We want to evaluate the use of four PDM microphones and want to know which evaluation board will work for this.
Also, the ADAU1761 has 8 digital channels. Does that mean support for 8 separate microphones?
I guess I need to understand a little more about what your needs are. So you want to evaluate the microphones, do you need to do very much processing in a DSP? Do you need to send this audio in digital format out to other hardware or will you just need to mix it out to stereo and analyze it in analog form?
Either way there are many different solutions we can offer.
1) Simply mix four channels of audio from four PDM mics and output it in analog form. The ADAU1772 has two stereo PDM microphone inputs and a small DSP that will easily provide some mixing capabilities and some external GPIO for mutes. This should be a simple platform for testing for you. It also has a headphone output on the chip. There is an evaluation board for this part. ADAU1772 Evaluation Board | Analog Devices
2A) If more involved DSP processing is needed then going the route of converting the PDM signals into TDM is probably the best way to do it. Then you can send the TDM signal to any DSP core that fits your needs. If you only need to convert four channels of PDM mics then I would use the ADAU1372 which is the codec only version of the ADAU1772, so no DSP. This will allow you to take in four PDM mics and then take a stereo signal back from a DSP to the stereo DAC and output it to headphones/line outputs.
2B) Again, if you need to use an external DSP for more processing, then look into this tidy little solution we have. This is the ADAU7002. It is simply a PDM to I2S/TDM interface. This part is capable of up to TDM-8. It will take digital signals from two PDM microphones and insert them into any of the 8 TDM slots when in TDM-8 mode. Of course it will also output in I2S or TDM4. It tri-states the data output driver during unused slots so using four of these parts will give you a solution for converting 8 digital microphones into one TDM-8 data stream.
Your question about the ADAU1761. Yes it can receive 8 digital channels using the serial port in TDM mode. It also has one digital microphone input so it can only take in two channels of digital mics. The only little detail is that you will not be able to use both the digital mics and the serial port since the PDM mic input needs to use the BCLK pin to clock in the data. This makes it not available for use with the serial port. There may be a way of doing this where the frequencies would be the same. I would have to study this and get back to you about it. I think the best solution is to use multiple ADAU7002's and come into the DSP using the serial port. Then you will still have the use of the ADCs built into the ADAU1761 if you choose to use them.
So as you see there are plenty of options.
Thanks for the reply,
We want to input the microphones to do some processing and then send digital data to another chip.
How is PDM changed to TDM for the eight digital channel inputs to the ADAU1761?
I do not know the difference between digital PDM and TDM for DSP processing of one or the other at the moment. The SigmaStudio modules do not appear to explain a lot of what is happening in going from one module to the next, nor exactly when PDM input might change to some other format, such as TDM.
Some processing might be FFT and looking for a vocoder type process, and also looking at the beam forming modules, which so far do not appear to be working.
Are there more documents on the beam forming modules?
Looking at the ADAU1450 and 1451 it looks like they can receive the PDM using the SDATA input, BCLK, and LRCLK pins for each microphone.
A question for this is do the left and right inputs have to be combined before going into the SDATA input? We want to use only four microphones so possibly can use left only or right only.
According to '2B' the SDATA ports are only for TDM input?
Looking at the ADAU7002, the PDM input looks like the left and right have to be combined first? Then we only need to choose the DSP we want to use.
In SigmaStudio it looks like an evaluation board has to be connected to try out any of the DSPs in an evaluation process. It would be nice to be able to target different chips for testing, especially if we already bought an evaluation board and may buy another one.
Hi Dave T.
I think I understand the options now.
Is there an evaluation board for the ADAU1450? or will the ADAU1446 eval board work as an equivalent to the ADAU1450?
We are looking to purchase another evaluation board quickly.
Sorry I missed your responses. We can continue the discussion on the newer thread you started.
Does Analog Devices have a vocoder?
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