high pass filter produces clipping

[rayc]

I was treating some problems with a mix & exported a mix track as a .wav to a diff program to apply a few different strategies.
There was some unwanted stuff happening below 20hz so I decided to run the file through a high pass filter set at 20Hz.
I did this & the visual image displayed areas of clipping that weren't there before the filter.
I've seen this a few times before.
I have some suspicions as to why this may happen but no knowledge.
The three images below shhow the file as exported
Then the paranters of the filter & the 3rd show the filtered file with clipping incidents marked automatically in green.
Would someone in the know please explain this to me.

 

[bouldersoundguy]

Sometimes a low frequency wave's positive peak will line up with a higher frequency wave's negative peak. When you filter out the low frequency the high frequency's peak comes back. Any time you limit the signal and normalize peaks then filter it you can have this kind of clipping.

 

[Ethan Winer]

the visual image displayed areas of clipping that weren't there before the filter.

I think the more usual cause of clipping after a filter is phase shift, which "rotates" a waveform. The average level is actually lower because of the content being removed, but the peak level can go up or down. Fortunately, modern plug-ins use 32-bit math, and most include a volume control. So if this happens on a file that's very near to clipping, you can just reduce the volume to avoid the clipping.

--Ethan

 

[rayc]

bouldersoundguy
ethan,
Thanks gents,
I figured it was something like phase cancellation.
the program I used is specifically written for vinyl restoration so isn't really full bells'n'whistles thing.
It does a great job at what is was written to do but I'll contact the author about the issue anyway.
THANKS AGAIN.

 

[Farview]

With any sort of filter, there is going to be a little bump around the corner frequency. Any eq, positive or negative, can raise the peak level of a file. So if you start with a file that is already near clipping, the eq will push it over the edge.

 

[Ethan Winer]

I figured it was something like phase cancellation.

To be clear, it's not phase cancellation but rather "phase rotation." I just created a 100 Hz square wave at -6 dB FS, then applied a 6 dB per octave high-pass filter at 600 Hz. You can see that the filter "rotated" the waveform so severely that the peaks rose by about 5 dB and almost hit clipping. The total energy is much less - the area under the curve so to speak - but the peaks rose substantially.

--Ethan

 

[bouldersoundguy]

I think the more usual cause of clipping after a filter is phase shift, which "rotates" a waveform.

I suspect both phenomena are contributing. Some experimenting with a linear phase eq might help to sort it out.

 

[rayc]

Thanks for the amplification on the theme fellows. Particular thanks Ethan for the visuals - I'm a visual learner & now have a much better grasp of the concept. My lack of depth in the topic is why I thought my way through to phase cancellation & no further.
I learnt a new thing - again thanks fellows.