I've seen a number of posts on this topic asking which is 'best' - USB or Toslink for connecting to a DAC. As ever, 'best' is not always easy to define, and "one man's meat is another's poison" and all that. I've been experimenting with kit and connections recently, and have the following to offer.
First, a very Noddy introduction to the two. From a data transfer pespective, each of these approaches has a distinct advantage and disadvantage: Toslink provides electrical isolation of the DAC from the source, but the clock controlling the data is in the 'push' device, normally a relatively poor quality device (leading to higher levels of 'jitter' when compared with the clock in DAC); asynchronous USB transfers the control of data flow to the (normally much better) DAC clock (reducing levels of jitter), but the two devices are (normally) connected electronically, so any electronic interference from the source device can affect the DAC. I say 'normally' for the USB connection, as there are power supplies available that provide the DAC with a separate ultra-linear DC supply that is isolated from the supply to the computer/streamer.
I normally stream music to my DACs (Audiolab 8200CDQ, Beresford Bushmaster, and Audiolab M-DAC) via iTunes and Airplay using Apple Airport Express (AE) units and Toslink optical, sourced using wired ethernet connections. I have 2 x gen 1 AE, and 1 x gen 2 Apple AE - more on that later. Intrigued by the difference that USB connection would make (or not), I recently bought a Raspberry Pi to experiment. Just to put the units in context, the AE are £100 each, whilst the RPi is about £50 by the time you add a power supply, cables, case, and SD Card. Using the excellent Volumio software on the RPi, I was soon able to stream iTunes to a DAC using the USB connection, but the RPi would pop, click, or drop-out every 6-8 seconds, and was too annoying to use. I tried a different (more expensive!) USB cable, and this did seem to make a (very slight) difference, reducing the pops/clicks to once every 9-10 seconds or so. However, there appears to be an issue using wired ethernet and USB out from the RPi, as I explained in a previous post on the Volumio forums:
http://volumio.org/forum/hip-pop-with-rpi-and-usb-dac-t662.html
Whilst the USB connection to the DAC was too annoying for regular use, I did notice that the sound from my main system seemed to be slightly different wth the USB connection (in between the pops and clicks!). 'All' that had changed was that
iMac -> router -> AE -> Toslink -> 8200CDQ -> amps & speakers had become
iMac -> router -> RPi -> USB -> 8200CDQ -> amps & speakers. So, not a massive change in the overall chain from source to DAC, and the amps, speakers, room and set-up remained identical.
Through USB, I thought I noticed that the soundstage seemed wider, and that somehow the music seemed clearer, but it wasn't easy to pinpoint what was different (if anything). I was also very aware that it might just be down to expectation bias, and that there may be no real difference at all, although I couldn't help but feel that it was 'better' via USB. At that time, I had only been using one track that I know extremely well - the live acoustic version of "Hotel California" off The Eagles' "Hell Freezes Over" (ripped using ALAC on iTunes).
It was when the next one of my usual testing tracks came on that I noticed a definite difference, and afterwards swapped back and forth between the AE (Toslink) and RPi (USB) a few times to ensure that what I was hearing was correct. The track is "Limit to Your Love" off James Blake's self-titled first album, and has a massive sub-bass element to the track. Via Toslink through the Apple AE, the sub-bass is very obvious (it's AWESOME!), but the specific notes of the sub-bass were difficult to pick out. Using USB from the RPi, the individual sub-bass notes (i.e. the difference in frequencies) are very clear and obvious. Similarly, the kick drum at the start of the Red Hot Chili Peppers "Dani California" is much clearer and the tone of the drum is very obvious, as are the notes on the the double-bass in Diana Krall's beautifully-recorded "Temptation". In every case, USB gave a clearer and more accurate presentation of the bass notes and (I believe) a slightly warmer and more realistic sound overall. However, that is much more difficult for me to quantify or give clear and definite examples - it is much more a 'sense' and 'feeling' that things were better with USB.
