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Does Quality of Coax Input Matter for DACs?

chris719

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Admittedly I am having trouble "unwrapping" what you wrote.

Yes, 15pF will be 100 ohms at 100MHz, but how many 100MHz sources of noise do you have that will induce even small amounts of common mode voltage? That common mode noise voltage will also need to present as a differential voltage at the receiver input where there is invariably a capacitor to shunt just such noise to ground. I can also use supplementary common mode filtering. Further, that 100Mhz noise would have to form a beat frequency with the underlying clock transitions to generate noise below the PLL cut-off. Not saying it can't happen, but it is what would have to happen.

A far worse issue with galvanic isolation is data induced jitter.

w.r.t. "RF holes", while that may be the case, it only needs to be the case at one end. You are also bringing in a very small piece of metal which only is an effective antenna at very high frequencies. On the transmitting end, there is no reason of course not to ground the BNC. On the receiving end, you can capacitively couple the shield to case with a small capacitance to eliminate RF, and get the side benefit of shunting the aforementioned common mode noise to ground.

Shields bonded at one end aren't good practice and a recipe to fail EMC. You can get away with a lot of crap for audio, I'm sure.

Yes, you can add some capacitance to the case but unless it's an annular feed-through type, it's going to be iffy. Amphenol does make BNCs like this but they are not commonly used. I've never seen a DAC use a common-mode choke on a coax input either.

We can debate the details, but optical is superior if you care about galvanic isolation. Since jitter does not practically matter as long as the PLL can lock and/or you have an ASRC after the receiver, I don't see much reason to use coax unless you need the distance or 192 kHz.
 

audio2design

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Shields bonded at one end aren't good practice and a recipe to fail EMC. You can get away with a lot of crap for audio, I'm sure.

Yes, you can add some capacitance to the case but unless it's an annular feed-through type, it's going to be iffy. Amphenol does make BNCs like this but they are not commonly used. I've never seen a DAC use a common-mode choke on a coax input either.

It's audio. The cap gets me the frequencies I need out almost no matter where I am connected. I am neither worried about emissions or susceptibility.
 

chris719

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It's audio. The cap gets me the frequencies I need out almost no matter where I am connected. I am neither worried about emissions or susceptibility.
Naive, and won't cut it for a real top-quality commercial product. For DIY, sure.
 

audio2design

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Naive, and won't cut it for a real top-quality commercial product. For DIY, sure.

Considering I spent decades actually developing commercial products the only thing naive is your comment.

Emissions in most audio products is rarely an issue even with processors and switch mode power supplies and not grounding the BNC to the case is not going to change that. The bandwidth for SPDIF is <10Mhz. It's easy to keep EMI emissions at >30Mhz low on the SPDIF. Keep in mind the coax ground is no different from an emissions standpoint than the often DC power input with often an unshielded no twist cable.

Wrt EMC susceptibility, SPDIF is a relatively low frequency connection so even the most rudimentary of filtering will keep out any likely RF.

On the professional side, two signals + shield of course eliminates any concern of grounding a shield impacting a signal. On the home side, the RF environment is relatively benign with most signals high frequency and spread spectrum. It's relatively easy to keep out any external RF that would impact performance especially in most DACS where high impedance nodes connected to long traces and wres simply do not exist.
 

KSTR

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Emissions in most audio products is rarely an issue even with processors and switch mode power supplies and not grounding the BNC to the case is not going to change that.
My take as well.

If anything, coaxial SPDIF is as likely a source for RFI (CM) as any other connector, from bad PCB layout.

As for (air-borne) RF in general, the moment you have any XLR or TRS connectors, or displays, ventilation slots, etc, the shielding effectiveness of a metal case is lost anyway.
 

JonP

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You're missing something. To use a transmission line you need a constant impedance across the line. Having a 75ohm BNC plug mated to a 75ohm BNC socket doesn't help you if your cable is 50ohm, or your source is 50ohm.

If you're going for perfect or ideal you *need* to check compatibility with not just the cable, but between the two piece of equipment you are connecting as well. Again this is why some equipment has multiple different outputs with different connectors and different terminations.
Oh, I had thought that 75ohm was the industry target impedance and 50 ohm cable would have been the mismatch?

