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I2S FM modulation of LMX2571

Steve Palmer
Intellectual 365 points
Other Parts Discussed in Thread: LMX2571

I'm working with the LMX2571 eval module and attempting to use I2S audio data to provide modulation data resulting in NBFM analog audio.  Does the I2S mode, (described in section 8.1.1/page 36 in the data sheet) support an I2S voice data stream as an input, resulting in NBFM as an output?  I have had some success with audio in, but it appears that I'm over-deviating.  I cannot find information on how to set or change deviation for this mode in the data sheet.  Even when the I2S values reflect very low audio levels, I've got very broad-banded outputs of several hundred KHz.  I've tried reducing the DEM value, modifying CHDIV1 and 2 and other registers without success.

Any information about how to implement the I2S mode would be appreciated.

Thanks,

Steve

  • 0
    TI__Guru** 107030 points
    Hi Steve,
    You cannot directly inject the voice data stream to the I2S interface. The voice data stream represents a streaming of voltage over time. But what you need to provide to the I2S is a streaming of frequency deviation over time. Furthermore, if you need narrow band FM, you need to over sample the voice signal by at least 20 times. For example, use a 96kHz sample rate to sample the voice. Convert the voltage at each sample point to frequency deviation and then write to the I2S interface. See datasheet Figure 52, Table 34 and Figure 53 for an example.