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Old 14th August 2005, 04:35 PM   #1
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Default USB to SPDIF Converter

Does anyone know where I can get a schematic for a USB to SPDIF converter, like the ones from empirical audio. Thanks!!! I like their concept but $500 is a bit steep. Preferably somethng that can plug into a Mac without needing drivers.
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Old 14th August 2005, 05:23 PM   #2
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The chip is the PCM2902 or PCM2906, works in win, Mac and linux without drivers, the schematic is on the datasheet.
You find also some schematics here in the Digital section of the forum.

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Paolo
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Old 16th August 2005, 06:06 AM   #3
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I used the PCM2704. Very straightforward. Made a PCB (called the HAGUSB), which I was planning on selling. But instead just put the circuit onto the motherboard of my new DAC. Anyway, here's the schematic. I left off the headphone amplifier output stuff. Just straight USB to clean S/PDIF. Output is 75 ohm with 1.2:1 tranny.

Click the image to open in full size.

Circuit is self-powered by USB cable, isolated from CD player via tranny. Windows XP (and I suppose Mac) recognized it as soon as I plugged it in. After 30 seconds or so of automatic configuration, my laptop was playing CDs through the USB port to my external DAC. No software. No drivers. No mess. Them folks at TI did a great job with this part.

Here's my test board.

Click the image to open in full size.

jh
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Old 16th August 2005, 06:52 PM   #4
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One thing I have been wondering about these USB to SPDIF parts: what clock controls the audio data rate? In other words, does the PCM2704 pull the data from the PC so that the 12 MHz crystal in your schematic determines the data rate (and hence the jitter on the SPDIF) or does the PC push the data into the PCM2704, in which case the data rate is set by a clock in the PC (thus causing SPDIF jitter by passing jitter from the PC clock)? Or is there some sort of ASRC involved?
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Old 16th August 2005, 07:15 PM   #5
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It is very much ASRC. The USB side is sent in data packets every couple of milliseconds. Then sits idle until next packet. Not sure if it is push or pull. The USB data rate is fixed, the packet rate is variable. There is networking stuff going on, like other items on bus, acknowledges, re-sends for error correction, etc. The TI chip buffers the data and then re-clock it out at a constant rate.

Only way they can get the right output frequency is via some synthesizer onboard. I haven't dug into it deep enough to figure out exactly how they do it, or how sensitive it is to jitter or the xtal charateristics. Nothing depends on the clock in PC, except to make sure the USB data rate is close enough so that it can be received without too many errors.

So asynchronous pulsed data in, constant data out.

jh
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Old 16th August 2005, 07:32 PM   #6
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From the PCM2702 datasheet:

The newly developed SpAct™ (Sampling Period Adaptive Controlled Tracking) system recovers a stable, low-jitter clock for internal PLL and DAC operation from the USB interface audio data

The same applies for PCM290x family (that has also the spdif input).
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Old 16th August 2005, 08:07 PM   #7
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So how about sample rate?

How does Windows drive it? 32, 44 or 48 khz depending on source, or directsound based sample rate conversion?

Is the Windows volume control working? If it does, possibly the sound is not 100% pure the source signal. (Some even say you never get more than 14bit if the Windows mixer is in play)

and how about higher Fs and bitrate? 96/24?

Thanks
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Old 16th August 2005, 08:35 PM   #8
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Default Re: USB to SPDIF Converter

Quote:
Originally posted by mp006ltk
Does anyone know where I can get a schematic for a USB to SPDIF converter, like the ones from empirical audio. Thanks!!! I like their concept but $500 is a bit steep. Preferably somethng that can plug into a Mac without needing drivers.
Edirol has just announced a new USB dongle, UA1-EX that supports 24/96 IO. I use the older UA5 product, the new drivers install and work well. Seems it will be $79.95 street price. Nice thing too it's both input and output. They also claim native Mac/PC USB driver support.

EDIT- I just noticed a catch, the SPIDIF is optical only so you might need a converter (I've seen them for $20 or so).
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Old 16th August 2005, 09:12 PM   #9
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Quote:
How does Windows drive it? 32, 44 or 48 khz
It does all of them. Sample rate data is included in the header of the audio packet information. The chip might even do 96k, I forget.

USB Data is data, no clock is sent. Only a few bits indicating what the clock rate should be. Output S/PDIF embeds a newly generated clock with the data.

I think the normal "sounds" of Windows is at 48k. That is, the chimes, clicks, bells, and other mouse & keyboard sounds. Only an audio CD is played back at the 44.1k rate.

jh
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Old 16th August 2005, 09:36 PM   #10
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Quote:
Originally posted by hagtech


It does all of them. Sample rate data is included in the header of the audio packet information. The chip might even do 96k, I forget.

USB Data is data, no clock is sent. Only a few bits indicating what the clock rate should be. Output S/PDIF embeds a newly generated clock with the data.

I think the normal "sounds" of Windows is at 48k. That is, the chimes, clicks, bells, and other mouse & keyboard sounds. Only an audio CD is played back at the 44.1k rate.

jh
Works also at 96 kHz (under XP) but unfortunately the bandwidth is limited to 24 kHz (as from datasheet.. max sample rate 48 kHz).

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