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stm32f4xx using mic at low systicks

Question asked by haeye.hauk on Feb 21, 2016
Latest reply on Feb 21, 2016 by Clive One
I'm using the stm32f4-discovery board for recording signals utilizing the microphone.
I'm configuring the clocks according to ST configuration tool "STM32F4XX Rev A system clock configuration v1.0.1".

when configuring according to HSE(8MHZ) and desired HCLK freq of 168MHZ, everyhting is OK.
but when using HCLK 84 MHZ freq,the recordings look almost good->there are many abrupt peaks,at sine wave recording.

also,according to the AN3997
"The I2S peripheral is configured as master in order to generate the correct clock (1,024 MHz) for the digital microphone. The 1,024 MHz clock is calculated from the output audio streaming (16 KHz) and the decimation factor (64) chosen for the demo (16000 Hz x 64 = 1.024 MHz). (Refer to AN3998 for PDM audio software decoding)."
There's a register that's supposed to be loaded with the 1.024MHz.
what is its name ? At the examples of the board recording, i didn't manage to find the mentioned configuration.

any recommendations ?
Thanks a lot !!!