirm's statement here is very important to understanding the quality of the signal that is being delivered to the devices using coax cable. The SPDIF spec calls for 75 ohm impedance for the signal chain. While the cable—probably—is very close to the nominal value, the RCA connector is nowhere near the correct impedance value.

RCA connectors—by virtue of their inner conductor to outer conductor diameter ratio and dielectric—are closer to 40 - 50 ohms. There are some companies that claim to make compression fittings in RCA style that are 'true' 75 ohm impedance, but that's simply not possible given the dimensions of the RCA connector. There will always be an impedance mismatch at both ends of the cable.

In many cases the output signal level is also nowhere near what the spec calls for and may exceed the spec's level by several times. The level mismatch is probably intentionally designed in to accomodate losses for longer than spec runs of out of spec cables—think about those cheap video cables with the yellow RCA connectors on them that used to come bundled with every VCR and DVD player with component outputs. So, despite all the cable arguments that surround analog audio, here in the 'digital domain', cables/connectors matter if you want low jitter.

A cable impedance caculator that has the math. You have to guess the dielectric constant of your connectors, but the basic premise still stands.
 
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amirm

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How are the buffers on Home Theater receivers? I bought a Topping D10 just for this purpose. I know others do as well.
I have measured coax input for some. Hard to generalize though. Here is an example:

index.php
 
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amirm

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@amirm's statement here is very important to understanding the quality of the signal that is being delivered to the devices using coax cable. The SPDIF spec calls for 75 ohm impedance for the signal chain. While the cable—probably—is very close to the nominal value, the RCA connector is nowhere near the correct impedance value.
There can be measurable degradations not only due to termination but even type of cable used. The audio precision actually only has BNC connectors so I had to use adapters to convert it to RCA. And this adds jitter (likely the reason AP's output has some). I have not measured an impact on the output of DACs however due to any of this.
 
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amirm

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Hi to all, I'm not the biggest expert in DACs architecture but maybe modern DACs are recloaking even their coaxial input and old classic DACs ( 44.1/16), are synchronized to the coax input and are influenced by the quality of the coax input ?
It is a requirement that the DAC stay in lock step with the input clock otherwise you get drift (and lip sync issues with video). There are some exceptions out there in some CD players which detected the input rate and then ran freewheel. But for 99.999% of devices, they must stay with the same cadence as the input. They can use their own clock but they need to change its rate up and down to stay with the input data rate.

Phased Locked Loops of various designs are used to get rid of input jitter. The challenge is that the better you make such a PLL, the slower it will move to lock to a new sample rate. This can be annoying to users who have to wait until music plays. Solution involves using nested PLLs (one slow and one fast), various digital and hybrid PLL schemes. The cheapest solution tends to have only filter above audible frequencies leaving jitter in audio band.
 

sam_adams

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There can be measurable degradations not only due to termination but even type of cable used. The audio precision actually only has BNC connectors so I had to use adapters to convert it to RCA. And this adds jitter (likely the reason AP's output has some). I have not measured an impact on the output of DACs however due to any of this.

Yes, the choice of cable is very important—particularly the dielectric of the coax. Proper termination is also one of the things that contributes to signal quality. Keeping the dielectric as close as possible to the termination of the center conductor and keeping the inner foil shield—if so equipped—as close as possible to the end of the dielectric at the inner conductor termination point helps to maintain the impedance.

I could guess, but you have the AP and would know better than I what the output/input impedance would be. With BNC connectors though, it would probably be closer to 50 ohms.

Even if one were to compensate for possible cable impedance mismatch, there is still the RCA connector to deal with. However serious the impedance of the cable/connector mismatch might be, we see that the chip manufacturers seem to have thought this all through and have designed in compensation in form of jitter reduction circuits to make up for this.
 

GWolfman

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Any other competitors for USB to S/PDIF?

I'd like to send audio from my Pi to an AVR. Or is there a better (i.e., more accurate) way to get audio from a Pi to an AVR?
 

Tks

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Is it possible to get TOSLINK tested? That’s the one I always wonder most about due to shortages of Toshiba quality connectors about a year ago.

It’s the input I see mostly varying among modern DACs (USB is almost always best performance).

Great article btw, informative for sure.
 

Holmz

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Yes, the choice of cable is very important—particularly the dielectric of the coax. Proper termination is also one of the things that contributes to signal quality.
Where in the review was that mentioned ?
 

garbz

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I'd like to send audio from my Pi to an AVR. Or is there a better (i.e., more accurate) way to get audio from a Pi to an AVR?
What you want is a HAT. I use this one: https://www.hifiberry.com/shop/boards/hifiberry-digi2-pro/. These not only provide S/PDIF outputs, but come with various other features. Some include DACs, the one I linked to allows you to install a DSP module to it for room correction, nearly all of them provide a dedicated audio clock back to the raspberry pi processor as well (since it doesn't actually include one).
 

JensH

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75 ohm BNC connectors are available, but judging from pictures of AP equipment, it looks like they use 50 ohm BNC connectors.
Here is a picture, which shows the difference between the two types:
50 and 75 ohm BNC

For SPDIF it probably isn't terribly important what connector it is.
 
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