ADAU1442 SPDIF output sample rate error

Brettcoupe Question

Feb 29, 2016 00:00

Hi

I have a ADAU1442 running as a slave to serial input 4 clock domain4 (TDM16) from another DSP IC(Master)

The ADAU1442 is running external XTALin from the master DSP at 24.576 mhz (not from xtal)

The ADAU1442 also has Serial 0-3 as ADC-DAC ICs (slave ICs).

The Start Pulse Bit is set to serial input pair 4.

I measured the LRCLK feeding the ADC-DAC is and its exactly 48K. The CCLK out is exactly 24.576mhz (512xfs)

However , SPDIF output should also be running at 48k. but it is running at 48.01k. More strangely is that any tone generator i add to the DSP schematic and output to the SPDIF runs at 92% of the programmed Tone frequency. IE a 500HZ sine generator measures 460hz on my test gear.

The same tone routed out the DAC channel measures correct frequency.

Note My test gear is accurate and calibrated. Its an Audio Precision (AP) unit.

Any Ideas why the SPDIF output is off frequency?

Thanks

Brett

Message was edited by: brett kelly

Audio Signal Processors

spdif

adau1442

SigmaDSP Audio Processors

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DaveThib Reply

Mar 1, 2016 00:00

Hello Brett,

A couple of things have me scratching my head. I think you do not have all the blocks in the system set to the same sample rate in SigmaStudio. But, this is what has me wondering. If the oscillator cell is set for 48kHz fs and the hardware is playing it back at 44.1kHz then a 500Hz tone would play back at exactly 459.5Hz. So that must be what is going on somewhere.

Step 1, In SigmaStudio, at the top of the window there is a box that is for the default sampling rate when you drop cells into the schematic. Set that to 48k, if it is not at 48k, and then to the right of that box there is a button that is to propagate the sampling rate to all the blocks in the schematic. So try that and recompile and reload the program.

I think the problem will still be there but lets eliminate this possibility.

So it seems to me that the SPDIF output is being played back at a slower rate. Are you going through a computer interface? Could it be set for 44.1 and is somehow playing it back at the wrong rate? The 1442 does not have much control of the SPDIF output format and user data bits. The non-audio data bits are all zero except for the validity bit which is set to 1. So what is reading the SPDIF?

Dave T

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Brettcoupe Reply

Mar 2, 2016 00:00

Hi Dave

Well I found the issue. My TEST GEAR!.. Its an Audio Precision. (well known and trusted and very expensive) Looks like it is out of Calibration all of a sudden.

I compared it against my Other test Gear. NTI and the NTI was correct.

Sorry for the alarm.

Regards

Brett

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DaveThib Reply

Mar 2, 2016 00:00

Hello Brett,

For the sampling rate you measured of 48.01kHz I was not worried about and I can see that as a calibration issue, but the 500Hz being 460Hz I would doubt it was that far off?

Now, I have seen signal integrity issue cause frequency problems when reflections start triggering as false edges. So that was a direction I was thinking of heading into next. So I am glad you seem to be up and running.

I hope the rest of the project goes smoothly!

Dave T

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