Before using Hifiberry, I have successfully recorded ultrasonic sounds with microcontrollers and specialized microphones. Using the same mic hardware, I tried recording ultrasonic sounds with the DAC+ ADC and the DAC+ ADC PRO and am unable to see the high frequency signals in most spectrograms. After analysis, we get a very clean spectrogram below 20 kHz and then we quickly lose all signal above. We've tried using Linux's arecord, audacity, python, you-name-it to try and obtain this high frequency data, but have had little success (and yes, we are setting the sample rate to 192 kHz). Rarely, we do get a recording that seems to have no low-pass filter, but it is random and very infrequent. Is this expected behavior and is there a way to permanently disable this filter? Thanks!
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3 comments
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HiFiBerry team The DAC+ ADC has an anti-aliasing filter in the input stage that limits bandwidth to about 60kHz, the DAC+ ADC Pro does not have any input filter. There is also no filter that can't be turned on or off. Therefore, I can't say what might be the problem in your setup.
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Stereo I probably experienced the same problem: a 24Khz Low pass filter on the DAC+ADC PRO when using Audacity.
However, I solved it by changing input "default" to sysdefault (or snd_rpi_hifiberry_dacplusadcpro:....)
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HiFiBerry team That could be definitely causing the problem as "default" might be almost anything including things like dmix or pulseaudio that re-sample internally to 44.1 or 48kHz. In this case, all higher frequencies will be lost.
Always make sure, you use the hardware device.
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