[aos]

That USB DAC in the DIY forum should cost you less than $100 but check in there, I believe some people listed how much it cost them to buy parts. Note that you'd have to modify what's there in order to add an actual SPD/IF out, coax or optical, and probably use a slighly different chip.

 

[ampgalore]

Harman kardon DAL-150.

One catch, the box is an mp3 decoder. Only mp3 songs played in the Windows media player can be outputted.

 

[aos]

There seems to be a commercial ASIO driver for USB. Interesting thing is that Elektor audio-dac project from 12/2000 is supported, but only at 44.1kHz (which is most important anyway). But that project from what I can find uses PCM2702 which is a variation of PCM2705 that I use and that works best at 48kHz. What the...? When I get home I'll download the demo and try it.

http://www.usb-audio.com/

 

[AdamZuf]

Quote:

Originally Posted by aos That USB DAC in the DIY forum should cost you less than $100 but check in there, I believe some people listed how much it cost them to buy parts. Note that you'd have to modify what's there in order to add an actual SPD/IF out, coax or optical, and probably use a slighly different chip.

i didn't meen the USB DAC. i refered to that USB->coax you talked about..

 

[aos]

The USB DAC project *is* what you'd use as a starting point to get the coax out. Basically, some of the chips have the digital out pin - TTL level. You'd feed that to an optical transmitter (trivial and simplest - but you want coax) or you'd have to implement TTL to COAX circuit with a few logic elements (schematics can be found on the web via Google). If you don't know how to do it maybe you can ask whoever designed the board to modify it in order to add coax. It's a useful thing to have so they should be interested.

 

[Wodgy]

Quote:

Originally Posted by aos Actually there just doesn't seem to be anything that works for certain with the standard USB interface - if I use laptop with winxp then it's 16/48 that is (probably) bit perfect - but 16/44 graphs for thd/imd look horrible. On the other hand 16/44 from windows 2k looks very clean, except that there is a hf roloff which reeks of filtering - but the graphs are so clean that the filter must be good. Any other resolution/bit rate and you can see evidence of signal processing all over the measurements. It seems to me that 16/48 is meant to be USB standard and that usually works well, but anything else, including 44kHz is being processed and the quality of processing varies wildly even with different windows versions.

I've noticed similar things (differing performance on Win2000 and XP), but I don't agree with your conclusions that 48kHz is the "native" mode for USB audio. The resampling you get oftentimes is an interaction between the Windows USB audio driver and kMixer, the Windows kernel mixer.

Since the PCM270x series uses the built-in Windows drivers, it's not clear if there's anything anyone can do to fix this, though your suggestion to use ASIO (or even Kernel Mixing) is worth a try. However, these USB devices should work properly with no resampling on Mac OS X and Linux. Linux in particular is anal about never resampling.

The best way to check if you're getting a clean digital output is to try playing back 44.1kHz DTS-encoded or Dolby Digital-encoded files to a home theatre receiver. This test stays entirely in the digital domain.

Looking at analog measurements to try and determine whether you're getting clean output from USB is near useless, IMHO, because it may only show gross distortions of the digital signal (i.e. more subtle distortions may be concealed). Several online "hardware review" websites have been caught publishing incorrect claims that certain driver/OS/hardware combinations are bit-perfect, when in fact they're not, because the reviewers were solely using analog measurements.

 

[aos]

I noticed that when resampling is done, it usually shows plenty of artifacts above the noise floor. A very good filter could avoid that though, so it isn't a good enough test, I agree.

Using different modes on that popular windows playback software (can't recall the name) did not help. I'll try Linux - that's a good suggestion.

 

[momerath]

Is anyone actively working on this right now? I'd like to help, but I dont know where to start. From what I've read, my understanding is that a PCM270x doesn't need drivers written and has spdif out, but that we don't yet know for sure whether it can output 44.1/16 bit-perfectly. I could try to build the hardware starting with the 2702-based usb dac design, but that is a daunting enough task for me that I wouldn't want to undertake it only to find that its not bit-perfect. If someone has done this and can report that it is bit-perfect, then I'd be happy to pay them well to make me one.

 

[audioengr]

I routinely install modified Transit hardware inside DAC's. I purchase the hardware OEM from M-Audio. I can easily create a USB to S/PDIF coax converter that has an optimized S/PDIF coax output tuned to precisely 75 ohms and is galvanically isolated. I can even install a short 75 ohm cable attached to the box, so that you just place it behind your DAC and plug it in. No extra cables required. I would probably install a more stable clock in the converter as well. The transit hardware comes with the S/W drivers and passes bit-perfect 16-bit and 24-bit/96kHz data.

I have been thinking about such a product. It would not be particularly pretty, but would be heavy-duty and use the best components. And BTW - powering the transit off the computer seems to have no impact on the sound quality, but I could also create a version with a wall-wart supply if thats what people wanted.

Steve N.
Empirical Audio
http://www.empiricalaudio.com

 

[sleepkyng]

why would i use that?

i might as well just buy the transit

 

[audioengr]

Because the transit only outputs Toslink. Besides, it needs mods to sound really good.

 

[thomaspf]

I could not find any advantage of that commercial USB audio driver over using asio4all with the Windows system driver.

With asio4all over the generic Windows USB audio driver you get bit perfect playback at 16/44.1 and as far as I can tell any other sampling rate or bitdepth up to 24/96 as well.

The one limitation that you are still stuck with is the lack of AC3/DTS pass through to an external processor but the last time I checked the commercial USB-audio can't do that either.

I understand the benefit of the commercial driver that comes with the M-audio cards is that you can switch it into a mode in which it will work with all PCM streams as well as AC3/PCM from a software DVD.

The idea of building an USB audio driver that implements the profile for an adaptive sink is a great idea. USB is fast enough to allow for asynchronous operation with flow control.

Cheers

Thomas

 

[sunshinedawg]

Quote:

Originally Posted by audioengr I have been thinking about such a product. It would not be particularly pretty, but would be heavy-duty and use the best components. And BTW - powering the transit off the computer seems to have no impact on the sound quality, but I could also create a version with a wall-wart supply if thats what people wanted. Steve N. Empirical Audio http://www.empiricalaudio.com

What would such a product consist of? I would think that you would use a turbo superclock. If you were to design something from scratch, what components would you use?

 

[00940]

for those who opened the transit : which USB receiver chip do they use ?

I'm curious because i'm working an USB DAC. The pcm2707 is very nice because it offers direct I2S out but TI made the SPACT clock system unbypassable and xtal clocks are about useless with that system. The only other chips with USB in-I2S out I've come across are from Philips but they require some programming (while the PCM2707 is hardware configurable).

Check here for a simple PCM2902 based project for Spdif out : http://hepso.dna.fi/misc/pcm2902/PCM2902_usb_dac.html

 

[ooheadsoo]

How about a nice USB to BNC device? This would be ideal, I think, and could replace my emu 1212m.