USB to SPDIF with Word Clock input??? - diyAudio
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Old 19th May 2008, 09:28 PM   #1
mr-mac is offline mr-mac  Scotland
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Default USB to SPDIF with Word Clock input???

Hi,

I was wondering if anyone could think of a fairly inexpensive. USB breakout box that has SPDIF output that will accept a word clock input?

Hopefully I want to connect a PC to my Pink Triangle Da-capo via the clock locked route the same as the Cardinal transport does.

However it isn't easy to find a USB device at a reasonable price that will accept a master clock input.

Any ideas greatly appreciated.

John

PS think the da-capo outputs a 128Fs master clock
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Old 19th May 2008, 09:57 PM   #2
giulio is offline giulio  United Kingdom
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emu 0404usb?
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Old 19th May 2008, 10:36 PM   #3
peufeu is offline peufeu  France
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ALL cheap USB audio devices use the same usual suspects chips like PCM270x which derive the clock from the USB port and don't support any form of digital input.

You will have to look into custom USB, firewire, or the good old PCI soundcard like RME.

I had a Terratec DMX 6Fire USB for a few days, inside there is the (also very usual) Cypress USB chip and a small FPGA, so it implements asynchronous USB. it is quite expensive and sounds absolutely horrible, really the worst soundcard I've EVER heard, but that's because of the $1 codec and the $0.2 opamps and the huge amount of jitter due to the veru crummy clocks with exceedingly noisy power supplies, not the USB async technology per se.

Conclusion : get a PCI soundcard
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Old 19th May 2008, 10:41 PM   #4
peufeu is offline peufeu  France
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Quote:
Originally posted by giulio
emu 0404usb?
The specs say :

Ultra-low jitter clock subsystem: < 500ps RMS in PLL mode (48kHz, Coaxial S/PDIF Sync)

This would rather qualify as "ultra high jitter" IMHO, lol. Just "acceptable jitter" should be below 10 ps... as for the Ultra I'll let you guess...
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Old 20th May 2008, 08:52 AM   #5
mr-mac is offline mr-mac  Scotland
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surely there must a be resonable USB or firewire solution that isn't stupid low quality.

The amount of noise that must be injected into an internal sound cards circuits through PSU and other interferance must be esspecially bad for a digital out signal.

A USB or firewire (esspecially one with it's own PSU which I could replace with a higher quality PSU at some point) must have such an advantage over internal to start with.

When I say cheap I am thinking up to £200 and also thinking second hand is also an option to get somthing better for the money. Besides does the quality of the DAC really make any diffrence at all if I am jsut using it as a clock locked digital out?

John
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Old 20th May 2008, 09:53 AM   #6
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Quote:
Originally posted by mr-mac
surely there must a be resonable USB or firewire solution that isn't stupid low quality.

The amount of noise that must be injected into an internal sound cards circuits through PSU and other interferance must be esspecially bad for a digital out signal.

A USB or firewire (esspecially one with it's own PSU which I could replace with a higher quality PSU at some point) must have such an advantage over internal to start with.

When I say cheap I am thinking up to £200 and also thinking second hand is also an option to get somthing better for the money. Besides does the quality of the DAC really make any diffrence at all if I am jsut using it as a clock locked digital out?

John
The PCM270x are very good considering what they do. With all the digital on the chip, even powered from USB, they are impressively good. Not good enough, of course, but I wouldn't call them stupid low quality.

Unfortunately doing a USB audio transfer in slave mode is much more difficult for implementers. I don't think you'll see an integrated IC like the PCM270x with that feature. Unfortunately, the USB Audio Class that these devices implement specifies a host-controlled clock, and there is no provision for slaving the clock to another device. As a result any solution offering that feature will need both drivers and firmware written. As far as I know there aren't any DIY solutions, and that leaves you with commercial products where the implementation details to the level you're looking for are not generally available prior to purchase. Given that there's no compelling reason to include this feature in a USB device, and the difficulty with which it would be implemented, I don't think you'll find too many choices.

The situation with Firewire is a bit better in that it's easier to implement. However, there are very few ICs out there that will do it, and the target market is much smaller.

I would also suggest that PCI is the way to go here. You get benefits like low latency, easily accessible circuit board, and a variety of devices that can do this out of the box. Given that some of the cards in your price range are used for real professional work and have impressive specifications, I would think the 'internal sound cards are inherently crappy' assumption doesn't hold a lot of water, especially if you're not trying to use the onboard analog sections. Remember that neither USB or Firewire isolates the grounds, so you're going to be getting some noise coming in anyway.

If none of that is good enough for you, peufeu's ethernet DAC project is probably your only real option. It aims to solve all of the concerns above.
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Old 20th May 2008, 10:12 AM   #7
mr-mac is offline mr-mac  Scotland
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Ok no probs, thanks for advice.

Was looking for USB/Fireware as I would have liked to be able to use it from a laptop rather than a desktop.

maybe I should look for a PCI card that has a break-out box that runs from it's own PSU. I could use an older laptop (IBM X21) as I can get a docking station that will allow me to use a half width PCI card. I didn't say internal cards are inherently crappy. I do still stand by the fact that any internal device really does have a lot to contend with inside a PC meaning it is always starting at a disadvantage.

Still a shame as I was rather hoping for a device that wouldn't tie me to using it with a single PC.

Not really wanting to go down a 10/100 DAC route as I have a Pink Triangle Da-Capo with DC PSU which I think is really one of the best DAC's I have heard.

Kind Regards

John
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Old 20th May 2008, 10:31 AM   #8
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Well, I don't want you to think that it's not possible - there are definitely devices out there that can do this. The problem is going to be whether or not they're good enough for you. Most don't publish jitter specs.

RME Fireface is out of your price range, but it definitely does this. Alesis has the io|14 and io|26 that probably can. M-Audio has Firewire 410 that's capable. There are probably more out there, but I can't find jitter specs for any of these. Maybe you can get a local music shop to lend you a display model or something and you could try it out?
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Old 20th May 2008, 10:48 AM   #9
peufeu is offline peufeu  France
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Actually you don't need anything fancy, here is how you do soundcard clock slaving :

- your DAC has a master clock (low jitter)
- your DAC outputs this clock in the form of a blank SPDIF signal containing silence. (this needs a SPDIF encoder chip like CS8406, pretty easy to do).
- your DAC receives a SPDIF signal and expects it to be synchronous with its own clock, so the synchronous reclocking works.

Now, take a soundcard, ANY soundcard that has SPDIF inputs and outputs, even your laptop's internal soundcard or your PC's mobo soundcard or a RME or really anything, provided that it doesn't do stupid things like using an ASRC or resampling everything to 48k, and that it has digital IN and OUT.

Plug your DAC's SPDIF out in the card's SPDIF IN, plug the card's SPDIF OUT in your DAC SPDIF in, and the card should sync, and it should work.
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Old 20th May 2008, 10:49 AM   #10
mr-mac is offline mr-mac  Scotland
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Is there a reason for jitter being part of my selection criteria.

If I send a Master Clock from the DAC which the usb/firewire device locks to and then sends the digital data to the DAC the dac doesn't use a recovered clock from the data. It uses it's own internal clock meaning source induced jitter shouldn't be an issue.

Or am I missing somthing?

John
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