For a small project I'd like to use a HiFiBerry DAC to output an FM signal with stereo + RDS. This means the signal contains frequencies up to 60kHz. I'm using a 192kHz sample rate. When I measure the resulting signal, I see that the frequency response is far from flat. Compared to the output level of a 10kHz tone, this is what I measure:
20kHz - 96%
30kHz - 92%
40kHz - 89%
50kHz - 82%
60kHz - 71%
70kHz - 48%
80kHz - 28%
To get a quality FM signal, it should be flat until at least 60kHz. Is there a way to improve this behaviour of the card? I had a look at the amixer output and saw this option:
"Simple mixer control 'DSP Program',0
Items: 'FIR interpolation with de-emphasis' 'Low latency IIR with de-emphasis' 'High attenuation with de-emphasis' 'Fixed process flow' 'Ringing-less low latency FIR'
Item0: 'FIR interpolation with de-emphasis'"
Would this have anything to do with the behaviour I'm observing? What do these options mean for the output signal?