USB to S/PDIF conversion

[zagisrule!]

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

 

[jeff mai]

Are you sure you searched? Here is a thread that has virtually the same name as yours:

http://www.diyaudio.com/forums/showthread.php?s=&threadid=62537

Regarding data formats, try reading the PCM2704 datasheet available on TI's web site.

 

[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. 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.

 

[analog_sa]

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?

 

[jeff mai]

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.

 

[BlackCatSound]

Who says you can't isolate I2S?

 

[jeff mai]

Who says you can't isolate I2S?

Fair enough, but I have yet to see any implementation that does this.

 

[pupsik46]

I have done USB-DAC using PCM2706 and AD1852, for galvanic isolation used ADUM1400.
Works fine.

 

[anbello]

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

 

[debibi]

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

 

[hifimaker]

It's tought to beat the performance of this USB to S/PDIF product from Roland. It supports 24/96 and only costs $80. It's tough to build and debug one for this price.

http://www.roland.com/products/en/UA-1EX/

-David

 

[Ardee]

One advantage of using SPDI/F is that the sample rate can be detected and switched automatically by reading the user bits. Are there any USB/I2S chips which can implement this?

 

[Archwn]

I believe PCM2706/7 are also have spdif out.

Just pull function select pin to 'high'

 

[Ardee]

Archwn - The 2707 gives the option of I2S or SPDI/F but, so far as I know, when using I2S there is no indication of the sample rate. To do so would need at least one pin dedicated to indicating this, assuming only 44.1 or 48 rates are used. I was asking if there are any chips which do have such an output. Sorry I did not make this clear.

 

[dangus]

There was a CM102S inside a Nexxtech "Digital Sound Adapter" 2616365, for what it's worth. (Got it at the store formerly known as Radio Shack in Canada for about $10) It looks

Another option is the ADSTech "Instant Music". It's based on one of those PCMsomething chips, and appears to be a quite faithful copy of the application note (same values in the low-pass filters, anyway). It does optical S/PDIF in and out, as well as RCA line in and out. The overall build quality is decent, and there's plenty of room for mods inside the case if you wanted to change electrolytics to film caps. List price is around $70 but places like Staples often have them on sale for less.

 

[Archwn]

Hi Ardee,

My knowledge is limited. Are you saying 3 channels BCK, LRCK, DATA in I2S are not enough for typical DAC says tda1541a tda1543 to do their work? Mine work fine.

pcm2707 support 48KHz max. though. I don't know how it manage 96kHz signal if fed from usb stream.

let me know if I got you wrong.

 

[Ardee]

Sorry about the delayed reply: reply notification seems to have got lost.

If you are going straight into a DAC chip and most filters, the clock rate will tell the chip what the sample rate is. I'm using a PMD200 filter chip, though, which needs to be told what the incoming sample rate is. I don't know why this should be. The PCM2707 only passes on this information via the SPDI/F status bits, so far as I can see.