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:
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:
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)
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:
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
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.
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:
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:
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)
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:
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
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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