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What's the deal with these USB -> Coax converters? Am I retarded?


Thaddy
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I understand the importance of wanting to bypass all of the Windows filters and crap to achieve bit-perfect output to feed a DAC, but what's with these converters out there with their own dedicated power supplies? Is that really necessary?

I'm not technical by ANY means, but what I do understand is that they are basically taking a digital signal (via USB) and...converting it...to...a digital signal (via Coax). How should that process have any affect on the sound? And if it does, doesn't that mean you just bought a crappy product with a nice external power supply?

Please, enlighten me, my head hurts.

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It seems there are a few factors. Matching impedance, syncronous vs. asyncronous USB, jitter, reclocking, and more small things that seem to affect some of those previously mentioned. The dedicated power supply may affect one or more of those, I'm not sure in how or in what way.

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One thing computers do very poorly is ground plus noise and other garbage riding on the power rails. I for one have a unit that offers the ability to use a separate PSU and there might be a small difference compared to powering it off the USB bus. Now my PSU cost me absolutely nothing (old Iomega Zip power brick) so why not use it...

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Maybe also its just a utility thing. Suppose someone has a computer without an s/pdif coax output, but does has a usb port. Suppose they have a Dac without usb input but with a s/pdif input. Suppose they are non technical and just want to play music from their computer on their Dac without installing additional software? A product such as the above meets the requirements.

Now if you are getting someone to buy such a thing, how do you maximise your profit ;)

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One thing computers do very poorly is ground plus noise and other garbage riding on the power rails.
Would TOSLINK optical then be the best digital cabling method from a computer to DAC in terms of isolation from errant electrical noise (assuming the PC has direct optical output, of course)?
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If you have a good DAC which has S/PDIF input (preferably BNC), you are using a cheap transport, or you want or are forced to use USB or optical, which are less than great, then these things help IMO. Ideally though, the last connection should be 75 Ohm BNC to BNC.

I am limited to USB or Optical with my PC, which is why I decided to pick up one of those coaxial HiFace dongles and a shielded 75 Ohm coax cable to run into my DL III DAC.

PS - Why BNC over coaxial?

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Basically, only a BNC connector used with a proper spdif 75ohm cable will properly transfer the digital signal. RCA, by virtue of it's physical dimensions, is not a proper 75ohm connector. I believe that the devices on each end need to be properly terminated as well, but my knowledge is a bit fuzzy in that regard.

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Basically, buying some device that is supposed to improve digital transmission from your transport is a bit silly if you're going to use a connection method that makes it worse by virtue of not being to spec. I'd say though if it's just a cheap adaptor, no big deal, but I'd say if someone decided to drop $500-1k on one of these things then using RCA would be fail. S/PDIF is only to spec if all the bits in the connection are 75 Ohms. Supposedly if the connection isn't to spec, then you're undoing some of the benefit of the converter by introducing signal reflections. RCA is just used on a lot of gear out of laziness or cheapness and probably because most people don't know any better.

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RCA is used on a lot of gear because it's become the de-facto standard (RCA audio -> RCA composite video -> RCA SPDIF was the general slippery slide down, IIRC). Not using it alienates a very large portion of a possible sales. Not including a BNC as well....that's just laziness/cheapness :)

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I was doing some brainstorming earlier about using a Squeezebox Touch modded with it's I2S lines connected to a LVDS driver to output I2S over HDMI/Cat5 (basically what PS Audio published), to a Buffalo II with a LVDS receiver. I think that'd be a decent transport method for a digital interconnect, eh?

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Hence the LVDS driver on one end and a LVDS receiver on the other. Slightly tricker than just tapping the lines and running it to a jack, but certainly a bit more robust as far as I can tell. Basically just following PS Audio's posted schematics for what they used.

I just haven't figured out where to source a panel mount HDMI jack (so far only found Neutrik's hdmi coupler) and some sort of breakout pcb for a 16-SOIC chip. Might have to try my hand at ordering a PCB if it comes down to it.

Firewire should have been the audio standard for computer use ^-^

Firewire? Wut's that? :P

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Would TOSLINK optical then be the best digital cabling method from a computer to DAC in terms of isolation from errant electrical noise (assuming the PC has direct optical output, of course)?

Or just make sure the receiving end is transformer coupled. No direct electrical connection.

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Aren't you still having the problem of a push protocol, rather than a pull protocol?

You're still driving the Dac's clock from the transport clock (albeit on discrete lines)( = PUSH) , rather than using the DAC's clock, to clock data from the transport ( = PULL).

Yes, you can buffer the data, but where's the flow control to prevent buffer overruns / underruns? Unless you then modify the dac's clock rate, in which case you have just re-invented wow and fluter in the digital domain :)

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Isn't the push vs pull problem more of a DAC and Transport design issue rather than an interconnect issue? I think I've seen some pro gear that have clock input/output for syncing, but I haven't seen anything of the sort within the consumer/audiophile world.

Would something like Tent Lab's tentlink mode be an example of a push? It's still using spdif, but the clocks in the DAC and transport are synced via a separate connection. The only limitation to it AFAIK are the crystal frequencies available.

What would you recommend as a non-defective protocol for transporting digital audio that's non-proprietary and DIY-able? I'm genuinely curious as I've been looking into the whole digital side of things as of late. :)

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One must remeber one thing about BNC, that there isn't just one standard out there. There is also a 50 ohm version which most of the higher end connectors seem to be designed for...

Would TOSLINK optical then be the best digital cabling method from a computer to DAC in terms of isolation from errant electrical noise (assuming the PC has direct optical output, of course)?

Or just make sure the receiving end is transformer coupled. No direct electrical connection.

That basically covers it but Toslink has its own problems but not the more advanced optical systems. I for one just use a transformer coupled BNC system with a true 75ohm coax.

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