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Old 21st September 2004, 02:03 AM   #11
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Default USB DAC

Hello guys,

I recently designed a complete USB upsampling DAC for an internationally renouned audio company using similar devices to what you have discussed. Also I have done a thorough design review of the BelCanto DAC2 and this unit should by no means be used as a basis.

Assuming my estimate of what the interfacing is to the PCM1794 (I didnt use this device) your design will not work.
I2S is a packaging format and is as different from SPDIF as you could possibly get. I2S uses three signals (LRCK, BCK and DATA) the SPDIF is an embedded serial packetised protocol over a single ended medium.

Again assuming the PCM1794 doesnt accept SPDIF directly (and I'd have to look at this) you need an additional conversion stage to convert SPDIF to I2S (or RJ/LJ). Asahi Kasei/Cypress stock Digital Interface Receivers and make several variations of these.

Clocking is a pain in the a**! I ended up having 14 different clock frequencies bouncing around the board and a few crystals. This area is where you get all the problems and the most opportunity for improvement!

Likewise powersupplies are a similar pain. I designed a proprietry active noise attenuator and used multiple regulation stages. BelCanto's DAC2 is terrible in this respect, and its layout is awful.

Siimilarly their attention to clocks is also lacking, and I still have no idea why they put on a huge great CPLD/FPGA??!! It doesnt need much in the way of processing and inherent latencies of FPGA/CPLD silicon worsen the effects of jitter.

There is nothing wrong with the analog output stage example given in the app notes. Instrumentation amplifiers produce considerably less noise, but the specs of the devices specified are more than adequate. Just start off with a decent layout and you get similar performance gains with no additional cost (inst amps = expensive).

I have had lengthy discussions with several authorities in the field and the topology of the I/V conversion stage is a hotly contested issue. Some promote the opamp based one, others the simple resistor. I found no noticable difference between the two.

One final note... You will hear the effects of jitter, as it produces vast distortion effects, the overall THD of a 'circuit' (using low THD parts) will not be as easily detected.

Feel free to email me if you have any further questions. Just put DIYAUDIO - USB DAC in the subject so that it is spotted amongst the spam.

Have fun!
Apollyon25
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Old 21st September 2004, 06:07 AM   #12
niles is offline niles  United States
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You appear well meaning, so I will take a pass on the skewer here and just suggest that you actually take a look first at the entire thread posted above to the head-fi forum before sharing any more of your wisdom here. After that, you might want to look at the data sheet for the TI/BB PCM2707.

Niles
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Old 21st September 2004, 07:24 AM   #13
00940 is offline 00940  Belgium
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Default Re: USB DAC

Quote:
Originally posted by apollyon25
Assuming my estimate of what the interfacing is to the PCM1794 (I didnt use this device) your design will not work.
I2S is a packaging format and is as different from SPDIF as you could possibly get. I2S uses three signals (LRCK, BCK and DATA) the SPDIF is an embedded serial packetised protocol over a single ended medium.

Again assuming the PCM1794 doesnt accept SPDIF directly (and I'd have to look at this) you need an additional conversion stage to convert SPDIF to I2S (or RJ/LJ). Asahi Kasei/Cypress stock Digital Interface Receivers and make several variations of these.
PCM1794 doesn't accept direct spdif. But the PCM2707 usb device is able to directly output I2S, not spdif. Why would we need another conversion ?

Our big problem now is : the pcm1794 needs the classical 3 I2S signal (LRCK, BCK and DATA) and the PCM2707 is able to provide those. However, the PCM1794 also includes a digital filter. This digital filter requires a system clock. This clock needs to be synchronized with the LRCK clock. The question is : is the sck output of the PCM2707 able to provide that sck a/ synchronized, b/ with low enough jitter ?

Niles : have you considered the passlabs D1 stage ?
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Old 21st September 2004, 04:59 PM   #14
niles is offline niles  United States
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I havent really looked at the D1 circuit yet. I would prefer to work with BJTs on this but there are other options for FET stages, such as Borbelly's. With any of them we must choose between a DC servo and AC coupling. Yet another I/V is the one in M.O Hawksford's I/V paper. Both the Borbelly and the Hawkford circuits include a servo.

What I like about Jocko's circuit and the variants put up in other threads, one of which you posted further up in this thread, is the elegant simplicity and that they are reputed to sound really good, especially compared to opamp solutions. I see on the other thread that you are considering the D1 stage. I will take a look at it.
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Old 21st October 2004, 02:30 PM   #15
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0940,

How is your project doing? Any luck?
I'm thinking of PCM2707 with a TDA1543 * 8... (Although I doubt I
could ever solder a PCM2707 )

Regards,
Bas
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Old 21st October 2004, 04:32 PM   #16
00940 is offline 00940  Belgium
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soldering isn't really that bad. The pins on the pcm2707 have more space between them than on a pcm2702 or pcm1794.

The project is honnestly quite in standby. I finally won't have the time to build anything and conduct tests before next july or so. Not tomorrow as you can see. In the meantime, I'm still trying to gather as much information as possible and to optimize the layout of the prototype (and hopefully, it could be the last version). Choice on output stage is still the problem number one.

The nice thing with using the PCM1794 is the simplicity of the clok stuff. Going with tda1543 could be more tricky. The digital section as I have it on my drive looks fine and I won't probably touch it anymore. I know some won't like this thing with oversampling, digital filters and opamps but it's not intented to be more than a personnal project this time.
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Old 21st October 2004, 08:43 PM   #17
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Quote:
Going with tda1543 could be more tricky.
Why is that...I was under the impression that I2S would be all that is necessary....???

Quote:
I know some won't like this thing with oversampling, digital filters and opamps but it's not intented to be more than a personnal project this time.
That's what audio is all about imo...you have to design for your own ears and tastes.
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Old 21st October 2004, 09:08 PM   #18
00940 is offline 00940  Belgium
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All my apologies.

My poor brain cells went crazy there. I mixed up the thread you can find below (an high end pc dac or something like that) and an awfully complicated balanced tda1543 I planned at one time to do.

You're right and it's time for me to sleep.
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Old 21st October 2004, 10:22 PM   #19
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Can the PCM2707 work in "slave" mode for the clock frequency? This way we could put the XO onto the DAC and let the PCM2707 send data only when it is needed.

Another question:
Do you know a link where I can find some information about how-to convert the 16-bit I2S signal to 18-bit I2S signal for AD1865?
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Old 22nd October 2004, 07:25 PM   #20
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Do you have any information about the TwinDAC plus? It only says it has USB interface and it reclocks http://www.twindac.com/
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