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PCM1865-Q1: Using the device in M/S mode for 6 microphones

Expert 5435 points

Replies: 5

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Part Number: PCM1865-Q1

Team,

our customer is considering designing in PCM1865-Q1 and using it in M/S mode.

We have a few questions:

  1. Datasheet suggests there is analog gain (-12dB- +32dB) + digital gain (0.5 dB – 11.5 dB). Do these two gains add up, since maximum gain is said in datasheet to be 32dB?
  2. Datasheet also suggests there is no need to include antialiasing filters on the input of ADC, since this is handled internally?
  3. How to set-up and use M/S mode for 6 microphones. How much I2S (SAI) buses are needed for this setup?

I can share block diagram of what we want to accomplish if it would be helpful.

Thanks for your support.

  • Hi, Bartosz,

    Please refer to below comments:

    1. The analog and digital gains are added, 32dB is the maximum gain of the analog PGA. The gain structure of the PCM1865 has a feature of automatic gain mapping which follows an hybrid architecture where the analog and digital gains are set according to an internal look-up  table which gives the best performance. Please refer to section 9.3.5 of the datasheet for detailed information.

    2. For general applications, the anti-aliasing filter is not required, but with systems with high out-of-band noise, we recommend to put a small filter as shown in section 10.1.5 of the D/S.

    3. For an application with six mics, two PCM1865 can be used, one master and one slave. The first one may capture data from four mics, while the other can record audio from two. The device which will capture two mics can be replaced with a PCM1863, which has same performance of PCM1865 but has only two channels. The number of ASI buses is determined by the serial interface format:
      • Assuming I²S is used, three serial data out lines are required, where LRCKand BCK  should be common; on this configuration, the device with four mics will send data through two digital outputs.
      • If TDM format is used, a single data out line would be enough as it will be connected to the digital outputs of the devices. both LRCKand BCK are same for two devices.

    Best Regards,

      -Diego Meléndez López
       Audio Applications Engineer

  • Expert 5435 points

    In reply to Diego Melendez:

    Hi Diego,

    thank you, it helped us moving forward.

    Based on your feedback, we're attaching two block diagrams (zip below) and would have the following questions:

    1. Are these both setups valid for PCM186x?
    2. As we need to capture audio from array consisting of 6 microphones which of these two approaches is better?
    3. Do we need to have buffers for clks (as in block diagram) for both cases?
    4. If we prefer to use microphones with differential outputs, is setup the same in that case?
    5. Do you have any reference schematics where PCM186x is used as a M/S
    6. What is the SW and driver support offering for PCM186x?

    Thank you,
    Kind regards,
    Customer

  • In reply to Bart:

    Hi, Bartosz,

    Thanks for the feedback, please refer to the comments below. 

    Both setups are valid for the PCM186x devices, the only comment I have is that the I²C bus should be shared between the devices as it seems from the diagram that the control interface of PCM186x is daisy chained.

    Both approaches are valid and should work properly, the decision to choose between one or the other is mainly driven by end-user preference or limitations of the system. Please keep in mind that each solution would require a specific serial interface configuration.

    Buffers are not really required as long as the ICs are placed relatively close, which I think is the case.

    If differential input is used, the setup should be the same in terms of digital audio interface. The only change would be on the input circuit architecture and device configuration.

    We have a reference design, TIDA-01454, which uses two PCM1864 (similar to PCM1865, but with 130dB performance) and 8-mic. You can look at that schematic and use it as reference.

    Unfortunately we are not currently supporting driver development for our devices, I know there are a couple drivers available on some repositories for Linux on the web, but right now we might not be able to help with any change or configuration of them. 

    Best Regards,

      -Diego Meléndez López
       Audio Applications Engineer

  • Expert 5435 points

    In reply to Diego Melendez:

    Hi Diego,

    many thanks for your feedback. It helps.

    Regarding differential input, as datasheet suggests, the only difference is in the input filter. Is that what we refer to when we state that the only difference is in input circuit architecture and device configuration?

    Is there any additional difference in input circuit besides that for using analog mems with differential outputs?

    Also, since reference designs doesn’t involve power supply circuitry (it is taken from one of the DSP boards), It would be useful to know what is the preferred way of powering this IC’s. Can they handle properly designed switcher or is a linear regulator a must?

    Thank you,
    Best regards

  • In reply to Bart:

    Hi, Bartosz,

    Correct, the input circuit connection is the only difference between using a single-ended or a differential input configuration. It is possible that the circuit requirements for a given mic changes, but for the PCM186x point of view, the only difference is on the signals provided to the input pins. 

    A clean power supply is required to have the expected performance of the device, it is a system-level decision to select a proper power supply for the ADC. We usually recommend to use linear regulators, but I don;'t see a problem with using a switching supply as long as you make sure a clean voltage rails is provided to the device. 

    Best Regards,

      -Diego Meléndez López
       Audio Applications Engineer

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