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PCM2707C: No analog audio

Carlos Moreno1
Intellectual 550 points

Part Number: PCM2707C

Hello,

I just completed a test circuit essentially identical to that in Figure 36 in the datasheet (section 10.2.2, page 33), except that I set FSEL = 0 to output I2S audio.  A second difference is that I did not include the filter/snub components (R5 to R10 and C11, C12).  I only added DC-block capacitors C9 and C10  (I put 470uF since I had some of those already in my stock of components at home).

The device is recognized by the Linux host PC  (shows as "USB AUDIO DAC", and the output of the command lsusb includes: "Bus 003 Device 007: ID 08bb:27c7 Texas Instruments").  I can see the I2S signals on an oscilloscope --- I see a clean 44.1kHz for the LRCLK, a 2.82MHz (64x 44.1k), and also a 11.29MHz (256x 44.1k).  I can even hear the audio if I connect the DOUT signal to a speaker  (of course, that gives me a horrifically distorted version --- but I can make out the sound).

However, the analog audio is completely dead  (the two output pins do have a DC --- approx. 1.6V), but there is absolutely no sound in there.   Any ideas why?

Notice that in my final design I will not care at all about that analog output --- I will use the I2S to go directly to a DSP chip;  but still, I'm concerned that the fact that I have no analog audio (when I think I should) may be a sign that I'm doing something wrong that could come back and bite me later.

Thanks,
Carlos
--

  • 0
    TI__Guru 57800 points

    Hi, Carlos,

    This is an expected behavior. The PCM2707C has only one output from the USB audio device perspective. When pin 9 (FSEL) of the PCM2707C is set as '0', the I²S interface will be used as the output of the device.

    In this operation mode, the output from the USB interface will be streamed through DOUT (pin17) of the I²S interface, and not the analog DAC output. The input of the I²S interface (DIN, pin 4) will serve as the DAC input, the source for the analog output.

    Best Regards,

      -Diego Meléndez López
       Audio Applications Engineer

  • 0 in reply to Diego Melendez
    Intellectual 550 points
    Hi Diego,

    This completely answers my question and addresses my concern (about the no analog audio possibly being a sign that there was something wrong with my design); thanks!

    However, I have to rant: I find that the datasheet does an extraordinarily poor job at making this clear --- at best (from what I can see, unless I'm missing something else), section 9.3.4 *vaguely* hints that this might be the case, and section 9.3.5, well, let's say that it states something that is consistent with this. The block diagram (Figure 27) does not show this --- FSEL only controls a multiplexer to select which signal, I2S or S/PDIF, gets connected to DOUT and the buffers that output some of the I2S signals. Not only it doesn't show that: it actually contradicts it in that according to the figure, there is no path from DIN to the DAC.

    There's nothing that explicitly says (or even strongly suggests) that the DAC needs to be enabled through FSEL; all direct statements about FSEL describe a selection between I2S vs. S/PDIF --- these two are closely related, as they are the two possible formats for digital output; how could one imagine that the DAC output is conditional to having selected S/PDIF??

    Oh well, I'll stop my ranting on account of being happy that I got the answer I needed and that it looks like my design works as I want it to.

    Cheers,
    Carlos
    --
  • 0 in reply to Carlos Moreno1
    TI__Guru 57800 points

    Hi, Carlos,

    No problem, I understand you and we apologize for the confusion. The PCM27xx fasmily of USB audio DACs is quite old and the content for the datasheets across the family has been reused in the past without taking a lot if care on the specific content for each particular device, an example is the the block diagram issue, which  probably is related to the reuse of a generic diagram. 

    I'm glad to hear your design will work as you want, please let us know if you have further questions. 

    Best Regards,

      -Diego Meléndez López
       Audio Applications Engineer