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Old 16th January 2007, 06:54 PM   #1
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Default USB to S/PDIF conversion

Hi all!

I would like to build a high-quality DAC for my headphones, but I have a few questions.

1. I want to use USB as the input. How do I generate USB output to send my music over USB?

2. I cannot find any good converters w/ USB interface. I presume I need to convert the USB signal to S/PDIF for a decent converter. How is this done? I am looking at the PCM2904/6 audio CODEC chip with USB input and S/PDIF output...is this the kind of device that I should use?

3. Where can I read more about data formats and their conversion? What is my sample rate limited to with USB2.0?




Thanks for any advice, if someone has a link to a page with info like this that would be great too...



-Matt
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Old 16th January 2007, 07:08 PM   #2
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Are you sure you searched? Here is a thread that has virtually the same name as yours:

USB to SPDIF Converter

Regarding data formats, try reading the PCM2704 datasheet available on TI's web site.
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Old 16th January 2007, 07:16 PM   #3
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There's no real point in converting a digital stream from USB to spdif because you'll then have to convert it again before feeding it into a DAC chip. You're better off with a parallel or I2S output to feed a DSP or a DSP micro with USB interface built in. What do you plan to use for the DAC?

There are all in one devices that do this. A little USB dongle with headphone and microphone jack for about $20. For that price its worthwhile evaluating just to see if it does serve your purpose. Quality might not be that great.

Sam.
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Old 16th January 2007, 07:29 PM   #4
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Quote:
Originally posted by samborambo
There's no real point in converting a digital stream from USB to spdif because you'll then have to convert it again before feeding it into a DAC chip.

I am beginning to wonder about that. Been playing with a USB/I2S converter recently and the results so far have been less than exhilarating. Could it be that converting to spdif and the subsequent PLLs help deal with the USB jitter more efficiently?
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Old 16th January 2007, 07:49 PM   #5
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Quote:
Originally posted by samborambo
There's no real point in converting a digital stream from USB to spdif because you'll then have to convert it again before feeding it into a DAC chip.
There is one point at least: you can use a pulse transformer for galvanic isolation. With USB - I2S there can be ground loop issues between PC and DAC, some requiring expensive optical USB cables to solve.
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Old 16th January 2007, 07:59 PM   #6
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Who says you can't isolate I2S?
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Old 16th January 2007, 09:17 PM   #7
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Quote:
Originally posted by BlackCatSound
Who says you can't isolate I2S?
Fair enough, but I have yet to see any implementation that does this.
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Old 17th January 2007, 06:16 AM   #8
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I have done USB-DAC using PCM2706 and AD1852, for galvanic isolation used ADUM1400.
Works fine.
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Old 17th January 2007, 11:48 AM   #9
anbello is offline anbello  Italy
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Quote:
Originally posted by pupsik46
I have done USB-DAC using PCM2706 and AD1852, for galvanic isolation used ADUM1400.
Works fine.

I think this is the right way.
Is it possible to see more details of your implementation?

ciao
andrea
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Old 13th July 2007, 10:21 AM   #10
debibi is offline debibi  United States
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Lightbulb How about CM102-A

S/P DIF would be the same signals if different USB devices output it.
So I think it makes small differences in sounding whatever you choose.

There are some IC to convert S/P DIF from USB data.

PCM2704,PCM2705,PCM2706,PCM2707
CM102-A+

PCM2706 and PCM2707 can output I2S, so if you use old DAC chip such as TDA1543, it'll be useful.
But it may be hard to solder their tiny legs.

CM102-A+ is DIP package, so it is easy to make.
If you want to get USB-S/P DIF converter cheaply, this may be good.

See accompanying site.
http://ta2020.huuryuu.com/cm102_e.html
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