audio over USB v. optical

RedGTiVR6 · 02-06-2007, 09:33 AM

Let's just say that you have an outboard unit (like a DAC) that will convert the audio stream from digital to analogue.

What do you see as the pros and cons to USB over optical?
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EDIT: Since this thread has taken on it's own identity, I'm going to post this up here at the top to help explain what's going on here and what my intent is with this idea.
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This product, this idea, is all designed as a high end solution that does not currently exist for the relatively small population out there that is interested in high end audio. It was never intended as a vast consumer solution, as there are already plenty of those avaliable.

Everyone needs to remember, when starting out designing a product, you cannot implement the features that everyone wants, and you cannot implement the one off features that one or two people want. This product needs to be useable to the most people possible (the most high end users at this point). As such, the list below has been set forth at this point in time.

We now have a list of desired features:

USB connection to the PC (this is because USB is most readily avaliable, Firewire will be next once this is underway)
5 pair of RCA outputs (again, this will be adjusted once we know things work)
- at least 4V outputs, prefered to have 8V
- socketed RCA sockets - so that the end user can opt to upgrade the sockets if desired
User replaceable OpAmps
- User replaceable caps would be nice, but not necessary.
12v DC power supply, able to be run on unregulated 12v DC would be even better.
Encased in an external enclosure - much like a DAC or an external sound card
Size is of concern, we need it to be small enough that it's easily able to be hidden. Nothing like the size of the DBX drive rack and the like. Something along the same size as the Alpine 701 brain.

with this we can now start to look at building a prototype. I've contacted Douglas and asked him to take a look at this thread and if he wanted to register to give us some input. We'll see. Else I've got a few other contacts I can go after.

JC-S60 · 02-06-2007, 10:17 AM

The biggest pro for an USB DAC over spdif is the fact that it is two-way communication with acknowledgement of received packets by the receiving end - and a re-send of the packet if it got lost along the way (no ack).

Simply put: there is no jitter caused by the transport of audio over USB, while spdif can have some jitter.

Joachim

durwood · 02-06-2007, 10:31 AM

I thought there still was jitter across usb since it relies on clock cycle. And even though USB is bi-directional I thought that audio over usb is still only one way? Am I wrong? Is there error checking in there somewhere?

scott_fx · 02-06-2007, 10:37 AM

i thought so too

durwood · 02-06-2007, 10:44 AM

Something I found although I know there is a lot more info out there. I remember reading a lot of it but just disregarded because I wanted the computer to do audio processing which ruled out USB DACs.

http://forum.stereophile.com/forum/s...7819&Main=7334

turbocad6 · 02-06-2007, 10:54 AM

I've seen arguments each way.... the only way I've seen to really eliminate jitter is to use an inboard pci or a solution that uses a word clock & a pci board to sync it, but... how much jitter? truth is, it'll probably be an inaudible amount, & if you don't hear it, is it really there?

this may be a good place to ask again, does anyone know if multiple usb dac's can be used, & there any reason this would be a bad idea?

RedGTiVR6 · 02-06-2007, 11:06 AM

I'm not sure on the multiple USB dacs.

I'm asking because I'm tossing around the idea of looking into having an outboard DAC built that would have multiple RCA level outputs with high voltages. Something that has replaceable opamps and possibly other components.

I'm wondering if it would be better to go over USB or over optical.

Seems to me that USB would be the best bet since not all computers have optical outputs.

What options would you think would be desireable for something like this?

I'm thinking about 10 outputs (5 pairs).

Heck, maybe even making the board it's self but making it a project type board. That way, if I wanted to use the WBT sockets, but someone else didn't want to spend that much money, it's all an option. Same with the DACs and with the other components.

Something where you can pick the RCA sockets you want to use. The caps, the opamps, etc.

But since I know literally nothing about doing somehting on this level, it's going to take some collaboration.

This would be something that's an audiophile upgrade for most folks with CarPCs. Once something like this is done, we could approach a developer about integrating all of the software into one program...or something that could be integrated into a front end.

durwood · 02-06-2007, 11:22 AM

I see. Can I bring up another point? What about Firewire? I know it's not as common as USB, but firewire has more bandwidth. Or do we even need to worry about bandwidth? Does Firewire suffer from Jitter? Why aren't there more Firewire Dac's? So many questions...

RedGTiVR6 · 02-06-2007, 11:27 AM

I think the initial version would need to be something that's capable of being used on the most PCs. If it works, then we can look at a FireWire version.

BTW - even though I'm working for StreetDeck/MP3Car.com right now, this isn't something I'm researching as part of the company. It's something I'd personally like to see designed and built. If it takes off then possibly we can have it manufactured. But I think this might be something that's done simply on a hobbiest basis currently.

durwood · 02-06-2007, 11:37 AM

Ok I pose another question for you:

What do you want handling the processing? Were you thinking let the CPU handle it (currently what we have available with the VST effects) or do you want the DAC to handle it all?

comet28024 · 02-06-2007, 12:16 PM

As turbocad said the best way to eliminate jitter is probably running a Wordclock.

This may be entirely not an option, but have you considered running ADAT. You'll have the wordclock to eliminate jitter and can run 8 channels easily.

Felix509 · 02-06-2007, 12:57 PM

Long time lurker, and my first post here... I have done research on this for my home system and have some documents saved on my compter, not sure where they came from exactly, i think from the USB DAC product description pages...

