There are some facilities to help with the upsampling needed for the DACs. Some modules need a hex file containing the value of the coefficients to be copied into the MiST(er) folder for compilation. The hex files are available in hdl/sound folder.
The module jtframe_fir is designed to operate on a stereo signal input to apply a FIR filter of up to 127 coefficients. It takes one BRAM block. As it needs about 256 clocks per sample, the input sampling frequency must be lower than clk/256.
IIR filter to remove the DC value of a signal.
This covers the usual situation of having two JT49 instances in a core. This module will add the two outputs, remove DC and filter out frequencies about 14kHz (for a JT49 clock of 1.5MHz). There is a qip file for this module.