3 ms·

Filtering out 50Hz/60Hz noise as a band stop filter is a common audio post processing step, precisely because it's noise from the power system. I wouldn't be s

Filtering out 50Hz/60Hz noise as a band stop filter is a common audio post processing step, precisely because it's noise from the power system. I wouldn't be surprised if there were phones that automatically applied this filter.

FPGAhacker 4y ago
Your point still stands but the noise here is effectively a higher frequency modulation of the base AC oscillation. In other words, the base oscillation of 50 or 60 Hz is not stable, but instead varies a little above and a little below at higher rates. The magnitude is significantly lower than the base mains frequency. So possibly not audible above any other noise in the recording, but present nonetheless.
- ok123456 4y ago
  These filters will take that into account since it never has exactly been 50/60Hz. It's just the expected mean. Maybe it's present in subsequent harmonics (e.g., 100/150/200Hz for 50Hz) as a systematic deviation from additive white noise. But, this would be difficult in practice to reliably isolate these signals via some form of component analysis.
myself248 4y ago
Of course, and a lot of microphones don't pick up that low anyway. That's immaterial because of harmonics. While the AC wave itself is nearly a perfect sinusoid, various devices introduce all sorts of nonlinear properties, and produce higher frequencies that are harmonics of the base powerline frequency, and thus vary right along with it. These are insidiously hard to remove. Your best bet would be to encode all the voices with a vocoder, ditch the background sounds entirely, and play the coded voices back. Which of course makes it obvious that the audio has been altered... I suspect that similar artifacts will be visible in lights too, since the brightness of an incandescent or neon source varies with the AC wave. Some fluorescents too, though LEDs all have DC drivers so they'd make it more difficult.

FPGAhacker 4y ago

Your point still stands but the noise here is effectively a higher frequency modulation of the base AC oscillation. In other words, the base oscillation of 50 or 60 Hz is not stable, but instead varies a little above and a little below at higher rates. The magnitude is significantly lower than the base mains frequency. So possibly not audible above any other noise in the recording, but present nonetheless.

ok123456 4y ago
These filters will take that into account since it never has exactly been 50/60Hz. It's just the expected mean. Maybe it's present in subsequent harmonics (e.g., 100/150/200Hz for 50Hz) as a systematic deviation from additive white noise. But, this would be difficult in practice to reliably isolate these signals via some form of component analysis.

myself248 4y ago

Of course, and a lot of microphones don't pick up that low anyway. That's immaterial because of harmonics. While the AC wave itself is nearly a perfect sinusoid, various devices introduce all sorts of nonlinear properties, and produce higher frequencies that are harmonics of the base powerline frequency, and thus vary right along with it. These are insidiously hard to remove. Your best bet would be to encode all the voices with a vocoder, ditch the background sounds entirely, and play the coded voices back. Which of course makes it obvious that the audio has been altered... I suspect that similar artifacts will be visible in lights too, since the brightness of an incandescent or neon source varies with the AC wave. Some fluorescents too, though LEDs all have DC drivers so they'd make it more difficult.