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Battle of S/PDIF vs USB: which is better?

amirm

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#1
I was recently asked about merits of S/PDIF versus USB for audio DACs. It has been said that S/PDIF was designed for audio whereas USB is a computer interface. And that makes USB noisier and less desirable interface.

I think most of you know my opinion on this. But just in case, I believe USB to be a superior and more "correct" interface for audio. Problem with S/PDIF is that it makes the source the "master," forcing the DAC to chase its timing. This means that if the source S/PDIF signal is not very clean, it can impact target DAC performance. Fortunately over the years S/PDIF interface has been perfected a lot and even in low cost implementation it can be excellent.

Still, there is no reason to have this antiquated architecture. Using asynchronous mode USB, the DAC can set the cadence using a high-performance clock and force the source, in this case a computer or streamer, to follow it.

Yes, there is some risk of noise here as USB is a much more complicated interface than S/PDIF. Dedicated processors are used to implement it and if not isolated from the sensitive DAC analog circuit (including its clock), we can still get polluted output. Fortunately this has also been sorted out for the most part and in high performance DACs it simply is not an issue.

Measurements
Words are one thing, data is another. So let's see measure the performance of S/PDIF and DAC on the same USB and see how they differ. Alas, USB seems to have taken the market by the storm and increasingly DACs have only USB input. I had to dig deep in my stash of DACs to find the ones that still have S/PDIF input. Here is their performance on the venerable J-Test signal at 48 Khz/24-bit.

The source is my usual HP z series laptop. In the case of S/PDIF I am using an external USB to S/PDIF converter from Audiophilleo. I bought it 5+ years ago so it is long in the tooth and it was costly at the time ($500). It is however very good quality.

First up is a DAC that i have not yet posted the review for it: the ifi iDSD. So this is a sneak peak into that. This DAC retails for around $449. This has a really funky and poor user interface in that there is no switch in there to select different inputs. It auto switches based on what it thinks is active. In my case, once I plugged it into USB for power, it would refuse to take input from S/PDIF. So for S/PDIF input I had to take the USB away forcing it to run on batteries. So it is possible the distortions that are seen are from the battery mode. Hard to say. Here are the results:

ifi idsd.png


What we see here is that while the general response is the same, using S/PDIF caused those jitter components to show up (in yellow spikes).

Winner: USB

Next up is another DAC I am testing, the Topping D30:

d3.png


Here the difference is quite pronounced. S/PDIF has noticeably higher noise floor to the tune of 13 to 14 db! So if you are worried about noise, S/PDIF interface has more of it with this DAC.

Winner: USB

Now let's look at Musical Fidelity V-DAC II which I tested recently. This is a now obsolete DAC so it reasons that is S/PDIF should be more mature than USB: (please ignore the ifi graph)

J-test.png


Looking at the Cyan and Green graphs we see that they are essentially the same.

Winner: Tie

Now let's jump way high in price point and look at my new Exasound E32 DAC which retails for $3,500. Review will be upcoming but for now, here is its performance on USB vs S/PDIF:

Exasound.png


As we see there is zero difference between the two interfaces as expected from a properly engineered DAC.

Winner: tie

Edit:
Here is a late addition, the Musical Fidelity V-DAC II
Musical Fidelity V-DAC II.png


The same theme continues: USB performs just as well as S/PDIF even in this older DAC.

Winner: tie

Edit 2:
another addition in the form of S.M.S.L. Mini DAC Sanskrit



S/PDIF output is full of jitter components so audibly worse than USB.

Winner: USB

Does it sound different?
No, none of these measurements indicate any audible difference. The measurements where different, show really small levels of noise and distortion. I can't make a case for audibility of any of it.

Conclusions
As seen from pure measurement point of view, there was no case where USB was worst than S/PDIF. Indeed the reverse was true in that S/PDIF was noisier in some DACs. So the default assumption that somehow S/PDIF has some advantage over USB is simply false.

