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Old 1st August 2004, 03:33 PM   #1
djd4n is offline djd4n  United Kingdom
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Default 2 Stereo in's, 2 Stereo out's, USB DAC

Hi

I'd like to build a 2 Stereo in's, 2 Stereo out's, USB DAC/ADC.

I'm pretty new to working with digital audio, but have experience of building analogue equipment and also computer programming experience.

I have looked at the PCM2902 and wondered if 2 of these could be used with one USB connection? Is there anything similar that would be able to achieve 2 Stereo in's, 2 Stereo out's with one USB connection?

Any help would be very much appreciated,

Thanks

Dan Morton
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Old 1st August 2004, 06:47 PM   #2
Nisbeth is offline Nisbeth  Denmark
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If two switchable inputs are enough, a simple DPDT switch will be enough. Depending on the loadimpedance, the 2902 might be OK with dual outputs. Otherwise a simple buffer stage with an opamp or a BUF634 would have that sorted in no time


/U.
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Old 1st August 2004, 07:14 PM   #3
djd4n is offline djd4n  United Kingdom
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cheers for the reply

i am actually looking to build a device with 2 separate stereo in's and 2 separate stereo out's, i.e. 4 stereo channels, 2 in, 2 out.

I think the best solution using the PCM2902 or similar would be to use 2 of them, in one box, as u can daisy chain usb devices together. This would mean that technically there are 2 usb devices in the box, but daisy chained one after the other.

Does anyone know if it would be possible to get these 2 devices to appear as one in windows or another operating system, customising the driver perhaps?

dan.
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Old 8th August 2004, 08:26 AM   #4
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Hello Dan,
I have some experience with the PCM2902 and PCM2906 (the same device but that reports to windows USB manager a limit current of 500 mA instead of 100 mA).
I have realized these 2 devices with the PCM 2906:

http://purebits.com/usbpreview.html

and tried to use more than one at the same time (attaching to different USB ports). I must say that it work! and you can have a multiple of 2 INs and OUTs..
Anyway I think that between the stereo channels of different devices the sync could be not perfect (although very acceptable for some uses like multitrack music recording of short songs) because the different devices use a different clock.
Also, each PCM2906 implements the "SpAct audio clock recovery architecture" and maybe that even if a single crystal is used for more PCM290x devices, the sync could be slightly different for each couple of channels.
All this in the hypothesis that you need to use at the same time 4 or more inputs and/or 4 or more outputs. Otherwise a simple switch and one single PCM290x can be used, as Nisbeth wrote..

For the rest, the project is very similar to the circuit recommended on the PCM2906 datasheet.. in the big unit (the X4) I added a variable gain preamplifier based on the OPA2350, used also as output buffer since can easily drive an headphone and can use the 5V power supply from the USB port.

Bye,
Paolo
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Old 8th August 2004, 12:24 PM   #5
alecwek is offline alecwek  Denmark
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but then i dont understand ur plan....are u gonna write ur own drivers......
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Old 8th August 2004, 12:48 PM   #6
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The nice thing of PCM290x is that it does not require drivers! USB audio specifications are included in the low level drivers of the operating system.
Just attach it to the USB port and a new USB audio device will be recognized and ready to use. And this works in all windows systems supporting USB, Mac and Linux.
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Old 9th August 2004, 01:08 AM   #7
alecwek is offline alecwek  Denmark
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cool....
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Old 9th August 2004, 01:19 AM   #8
alecwek is offline alecwek  Denmark
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very cool...i checked out the data sheet...only 40somethin k at 16 bit...not great but still cool to try....dyou know more about the windows architecture....what happens if u cram all ur usb ports with these things.....does windows recognise it as one device....with many ins and outs....that would be great......
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Old 9th August 2004, 09:45 AM   #9
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don't know much about the low level drivers.. it's a very complicate matter..
Attaching more units in different USB ports, windows will enumerate them as: USB device, USB device [1], USB device [2],.. (the exact name given depends on your windows language settings).
Each device has 2 channels in and 2 out. BTW, also any other multichannel soundcard is shown in windows as a list of stereo single devices. The difference is that a single multichannel soundcard have a single clock and can start in sync the rec or play on more than 2 channels (if the software that use it is well done), while with a bunch of PCM290x you may have small time differences between the different devices. I made some tests with the trial of the multitrack software SONAR, playing the same song in different tracks with different USB devices simultaneously: some slight phase difference could be noticed at the end of the song, but if in the different tracks there are different sounds, like happens in real use, the effect is negligible.
A thing to try, if you want to increase the sync in a multitrack architecture, is using a single crystal for many PCM290x, but remember that they are anyway different devices, each one with its "SpAct"..

The sound quality of the PCM290x is good. Maybe not so good as other converters, but very acceptable and better than most built-in soundchips in many laptops or motherboards. After all it's a Burr-Brown chip, so a minimum sound quality is assured..
In a preliminary prototype of my device I measured (with a loopback configuration) a SNR of 81 dB, THD 0,0025% and THD+N 0,01%. Now I'm trying some improvements.. (mainly better quality caps). I'll let you know..
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Old 10th August 2004, 12:37 AM   #10
digi01 is offline digi01  China
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djd4n,this is my friends project.USB+headamp pocket.

wish that have some helps


yours digi
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