QUOTE::

Basically the DAC has a single digital USB input. USB unlike SPDIF is bidirectional and therefore has error correction and buffering on both sides. This happens automatically so the data on the disk is identical to what is going out all the time. Also since this interface is asynchronous the clocking problems associated with SPDIF go away. What happens is... On power up of the computer the 2 devices negotiate services. In this case the USB DAC tells the computer it can do 16 bit audio at 32K, 44.1K and 48K. Since the USB receiver only has to handle these 3 frequencies, the clocking to the separate DAC IC has almost no jitter. SPDIF actually has to be synched to the exact frequency of the transport (i.e. if the transport is working at say 44.0896K instead of 44.1K the dac has to sync to that frequency). Therefore the jitter problems of SPDIF almost go away using USB. So using USB we have a zero error protocol to link the computer to the DAC and very low jitter what else.....

scott_fx · 02-06-2007, 01:05 PM

Quote: Originally Posted by RedGTiVR6

I'm not sure on the multiple USB dacs.

I'm asking because I'm tossing around the idea of looking into having an outboard DAC built that would have multiple RCA level outputs with high voltages. Something that has replaceable opamps and possibly other components.

I'm wondering if it would be better to go over USB or over optical.

Seems to me that USB would be the best bet since not all computers have optical outputs.

What options would you think would be desireable for something like this?

I'm thinking about 10 outputs (5 pairs).

Heck, maybe even making the board it's self but making it a project type board. That way, if I wanted to use the WBT sockets, but someone else didn't want to spend that much money, it's all an option. Same with the DACs and with the other components.

Something where you can pick the RCA sockets you want to use. The caps, the opamps, etc.

But since I know literally nothing about doing somehting on this level, it's going to take some collaboration.

This would be something that's an audiophile upgrade for most folks with CarPCs. Once something like this is done, we could approach a developer about integrating all of the software into one program...or something that could be integrated into a front end.

hey red,

you may want to contact Douglas; the guy who built the hpdac that i have. He designed and built that dac/amp with good results. maybe he could steer you the right way

turbocad6 · 02-06-2007, 01:18 PM

Quote: Originally Posted by Felix509

Long time lurker, and my first post here... I have done research on this for my home system and have some documents saved on my compter, not sure where they came from exactly, i think from the USB DAC product description pages...

QUOTE::

Basically the DAC has a single digital USB input. USB unlike SPDIF is bidirectional and therefore has error correction and buffering on both sides. This happens automatically so the data on the disk is identical to what is going out all the time. Also since this interface is asynchronous the clocking problems associated with SPDIF go away. What happens is... On power up of the computer the 2 devices negotiate services. In this case the USB DAC tells the computer it can do 16 bit audio at 32K, 44.1K and 48K. Since the USB receiver only has to handle these 3 frequencies, the clocking to the separate DAC IC has almost no jitter. SPDIF actually has to be synched to the exact frequency of the transport (i.e. if the transport is working at say 44.0896K instead of 44.1K the dac has to sync to that frequency). Therefore the jitter problems of SPDIF almost go away using USB. So using USB we have a zero error protocol to link the computer to the DAC and very low jitter what else.....

that quote looks like it may be from the wavelength site.... they seemed to say the same thing I think...

I have searched all over, & just about all of the really high end outboard dac's are just 2 channels....

I have found some really awsome external usb dac's, but nothing with a lot of channels...

I also considered using optical out into a real rack mount dac that has muliple channels, like an apogee... the concern there is, the rack mount will be designed for 110 & the power supply is critical... it would need to be re engineered in the power supply area for sure, but this may be an option? kinda like the ranes used to be done for eq....

at this point I think that all eq & xovers should be on the pc, but I did consider also going passive for the xovers which would lighten the load significantly on the processor, but then phase shift won't be able to be addressed & time alignment, which is critical at least for the sub to me, plus passive is wasteful power wise.....

theres also an outboard firewire dac that has multiple channels & also does some processing... it's called a power core system by tc... looks like it runs plugins on it's own processor & it has ram on board, but I'm not sure if the software for that would meet our needs, it's more designed for recording, which it seems most of the high end stuff is more geared towards... studios don't do what we do with multiple channels for one set of speakers on the output side.... they do mention it running vst plugins, but I'm not sure if that means theres or any...

they do make a smaller more portable power core too, might be the traveler? or I may be just thinking of the motu..., just looked it's called power core compact

scott_fx · 02-06-2007, 01:19 PM

Quote: Originally Posted by Felix509

Long time lurker, and my first post here... I have done research on this for my home system and have some documents saved on my compter, not sure where they came from exactly, i think from the USB DAC product description pages...

QUOTE::

Basically the DAC has a single digital USB input. USB unlike SPDIF is bidirectional and therefore has error correction and buffering on both sides. This happens automatically so the data on the disk is identical to what is going out all the time. Also since this interface is asynchronous the clocking problems associated with SPDIF go away. What happens is... On power up of the computer the 2 devices negotiate services. In this case the USB DAC tells the computer it can do 16 bit audio at 32K, 44.1K and 48K. Since the USB receiver only has to handle these 3 frequencies, the clocking to the separate DAC IC has almost no jitter. SPDIF actually has to be synched to the exact frequency of the transport (i.e. if the transport is working at say 44.0896K instead of 44.1K the dac has to sync to that frequency). Therefore the jitter problems of SPDIF almost go away using USB. So using USB we have a zero error protocol to link the computer to the DAC and very low jitter what else.....

welcome felix. thanks for the useful post