Given the convenience of USB when using computers and networked audio players and streamers, I see no reason to use S/PDIF. You get a superior architecture with no need to get yet another dongle to create S/PDIF out of your computer.

The other great thing about USB is that it will be here for all time. It is so prevalent in computer world that it has no chance of dying.

Not covered is higher functionality of USB with respect to support for uncompressed multichannel, DSD, etc. and using USB becomes a no brainer.
 
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watchnerd

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#2
To be honest, I think USB vs S/PDIF is yesterday's battle.

When I look at the number of all-in-one / "super-integrateds" (integrated amp/renderer with digital/network inputs) coming to market (Auralic, Hegel, NAD, Technics, Devialet, NAIM, Classe, etc, etc) the new question is:

Ethernet vs WiFi
 

Thomas savage

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#3
To be honest, I think USB vs S/PDIF is yesterday's battle.

When I look at the number of all-in-one / "super-integrateds" (integrated amp/renderer with digital/network inputs) coming to market (Auralic, Hegel, NAD, Technics, Devialet, NAIM, Classe, etc, etc) the new question is:

Ethernet vs WiFi
True but as I walked blindly into audio I was alway told how terrible USB was, how noisy it was and how it was not designed for audio and needed all this 'extra' stuff ( widgets) to make it good.

Clearly that's all ....., bollocks .
 

watchnerd

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#4
True but as I walked blindly into audio I was alway told how terrible USB was, how noisy it was and how it was not designed for audio and needed all this 'extra' stuff ( widgets) to make it good.

Clearly that's all ....., bollocks .
That's true, it wasn't designed for audio.

It was designed for digital.

Oh, wait....
 

BE718

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#5
To be honest, I think USB vs S/PDIF is yesterday's battle.

i
Looking at the proliferation of USB cleaners on the market, I would say not
 

Old Listener

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#6
audiophile fads come and go. However, no all audiophiles are in sync.

Async mode was the thing a few years ago. Galvanic isolation was the next new thing for a few months.

USB is soooo last year. It is hopeless. SPDIF is retro cool. Ethernet is modern and hip.
 

Purité Audio

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#7
Really interesting article A.
As always good design and implementation triumphs over medium.
Keith
 

Jinjuku

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#9
Too many people look at audio as a supply side issue. Audio is data, video is data, your email to mom is data, your word document is data.

What has always mattered in any of these scenarios is the peripheral that takes that data and interfaces us humans with it:

Monitor, Printer, Speakers, Email Client etc...

That a certain segment in this hobby don't get it despite all the parallels in their everyday life has always baffled me.
 
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#10
I am a big believer in USB being a much better interface. But to say that it without problems is not really true. I don't really know where the problem is, but many times using a PC or laptop to connect to a DAC via USB there definitely is some noise that makes it's way through to the DAC and beyond. Most times the noise is it is a 60hz hum that almost always seems to come from the wall wart the laptop is using, Other times it is a hum that just doesn't go away. Galvanic isolation fixes the problem but even then sometime the DAC doesn't sound as good as if you went via SPDIF. Now I in no way feel that SPDIF is better, but it seems that many times the people who designed these DACs are better at taking the signal from one interface than another. I feel it is mostly has to do with the DAC manufacturer/designer not being familiar enough with USB to be able to reduce or eliminate noise from the the USB ground wire from making it's way into the analog signal. I don't know how you can test for that though.

Hey my first post!
 

Thomas savage

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#11
I am a big believer in USB being a much better interface. But to say that it without problems is not really true. I don't really know where the problem is, but many times using a PC or laptop to connect to a DAC via USB there definitely is some noise that makes it's way through to the DAC and beyond. Most times the noise is it is a 60hz hum that almost always seems to come from the wall wart the laptop is using, Other times it is a hum that just doesn't go away. Galvanic isolation fixes the problem but even then sometime the DAC doesn't sound as good as if you went via SPDIF. Now I in no way feel that SPDIF is better, but it seems that many times the people who designed these DACs are better at taking the signal from one interface than another. I feel it is mostly has to do with the DAC manufacturer/designer not being familiar enough with USB to be able to reduce or eliminate noise from the the USB ground wire from making it's way into the analog signal. I don't know how you can test for that though.

