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PCM1803A timing

Benoit St-Jean
Expert 1030 points
Other Parts Discussed in Thread: PCM1803A, PCM1803

Hello, I've never worked with the PCM1803A A/D converter and I'm curious about the timing. The 1803 datasheet talks about pin 11 being BCK, pin 10 being LRCK and pin 12 being DOUT. On the I2S bus specifications from Philips, it simply talks about SCK which is the SERIAL CLOCK, WS which is WORD SELECT and SD as SERIAL DATA.

If I amalgamate both documents, I would presume the following:

PCM1803A BCK = I2S SCK

PCM1803A LRCK = I2S WS

PCM1803A DOUT = SD

I currently only have a 27mHz crystal which enables me to generate a PWM for the BCK / SCK clock at a frequency of 1.687Mz (50% duty). I believe that will be good enough for lower-quality audio sampling but please correct me if I'm wrong.

Now, regarding the LRCK / WS signal, I'm not sure exactly if I understand how to use it / time it with the BCK / SCK clock?

Does anyone have any simple and quick way of explaining other than referring me back to the documentation? I will re-read it but personal experience is sometimes better than manufacturer's explanations.

Thanks.

  • 0
    TI__Guru**** 189401 points

    Hi, Benoit,

    I'm not sure how the names got changed, but you've matched them up correctly.

    It depends on your system, but if you want to make the PCM1803A the master, you can use your crystal to generate an SCKI to the chip, and Table 1 provides the suggested frequencies for SCKI.

    Basically, BCK is the timing for each bit of data. LRCK tells you whether those bits are for left channel, or right channel. And, DOUT is obviously the data.

    -d2

  • 0 in reply to Don Dapkus
    Expert 1030 points

    Hello Don,

    Thanks for the reply. I believe I have something working here because I was able to generate a 3MHz 50% duty PWM using a PIC18F. If I look on the DOUT pin, I can clearly see some data. I am currently only using the left channel. Connecting the left input to GND (to simulate total quietness), I see a simple pulse recurring around every 96 kHz.

    If I let the channel input floating, then I see varying data which is a good sign. If I zoom-out on the SEC/DIV scale on my oscilloscope while I have the input floating, I can see a pattern of data very synchronously timed so I think I have the digitized audio data I am looking for. However, now what do I do if I want to listen to it? Is that audio straight SPDIF? Can I connect the digitized signal directly in an SPDIF input?

    Sorry, I am new to audio development and new to the PCM1803 but I believe I am on the right track.

    Also, let's say I cannot achieve a perfect 2.5MHz clock on the PWM for the BCLK pin on the 1803, how do I know what audio quality I have? I have all the TI documentation for the 1803 with me but I think I need someone to guide me a bit.

    Thanks again,

    Benoit

  • 0 in reply to Benoit St-Jean
    Expert 1030 points

    Actually, another question - still trying to follow the documentation but I'm not too successful... How would I lower my bitrate to have the lowest audio quality? Do I simply need to lower the frequency of my PWM? Right now I think I'm at the maximum speed but the problem I have is I don't have a way to listen to my audio and I think it might be too fast for me to try and parse it. But if I lower the bitrate to the lowest quality (slowest bitrate), then at least I might be able to do something with the data.

    Thanks again,

    Benoit