In my testing system (RPi -> USB -> 8200CDQ), I didn't notice any electrical interference or loss of SQ using USB, and I was only using a cheap power supply to the DAC. That said, the 8200CDQ has its own mains power supply, so it is not drawing any current from the power supply to the RPi. If you were to use a DAC that draws power from the supplying USB connection, that would almost certainly benefit from a better power supply than the cheap one that I bought for the RPi. I have also experimented with an on-board DAC for the Raspberry PI, but that is the subject of another story - more via the links below if you're interested.
http://www.whathifi.com/forum/hi-fi/my-raspberry-pi-hifiberry-music-server#comment-2933830
http://volumio.org/forum/rpi-hifiberry-dac-t879.html
I mentioned the Apple Airport Express units above. They are very handy devices, and I'm sure that many people (like me) love the simplicity and true 'plug and play' nature of Apple products. However, the gen 2 AE is MUCH worse in terms of jitter than the gen 1 devices. How do I know? Because the 8200CDQ continuously loses 'lock' with the gen 2 AE, but works just fine with the gen 1 boxes. The M-DAC has a function to alter the tolerance to jitter, and it needs to be set to 'high' for the gen 2 box, but will work on any setting with the gen 1 AE units. I should mention that all of my testing was carried out using the gen 1 box, i.e. the lower jitter device, so if you were to swap from a gen 2 AE via Toslink (assuming that your DAC could deal with the jitter, as the Bushmaster can) to USB, you may get an even bigger difference (improvement). Conversely, if you are connecting your DAC directly to a high-quality computer, the difference that you observe may be much smaller than for me, where the 'weak link' appears to be the AE hanging off the end of my router and house-wide gigabit (CAT-6) ethernet LAN.
Summary. For me, USB is definitely a little bit better than Toslink, mainly because lower frequencies are definitely better reproduced (on my system), and the overall sound is slightly warmer and more natural, although that is much less obvious, and some of it may just be in my head. The whole testing process was really interesting, and has led me to invest in new boxes and kit to ensure that I can get the 'best' supply of digital data into my DACs. I hope that's been useful for people, and helps to inform their streamer/DAC choices.
Enjoy the music!
First, a very Noddy introduction to the two. From a data transfer pespective, each of these approaches has a distinct advantage and disadvantage: Toslink provides electrical isolation of the DAC from the source, but the clock controlling the data is in the 'push' device, normally a relatively poor quality device (leading to higher levels of 'jitter' when compared with the clock in DAC); asynchronous USB transfers the control of data flow to the (normally much better) DAC clock (reducing levels of jitter), but the two devices are (normally) connected electronically, so any electronic interference from the source device can affect the DAC. I say 'normally' for the USB connection, as there are power supplies available that provide the DAC with a separate ultra-linear DC supply that is isolated from the supply to the computer/streamer.
I normally stream music to my DACs (Audiolab 8200CDQ, Beresford Bushmaster, and Audiolab M-DAC) via iTunes and Airplay using Apple Airport Express (AE) units and Toslink optical, sourced using wired ethernet connections. I have 2 x gen 1 AE, and 1 x gen 2 Apple AE - more on that later. Intrigued by the difference that USB connection would make (or not), I recently bought a Raspberry Pi to experiment. Just to put the units in context, the AE are £100 each, whilst the RPi is about £50 by the time you add a power supply, cables, case, and SD Card. Using the excellent Volumio software on the RPi, I was soon able to stream iTunes to a DAC using the USB connection, but the RPi would pop, click, or drop-out every 6-8 seconds, and was too annoying to use. I tried a different (more expensive!) USB cable, and this did seem to make a (very slight) difference, reducing the pops/clicks to once every 9-10 seconds or so. However, there appears to be an issue using wired ethernet and USB out from the RPi, as I explained in a previous post on the Volumio forums:
http://volumio.org/forum/hip-pop-with-rpi-and-usb-dac-t662.html
Whilst the USB connection to the DAC was too annoying for regular use, I did notice that the sound from my main system seemed to be slightly different wth the USB connection (in between the pops and clicks!). 'All' that had changed was that
iMac -> router -> AE -> Toslink -> 8200CDQ -> amps & speakers had become
iMac -> router -> RPi -> USB -> 8200CDQ -> amps & speakers. So, not a massive change in the overall chain from source to DAC, and the amps, speakers, room and set-up remained identical.