Hey my first post!
Welcome aboard :)
 

DonH56

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#12
Nice job, Amir.

I suspect some of the difference is the way the DACs implement the interfaces. There are plenty of USB interface chips and plenty of articles on how to isolate the USB side from the audio side. I suspect there are fewer options for S/PDIF and it is less a focus for many consumer DACs. Buffering and reclocking should fix the clocking issues for S/PDIF (?) I have not looked at USB interface specs but it may also be that errors and data packets are better handled in the USB protocol than the S/PDIF protocol. Since my day job is way down at the Phy (analog) interface I don't look much at protocol stuff.

For me, the bottom line is I might as well use USB, since most all of them do better with it and most PCs and audio components have a USB interface.
 

amirm

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#13
Criticism was raised elsewhere for my use of USB to S/PDIF converter. That is people somehow wanted native S/PDIF as a source. I can't think of a better one than my lab grade Audio Precision analyzer to create same. So I went back and re-tested two of my DACs again:

Exasound ex32:

Exasound with AP source.png


If I really, really squint maybe AP generated S/PDIF has a hair lower noise but clearly lost in the noise. :)

Next I tested the d30 which showed a marked improvement with USB:

d3 with AP as source.png


Here we easily see that USB has maintained its lower noise floor. And that S/PDIF as produced by my adapter are the same as far as general noise level. The AP signal does have lower jitter at low frequencies hugging the main signal in the form of those tiny pulses compared to my adapter. But the general noise level remains the same. And regardless, USB is cleaner than it by the same 14 db.

Conclusions
The same message remains. If you have a good DAC, its performance will not vary with USB or S/PDIF. Some DACs may show better or worse performance. Based on my set of DACs, USB is the winner there and at any rate, none of this is audible differences.
 

Sal1950

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#14
I've always struggled with a ground loop hum when using USB or S/PDIF over coax between my tower and DC-1.
I could solve it by doing some things I'm not totally comfortable with, like floating all the power grounds, etc.
Toslink neatly solves it without any negatives I can see here.
 

watchnerd

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#16
I've always struggled with a ground loop hum when using USB or S/PDIF over coax between my tower and DC-1.
I could solve it by doing some things I'm not totally comfortable with, like floating all the power grounds, etc.
Toslink neatly solves it without any negatives I can see here.
There are those who say Toslink sucks because it often has a cap on upper resolution (usually like 96khz) and often has higher jitter.

Me, personally, the only issue I have with optical is the stupid jackhole.
 

Cosmik

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#18
In terms of 'unwanted spectral products' in S/PDIF, differences could show up merely because of tiny differences in clock frequencies that vary from minute to minute due to temperature differences. If the source clock just happens to be matching the DAC's at that particular moment, no adjustments have to take place. If it drifts by 0.01%, suddenly the DAC begins having to adjust itself to match. Inaudible of course, but it might show up in the measurements. In other words no single measurement can ever be definitive with S/PDIF and similar unidirectional systems.
 

Sal1950

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#19
Sal1950, did you accomplish this by installing a sound card with Toslink in the computer, then connecting the computer to the DAC via Toslink?
No, the motherboard had the outputs from the onboard chip.

There are those who say Toslink sucks because it often has a cap on upper resolution (usually like 96khz) and often has higher jitter.
I have no problem doing 192khz over toslink here, jitter is handled by the ASRC reclocking in the DC-1

Me, personally, the only issue I have with optical is the stupid jackhole.
AMEN to that, who ever designed the piece of junk should be horsewhipped. The Apple solution of running the optical tube down the center of a headphone jack was a great improvement.
 

RayDunzl

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#20
Isn't there a Coaxial vs Optical argument to investigate?

disclaimer: *plugs stuff in if it works I figure it works
 
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