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ADMP441, SSM2529 Speech transmission

I am constructing a hearing aid for my bachelor degree project. This hearing aid will consist of a microphone with a Nordic Semiconductor nRF2460 2.4 GHz transmitter, and a receiver of samë model with a speaker. This device should filter out any unwanted frequencies outside of speech, that is anything outside of 1-5 kHz. We want to sample at 12 kHz. I and my group are designing the PCB's and power supply ourselves. For simplicity we want mostly digital signals on our PCB's. Therefore we want to use the ADMP441 which has an ADC included with an I2S interface. Our transmitter and receiver (which are the same IC), support 16 bit I2S. The SSM2529 will be our DAC and amplifier stage, and also the filter stage.

The question is, can I use the ADMP441, which supports 24 bit, to send a 16 bit signal to my 16 bit compatible transmitter?

  • The ADMP441 will always output 24-bit audio data. This may be compatible, though, with your transmitter's I2S input. The transmitter may be able to just use 16 of the 24 bits that it receives from the microphone. I looked at the nRF2460 data sheet to see if there were any issues with this interface and I wasn't able to find all of the information that I needed about it's I2S input. In I2S mode, the nRF2460 will be compatible with the ADMP441 as long as it is generating an SCK signal that is always 64x the rate of WS (sampling frequency). The nRF2460 data sheet is not clear on what the rate of its CLK pin is in master mode. If you determine that it will output a CLK at 64x fs, then the ADMP441 will work fine for your application.

  • Okay, thank you. We might have to substitute the ADMP441 for a separate mic and ADC. I passed the question on to Nordic. About the SSM2529, I quote page 14 in the datasheet:

    "The interface is capable of receiving I2S, left justified, right justified, PCM/TDM, or PDM input formats. The number of data bits must be set when in right-justified mode only."

    This does mean that it can receive whatever bit wordlength i send to it, including 16 bit, right?

    Edit: I forgot to ask if this is I2S 16 bit. This is the standard the bluetooth receiver/transmitter is compatible with.

  • Also, is there a preferred microcontroller to configure these IC's? I have experience with Atmel's AVR, but it seems ARM is the standard here.

  • Have you confirmed with Nordic what is the ratio of the CLK frequency to the frame rate? I just want to be clear that if it's 64x, then your design should work fine with the ADMP441.

    I don't think that there's any preferred microcontroller. I'd say that, if you're comfortable using an AVR, and it has the capability to set up the amplifier (the ADMP441 doesn't need any microcontroller setup) as you need it, then go with that.

  • Nordic Semi told me this might prove difficult, as the nRF2460 operates at 256xFs.

    We're rethinking our whole design because of soldering problems with very small components.

    We're thinking about the AD1934 as DAC and AD1871 as ADC. It would seem that the nRF2460 demands a sampling rate of 32 kHz (waiting for confirmation from Nordic).

    Question: Can I operate the AD1934 and AD1871 at Fs = 32 kHz with a clock at 256xFs? I checked out the datasheets, but it's all a bit confusing to me which requirement precedes the other.

  • I think that there is some confusion about the clocks that we're talking about. It makes sense that the nRF2460 operates with a 256 x fs master clock, but that's probably not the rate of the CLK pin (the I2S port bit clock). Even if the nRF2460 must run at a 32 kHz sampling rate, as long as the CLK output is 64 x 32 kHz (2.048 MHz), then then ADMP441 will work directly with that I2S input port.

  • I would recommend that you look at the AD1938; this is the full codec version of the AD1934. This codec, as well as the AD1934 DAC, work well in the 32 kHz rate environment, using the PLL at 256 x Fs clock rate.

  • Thank you both for replying.

    ColemanR: I've looked at the AD1938. When you say "full codec version", does this mean that I can use the AD1938 in two instances as an ADC on the transmitting side, and as DAC on the receiving end?

  • The AD1938 has 4 ADC channels and 8 DAC channels; the ADCs and DACs can be clocked from the same source (MCLK PLL, LRCLK PLL or PLL Bypass), or separate sources if that suits your needs. It is a very flexible part, allowing you to use the ports as though you have separate ADCs and DACs. I have reviewed designs where the ADCs are clocked from ALRCLK through the PLL and the DACs are clocked in PLL Bypass directly from a 512 x Fs MCLK. Please let me know if you have specific questions about your application.



  • Okay, I ordered 2 of the AD1938. I'm thinking of using a TDA2822M as a power amp for my 4 ohm speaker. I figured since the AD1938 requires filtering before audio output, I would filter out anything outside of speech frequencies after the DAC stage. At page 30 in the AD1938 datasheet you recommend what appears to be a Sallen-Key lowpass filter. I am however unsure of what the components to the right of node 1, at the output of the opamp, are for. Could you explain this?

    Also, is it necessary to use an active filter (maybe it helps to reduce noise, create buffer for impedances)?


    - Eystein