I'd like to see how much hysterisis induced jitter difference 2' of 50 ohm cable adds to a 75 ohm system. Almost would bet the amount might be difficult to measure.
Then again, probably shoudnt bet against crappily designed tx and rx circuits popping up over the past couple of decades.
 

fastfreddy666

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Just rip your CD collection to FLAC and you don't have to worry about jitter your vintage CD player might output via SPDIF (TOSLINK/COAX) This is also a good way to keep your CDs in "mint" condition. (I also ripped some of my more obscure vinyl records for this reason. No I don't think it sounds better. The sound quality is objectively worse)

I suggest to use a program like exact audio copy. (I used it for many years): https://www.exactaudiocopy.de/en/ (1)

(It's FREE. The creator, Andre Wiethoff, isn't in it for the money. He's a breath of fresh air in our (overly) capitalistic world. But if you appreciate his hard work you should donate some of your disposable income. I know you have it. You wouldn't be visiting this forum otherwise. And while you're at it. You should support Amirm's work too. Just click at donations in the menu above and you know what to do...

I'm digressing. The program is still in development. It uses a fairly new technology called Accuraterip: http://www.accuraterip.com/

This takes Audio CD ripping to the next level by verifying ripped tracks against an Internet database, making sure they are error free.
You need some CDs for calibrating the drives but once that's done you'll never have to do that again. (go to the website for more info)

But if you're lazy like me you could just buy dBpoweramp which also uses the same algorithm. Yes it will cost you $39. It can (batch)convert to mp3 (yikes), AAC or OPUS and several other formats too. If you already used another program and want to verify that your collection is error free. Perfect tunes is included and this cannot only verify your tracks (only lossless formats of course. This does not work on lossy formats) but can also add missing album art, mass ID Tag and remove duplicate tracks.

Ripping your collection is a tedious, time consuming, job. But when you're done. You could (should) move the resulting files to a NAS/File Server/Cloud and stream it from there. You could also use Roon of course but this will cost you $12,99 a month. Roon is an excellent way to order your music and listen to streaming services. I refuse to use it because and I quote "Roon and MQA (Master Quality Authenticated) technology are firm friends" I'm not. Welcome to the wonderful world of capitalism.

I already painstakingly tagged everything myself. FLAC was released on July 20, 2001 and I didn't have access to all those sophisticated online databases like FreeDb (unfortunately defunct ) yet and I had to "tag" everything by myself. Nowadays you could use something like the extensive database of Discogs.

If you like to support an artist. Buy the CD. And if your funds are limited listen to Spotify. And if the Corona crisis is finally over. Visit a concert!
I know I will. I'm really in the mood for some open air festival after two years of watching live streams. I'll will see you there.

O and I beg you. Please don't use these software programs for making illegal copies of copyrighted and/or protected works.

I'm not completely anti-capitalist but we could dial it down a bit. If I only knew how....

(1) Some documentation: https://www.exactaudiocopy.de/en/wp-content/uploads/2007/03/eacdoc.pdf

Enjoy the music
 
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mansr

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I'd like to see how much hysterisis induced jitter difference 2' of 50 ohm cable adds to a 75 ohm system. Almost would bet the amount might be difficult to measure.
It is. I tried.
 

chris719

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Considering I spent decades actually developing commercial products the only thing naive is your comment.

Emissions in most audio products is rarely an issue even with processors and switch mode power supplies and not grounding the BNC to the case is not going to change that. The bandwidth for SPDIF is <10Mhz. It's easy to keep EMI emissions at >30Mhz low on the SPDIF. Keep in mind the coax ground is no different from an emissions standpoint than the often DC power input with often an unshielded no twist cable.

Wrt EMC susceptibility, SPDIF is a relatively low frequency connection so even the most rudimentary of filtering will keep out any likely RF.

On the professional side, two signals + shield of course eliminates any concern of grounding a shield impacting a signal. On the home side, the RF environment is relatively benign with most signals high frequency and spread spectrum. It's relatively easy to keep out any external RF that would impact performance especially in most DACS where high impedance nodes connected to long traces and wres simply do not exist.

I've taken dozens of products/systems in other industries through EMC testing to standards beyond the requirements of commercial AV gear as per IEC 60950 or 62368 or whatever is commonly tested to. I'm glad you think there are not commonly issues with SMPS and processors, or that spread spectrum modulation makes emissions benign.

Most "hi-end" audio stuff is probably not even tested. I've heard a lot of commercial audio devices demodulate GSM and play tones back when 217 Hz TDMA was still in use.

Anyway, I appreciate the fact that it is not a functional issue for most products. I still maintain that an optical connection is more ideal if you want complete galvanic isolation.
 

chris719

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My take as well.

If anything, coaxial SPDIF is as likely a source for RFI (CM) as any other connector, from bad PCB layout.

As for (air-borne) RF in general, the moment you have any XLR or TRS connectors, or displays, ventilation slots, etc, the shielding effectiveness of a metal case is lost anyway.

