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Old 17th July 2012, 04:27 PM   #1
q-rz is offline q-rz  Austria
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Default PCM1704 sidebands around harmonics

Hi all DIYers!

First of all, sorry for my awful english...

Recently, I've built my first DAC with PCM1704's. The PSU consist of LM317's followed by simple shunt regulators. The input stage contains an FTDI USB chip, an Actel FPGA and a Tent Labs clock. The I/V converters are OPA627's, they will be changed to some discrete design at a later time. I'm planning to write an ASIO driver to be able to use it as an audio device with foobar or some other player. The only thing I currently have is a command-line application which sends out a wave file to the DAC. The sound is fine, but I'm sure there are million ways to make it better. I've measured an 1khz 24bit sine @ 44.1khz with my Terratec DMX 6Fire USB interface, and noticed that there are "sidebands" around the harmonics. Do you have any idea what could be the cause? I thought that jitter can only create sidebands around the fundamental frequency. Maybe I am totally wrong...

Cheers,
Peter
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Old 17th July 2012, 04:45 PM   #2
kevinkr is offline kevinkr  United States
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USB interface not asynchronous i.e. data clock frequency dependent on USB timing?

Anything using the Tenor TE7022L and other similar USB receiver ICs are not asynchronous even with custom drivers as far as I can tell. (I have a device that uses one of these chips)
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Old 17th July 2012, 05:20 PM   #3
q-rz is offline q-rz  Austria
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Hi Kevin,

Thanks for your reply. I use the FT232H from FTDI, which is not an audio device itself. It's basically nothing more just a small FIFO. The audio samples are "pulled" from this buffer by the FPGA, which is clocked with the Tent clock (11.2896mhz). Reading from and writing to the buffer are asynchronous processes.

Greetings,
Peter
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Old 29th July 2012, 02:03 PM   #4
regal is offline regal  United States
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Quote:
Originally Posted by q-rz View Post
Hi Kevin,

Thanks for your reply. I use the FT232H from FTDI, which is not an audio device itself. It's basically nothing more just a small FIFO. The audio samples are "pulled" from this buffer by the FPGA, which is clocked with the Tent clock (11.2896mhz). Reading from and writing to the buffer are asynchronous processes.

Greetings,
Peter
Try realignment of the output of the FT232H with your clock and digital flipflops (even though you are clocking the FTDI with the tent clock it may be adding significant jitter since the chip itself runs at a much higher speed.)
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Old 29th July 2012, 03:33 PM   #5
kevinkr is offline kevinkr  United States
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Quote:
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Try realignment of the output of the FT232H with your clock and digital flipflops (even though you are clocking the FTDI with the tent clock it may be adding significant jitter since the chip itself runs at a much higher speed.)
I think this is good advice, and should be comparatively easy to try.. Perhaps reclock just ahead of the D/A converter IC? I'm thinking about what might be happening to the clock in the FPGA..
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