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hum filter

PostPosted: Wed Jan 12, 2005 6:46 pm
by citguy
I have not been able to use the hum filter to advantage without affecting the quality of the recorded material. The hum is probably present due to a "ground loop" or other bad connection (60 cycle) and is not severe but can be noticeable. Using the hum filter usually degrades the recorded material somewhat. I have not attemted to use the graphic equalizer feature but have "played with it". Any advice is welcome.
Stan (citguy)

PostPosted: Thu Jan 13, 2005 9:29 am
by Derek
I'm surprised you say the hum filter is affecting quality. This filter is usually fairly benign in its operation. Of course, it will tend to supress low frequencies but as these are contaminated with the hum, it shouldn't be all that noticable. What degradation are you hearing?

It's possible that you are using an old version of Wave corrector (v2.5). This version had a bug that caused the hiss filter to be applied whenever you applied the hum filter.

I wouldn't advise using the graphic equalizer to remove hum. It has poor resolution at low frequencies and any adverse effects you are hearing with the hiss filter are likely to be as bad or worse with the graphic equalizer.

With hum, it is always best to remove it at source if at all possible. Any audio restoration process will cause some degree of degradation so a hum filter will always be a second best option.

PostPosted: Thu Jan 13, 2005 5:24 pm
by citguy
Thanks Derek. Does the hum filter work at predetermined frequencies or does it do a profile and then remove hum?
Stan

PostPosted: Thu Jan 13, 2005 5:42 pm
by Derek
It sort of does both. :wink:

It uses a profile but restricts the operation to frequencies up to about 250Hz.

The advantage of using a profile is that the fierceness of the filter depends on the amount of hum present; if this is small then the filter has very little effect but if there is a lot of hum then it will have a much bigger effect. Theoreticaly, the filter reduces the hum by about 20dB.

PostPosted: Sat Mar 19, 2005 6:58 pm
by jpr
This discussion about the hum filter has me confused. I have version 3.1r1 and my help file decribes the hum filter as using a time domain subtraction method, not a filtering operation. With the cancellation method, if the profile gathered from the lead-in/lead-out period is accurate, then there should be no effect on the music at all. But a notch filter at 60 Hz would definitely punch a hole in the music spectrum. Which is it?

I have a small amount of hum induced by my turntable motor so there's not much I can do to eliminate it. I never really noticed it until I started listening with headphones at high volumes while running Wave Corrector. It would be nice to get rid of it if the music will not be degraded.

PostPosted: Sat Mar 19, 2005 7:20 pm
by citguy
I have been using the "hum filter" more often since the original discussion and finding that if the hom is not too predominent the filter "imporves" the overall recording (I am working with tape). I even tried two passes with the hum filter in one case with improvement on both passes. I of course am mainly doing this to "archive" old tape recordings and am not as concerned as some members with absolutely stellar sound quality. As JPR mentioned, the filter does cut into the lowest audio frequencies.
Stan

PostPosted: Mon Mar 21, 2005 10:02 am
by Derek
jpr wrote:This discussion about the hum filter has me confused. I have version 3.1r1 and my help file decribes the hum filter as using a time domain subtraction method, not a filtering operation. With the cancellation method, if the profile gathered from the lead-in/lead-out period is accurate, then there should be no effect on the music at all. But a notch filter at 60 Hz would definitely punch a hole in the music spectrum. Which is it?


The hum filter does use a subtraction method although it is not true to call it 'time domain' The subtraction takes place in the frequency domain.

The program measures the frequency components of the hum (in the noise profile) and subtracts these components to remove the hum.

If there is musical content with the same components as the hum, then effect will be dependant on the relative amplitudes of the hum and the music. This is because the hum and the music will add by vector addition and depending on the relative phase of the hum and music components, they might add or subtract. If the music amplitude is much greater than the hum, then the effect of the hum (and the hum filter) will be small and therefore you won't notice significant degradation. However, if the hum and the music are of similar amplitude, then you may notice some degradation as the filter will under- or over-compensate depending on the relative phases.

I hoe this hasn't left you more confused than you were to start with. :)

PostPosted: Mon Mar 21, 2005 3:40 pm
by jpr
Thanks for the explanation. I understand how it works now. Given that, I think I will refrain from using it. In my case the hum is not bad enough to risk any degradation of the music.