I'd say this is a pretty big generalization as far as the enclosure goes, really depends how it's executed. Also, if you have a source of radiated emissions it is very likely susceptible as well.
 
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audio2design

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IEC60950 is a safety standard for IT equipment. IEC62368 is a safety standard harmonized for AV equipment and IT equipment. Neither is an EMC standard. Who is naive? It is not that I think those things are not normally issues. I know they are not normally issues in an audio context with basic galvanic isolation and typical designs.

TDMA has not been in use for some time. It had to be quite close to the equipment and you needed a high impedance analog node with some length of wire. DACs as I noted don't have high impedance nodes with long wires and we aren't inclined to embed our cell phones into our stereo.

Some high end gear is not tested. A lot of midrange is tested for emissions. You don't legally need safety compliance for import. You do need FCC compliance (which is only emissions).

Optical is more isolated but that does not make it relevant.
 

chris719

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IEC60950 is a safety standard for IT equipment. IEC62368 is a safety standard harmonized for AV equipment and IT equipment. Neither is an EMC standard. Who is naive? It is not that I think those things are not normally issues. I know they are not normally issues in an audio context with basic galvanic isolation and typical designs.

TDMA has not been in use for some time. It had to be quite close to the equipment and you needed a high impedance analog node with some length of wire. DACs as I noted don't have high impedance nodes with long wires and we aren't inclined to embed our cell phones into our stereo.

Some high end gear is not tested. A lot of midrange is tested for emissions. You don't legally need safety compliance for import. You do need FCC compliance (which is only emissions).

Optical is more isolated but that does not make it relevant.

For some reason I thought those referenced the EMC standards, CISPR 22, 24, and now 32 I guess. Of course, they would need FCC Class A/B if they are going to import to the US legally.

I agree these are not audible issues. I never claimed they were. Audible is a low bar, though. There are a lot of intentional radiators and high speed digital ICs in some audio devices today. They might not have +30 dBm peak output like a phone, but I'm sure judging by the ferrites I've seen sprinkled in some teardowns that it didn't all pass testing on the first go. Wonder how ESD testing would go as well on some of these products.

Optical is just as relevant or irrelevant as coaxial SPDIF, I'd say. In fact, I'm pretty sure most TVs today that still have SPDIF out have ditched the coax. LG C1/G1 have no coax, nor do Sony's 2021 range, but they do have optical.

Again, in the context of the thread and spirit of my original reply, why not use optical if you are going to subject yourself to SPDIF anyway? One fewer interchassis galvanic connection is probably a good thing.
 
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audio2design

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I'd say this is a pretty big generalization as far as the enclosure goes, really depends how it's executed. Also, if you have a source of radiated emissions it is very likely susceptible as well.

I would say that is a pretty big generalization that is far more often wrong than right. Power connections (in and out) would be an obvious spot where emissions are far more of an issue than susceptibility. Actually I would say for anything used in a consumer environment, there is a far higher chance of radiating then being susceptible to radiation in any practical way, even at the levels in IEC/EN 61000-6-2 & CISPR 35 for 61000-4-3/4-6/4-8 where susceptibility would be far less likely than radiated.

but I'm sure judging by the ferrites I've seen sprinkled in some teardowns that it didn't all pass testing on the first go.

Well at a penny or two (+/-), and from experience, I "sprinkle" ferrites all the times as would most good engineers. Slowing down power MOSFET edge speeds without taking a performance hit, high frequency power supply noise rejection to improve RF performance, reduce high frequency ringing on diodes, for differential and common mode rejection on data lines leaving the enclosure, etc. Layout and architecture cannot fix everything, and time to market is more important than saving a penny, even when you are making a few million.

Naive, and won't cut it for a real top-quality commercial product. For DIY, sure.

And I will go back to what I said before. The naivety is yours for taking a requirement for one industry / product and assuming it is necessary or correct for products in other industries.
 

garbz

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Oh, I had thought that 75ohm was the industry target impedance and 50 ohm cable would have been the mismatch?
75ohm was the target impedance for S/PDIF. There's no standard however for master clock distribution. But you're right, the relevance of this mismatch over the distances being discussed is somewhat irrelevant, especially considering a modern DAC's jitter resistance.

No, this is a common trope that is repeated on audio forums but is incorrect.

Galvanic isolation of coax SPDIF is not perfect. First, there is the interwinding capacitance of the transformer to account for. The Newava S22083 has 15 pF from pri to sec. This is around 100 Ohms at 100 MHz.

