subsonic offset

Another curiosity. I obtained a file, which I want to massage a bit. I believe it was copied from cassette tape but I don't know anything about the circumstances or the equipment used. While it is easy enough to work with, it has a characteristic unfamiliar to me.

There is a DC offset of almost 2%. This is clearly visible in waveform view anywhere there isn't other audio signal, but I first noticed it in Spectral View where I commonly examine things. There it looks like a great deal of low frequency noise (from 0 to 750Hz but especially strong from about 50 to 550Hz).  It doesn't  actually sound noisy, however. Frequency Analysis also shows quite a bit of very low frequencies, although not so much as most LP recordings I've looked at.

I can use the Normalize or Amplify transforms to remove the DC offset. This has little effect on the subsonics shown in the Frequency Analysis Graph.

Rather than either of those transforms, I can just apply a subsonic filter, such as the rumble filter preset in Scientific Filters. This removes the very low frequencies and also corrects the DC offset.

That is the part that sparks my curiosity. Is it that the DC offset just happens to effect only the subsonics, so that getting rid of that part of the spectrum incidently cures the offset, or is there some intrinsic relationship between the DC offset and very low frequencies, or ...  what?

DC offset is rather artificial - any assymetrical LF is likely to alter the amount indicated quite considerably, as it's only an estimate of how far the average base line is shifted. The only time it is really relevant as a measure is at the start of a recording, where there is no recorded sound, and you've just got the baseline shift caused by inaccuracies in the A-D converter itself, rather than the signal.

Is it that the DC offset just happens to effect only the subsonics, so that getting rid of that part of the spectrum incidently cures the offset, or is there some intrinsic relationship between the DC offset and very low frequencies, or ...  what?

When you consider that an extremely low frequency will effectively look like DC you've almost answered your own question. Most DC offset removal filters are simply sub-sonic filters with the turnover point set to a very low frequency.

Cheers

James