Bandpass increases peaks. Why?

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stevep
Posts: 1
Joined: Tue Jul 11, 2006 10:30 pm

Bandpass increases peaks. Why?

Post by stevep »

This is consistant with any song taken from an audio CD as aiff or flac. I have used Bandpass/Stop to limit frequency to 20-20000 (or 30-16000), just because I can, but also with the idea of eliminating frequencies that an MP3 won't use, thereby possibly reducing artifacts.

However, after saving the file and reopening it, the peaks are higher, as much as 2db. The average volume (using Match Volume to see it) remains the same.

Any response is appreciated. Thank you.
Blandine Catastrophe
Posts: 253
Joined: Sun Jul 24, 2005 3:25 pm

Post by Blandine Catastrophe »

Possible explanations, but not sure about it.

That can be due to some phase changes inside the signal. Some frequencies are delayed more than other, then some peaks will be in the same direction (positive or negative) and synchronized, though they were in the opposite direction or one had its peaks when for the other part of the signal it was crossing the zero. I try to manage some phase delays, and I have the feeing that bass frequencies are more late than high frequencies. Not sure about the bandpass filter, but sure for the graphic equalizer.

Other possible explanation: The part removed was included in the compressor/limiter used to make a constant level on the CD. When removed, for a same average level, they are missing to make a constant level. If you have higher peaks for a same average, that means that you have also lowered parts. The lowered parts should have included more removed frequencies and the increased parts less of them. However, I have another software with a bandpasss filter, what will be shown as vertical cut on the spectrum after processing. And in this case, there is no significant change of the envelope.
Gloup? :-°
DougDbug
Posts: 2172
Joined: Wed Feb 16, 2005 3:33 pm
Location: Silicon Valley

Post by DougDbug »

Blandine is right. There are no perfect filters in hardware or software.


Another possible explanation: Most filters are not flat in the pass-band. In order to get a better cut-off slope, there is sometimes a 1-3 dB peak. I believe there are phase error compromises too. It's up to the filter designer to choose a good compromise, and I'm not sure what filter-types are used in GoldWave.

Just to make sure you don't get clipping, Maximize (normalize) the file before saving. GoldWave can temporarily hold values over 0dB, but 0dB is the limit for 16-bit WAV spec.
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