Second, trying to galvanically isolate an interconnect that uses a shielded cable forces a compromise. Imagine you have a Class I appliance with metal enclosure. That enclosure is connected to PE as required for safety. When you add a BNC or RCA connector for your coax, you need to decide if it's an insulated BNC or one that connects to chassis.
I think you've got your heard so far buried in theory you've lost sight of what we are doing. Of course the galvanic isolation of a transformer is imperfect. But it doesn't need to be perfect to achieve it's goals, especially not at 100MHz. The point is to prevent circulating ground currents between chassis in frequencies that are audible, or which can effect electronics to produce an audible result.

Secondly picking appropriate connections to your application isn't a compromise. It's just a common engineering practice that recognises you can't do something in isolation (pun intended).

Catching up on the rest of the thread you seem to have a heavy focus on EMC compliance. This is not the purpose nor is it relevant to the galvanic isolation of S/PDIF inputs.
 

chris719

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I would say that is a pretty big generalization that is far more often wrong than right. Power connections (in and out) would be an obvious spot where emissions are far more of an issue than susceptibility. Actually I would say for anything used in a consumer environment, there is a far higher chance of radiating then being susceptible to radiation in any practical way, even at the levels in IEC/EN 61000-6-2 & CISPR 35 for 61000-4-3/4-6/4-8 where susceptibility would be far less likely than radiated.



Well at a penny or two (+/-), and from experience, I "sprinkle" ferrites all the times as would most good engineers. Slowing down power MOSFET edge speeds without taking a performance hit, high frequency power supply noise rejection to improve RF performance, reduce high frequency ringing on diodes, for differential and common mode rejection on data lines leaving the enclosure, etc. Layout and architecture cannot fix everything, and time to market is more important than saving a penny, even when you are making a few million.



And I will go back to what I said before. The naivety is yours for taking a requirement for one industry / product and assuming it is necessary or correct for products in other industries.
Just because the standards are low in audio doesn't mean bad practices are good. Chill out.

I meant the usual bodges btw, clamp-on devices clearly added after the fact, copper tape, spray coatings, etc.

Yes, we know you're some god of audio design, congratulations.
 

chris719

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75ohm was the target impedance for S/PDIF. There's no standard however for master clock distribution. But you're right, the relevance of this mismatch over the distances being discussed is somewhat irrelevant, especially considering a modern DAC's jitter resistance.


I think you've got your heard so far buried in theory you've lost sight of what we are doing. Of course the galvanic isolation of a transformer is imperfect. But it doesn't need to be perfect to achieve it's goals, especially not at 100MHz. The point is to prevent circulating ground currents between chassis in frequencies that are audible, or which can effect electronics to produce an audible result.

Secondly picking appropriate connections to your application isn't a compromise. It's just a common engineering practice that recognises you can't do something in isolation (pun intended).

Catching up on the rest of the thread you seem to have a heavy focus on EMC compliance. This is not the purpose nor is it relevant to the galvanic isolation of S/PDIF inputs.

It comes as part of the package if you are caring about spurs -130 dB down as it seems this thread does.

Again, why would you prefer coax if jitter is not an issue?
 

audio2design

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Just because the standards are low in audio doesn't mean bad practices are good. Chill out.

I meant the usual bodges btw, clamp-on devices clearly added after the fact, copper tape, spray coatings, etc.

Yes, we know you're some god of audio design, congratulations.

Over designing really does not benefit anyone. The standards are much the same as any other product that goes into the home. Not a god just not calling someone else naive out of ignorance.
 

chris719

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You keep repeating that claim, yet you persistently refuse to give a single example of actual equipment that suffers measurably from this so-called problem.

Don't bother, he's stuck in 1995 with Cirrus and Yamaha receiver ICs.
 

chris719

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Over designing really does not benefit anyone. The standards are much the same as any other product that goes into the home. Not a god just not calling someone else naive out of ignorance.

This is a forum that fetishizes over-designing. I wouldn't call doing the right thing over-designing either.
 

audio2design

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It comes as part of the package if you are caring about spurs -130 dB down as it seems this thread does.

Again, why would you prefer coax if jitter is not an issue?

The spurs are at <20KHz. RF would both have to be at a high enough level, find a sensitive node that is not shunted for high frequencies (and sensitive to them), and normally have an AM modulation component at audio frequencies in order to impact the audio band. Periodic transmissions can do that, but spread spectrum greatly decreasing impact and that is pretty much standard for almost everything of any RF power today in a home. Turntable inputs can be sensitive to very close AM radio transmissions because they are high impedance but we are talking DACs.
 
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