Setup shelve filter for ES8388

[impunchi]

Hey, Im using a custom board with an ESP32-WROOM kit to output a bluetooth audio stream via i2s through the audio codec ES8388. I've used the configuration file of the LyraT v4.3 board and the play_bt_music_example from the esp-adf repository as an initial starting point. Everything looked as it works as expected, but I've measured the output at the ES8388 with my oscilloscope and found out that my signal seems to be filtered. Especially frequencies lower than 100Hz are nearly not existent. After I looked up the datasheet of the audio codec i found registers 30 - 37 which can be used to control the a and b coefficient for a shelving filter. The problem is that there is no, or at least ive not found any, documentation of how the registers are representing the coefficient or what kind of transfer function is used to filter the signal.

Has any one experience with the filtering registers of the ES8388 or am I missing something? It seems like i cant find a way to boost the bass frequencies...

I would already be satisifed to setup the coefficients to give me an unfiltered audio signal out of the ES8388 because the signal is passed into another audio board which handles gain and filtering. My application only involves an incoming bluetooth stream from the esp32.

Every help is appreciated!