Through USB, I thought I noticed that the soundstage seemed wider, and that somehow the music seemed clearer, but it wasn't easy to pinpoint what was different (if anything). I was also very aware that it might just be down to expectation bias, and that there may be no real difference at all, although I couldn't help but feel that it was 'better' via USB. At that time, I had only been using one track that I know extremely well - the live acoustic version of "Hotel California" off The Eagles' "Hell Freezes Over" (ripped using ALAC on iTunes).
It was when the next one of my usual testing tracks came on that I noticed a definite difference, and afterwards swapped back and forth between the AE (Toslink) and RPi (USB) a few times to ensure that what I was hearing was correct. The track is "Limit to Your Love" off James Blake's self-titled first album, and has a massive sub-bass element to the track. Via Toslink through the Apple AE, the sub-bass is very obvious (it's AWESOME!), but the specific notes of the sub-bass were difficult to pick out. Using USB from the RPi, the individual sub-bass notes (i.e. the difference in frequencies) are very clear and obvious. Similarly, the kick drum at the start of the Red Hot Chili Peppers "Dani California" is much clearer and the tone of the drum is very obvious, as are the notes on the the double-bass in Diana Krall's beautifully-recorded "Temptation". In every case, USB gave a clearer and more accurate presentation of the bass notes and (I believe) a slightly warmer and more realistic sound overall. However, that is much more difficult for me to quantify or give clear and definite examples - it is much more a 'sense' and 'feeling' that things were better with USB.
In my testing system (RPi -> USB -> 8200CDQ), I didn't notice any electrical interference or loss of SQ using USB, and I was only using a cheap power supply to the DAC. That said, the 8200CDQ has its own mains power supply, so it is not drawing any current from the power supply to the RPi. If you were to use a DAC that draws power from the supplying USB connection, that would almost certainly benefit from a better power supply than the cheap one that I bought for the RPi. I have also experimented with an on-board DAC for the Raspberry PI, but that is the subject of another story - more via the links below if you're interested.
http://www.whathifi.com/forum/hi-fi/my-raspberry-pi-hifiberry-music-server#comment-2933830
http://volumio.org/forum/rpi-hifiberry-dac-t879.html
I mentioned the Apple Airport Express units above. They are very handy devices, and I'm sure that many people (like me) love the simplicity and true 'plug and play' nature of Apple products. However, the gen 2 AE is MUCH worse in terms of jitter than the gen 1 devices. How do I know? Because the 8200CDQ continuously loses 'lock' with the gen 2 AE, but works just fine with the gen 1 boxes. The M-DAC has a function to alter the tolerance to jitter, and it needs to be set to 'high' for the gen 2 box, but will work on any setting with the gen 1 AE units. I should mention that all of my testing was carried out using the gen 1 box, i.e. the lower jitter device, so if you were to swap from a gen 2 AE via Toslink (assuming that your DAC could deal with the jitter, as the Bushmaster can) to USB, you may get an even bigger difference (improvement). Conversely, if you are connecting your DAC directly to a high-quality computer, the difference that you observe may be much smaller than for me, where the 'weak link' appears to be the AE hanging off the end of my router and house-wide gigabit (CAT-6) ethernet LAN.
Summary. For me, USB is definitely a little bit better than Toslink, mainly because lower frequencies are definitely better reproduced (on my system), and the overall sound is slightly warmer and more natural, although that is much less obvious, and some of it may just be in my head. The whole testing process was really interesting, and has led me to invest in new boxes and kit to ensure that I can get the 'best' supply of digital data into my DACs. I hope that's been useful for people, and helps to inform their streamer/DAC choices.
Enjoy the music!
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