I'm wondering how to interpret artifacts on spectrograms like the following which shows the recording of a linear frequency sweep.
In this case:
- The recording is done with a cheap microphone and cheap sound card, and playback was done using an off-the-shelf system, which could have its own issues.
- There are obviously harmonics which can be seen at the beginning of the chirp, but I can't remember where these can come from, then also those "X" patterns that are catching my attention.
- There are some fixed-frequency perturbations, some constant, some pulsed and periodic
I'm looking for hints, ideas to perform root cause analysis, pointers to common and less common phenomena causing unwanted artifacts.
Collected hints so far:
- Playback issue: speaker saturation or sound clipping, which can cause harmonics, and can be witnessed using another recording device.
- Playback issue: clock desynchronization can cause waveform skips which cause very short pulses (of one sample).
- Short pulses (not on the attached spectrogram, they're short vertical lines) can be caused by power line transient perturbations.
- Missing anti-aliasing filter can cause capture of aliases of non-audible signals.
- Out-of-band noise caused by sigma-delta ADCs (ref) which can also be filtered by low-pass filter.
- Audion noise caused by capacitors in power circuits, eg. in LCD panel amplifiers.