Recording Record - FFT filtering of warps

[AlexM]

Hi,

I am in the process of ripping my vinyl for use with a squeezebox server solution.

Although not really audible when listening via the pre-amp, any warps are clearly visible in the waveform of the recording.

Would using a high-pass filter in Audacity at, say, 20hz help to reduce the effects of these slight warps?. I'm not sure what order the high-pass filter is in Audacity, or what the filter frequency should be set to.

Any ideas?

Alex

[Reid Malenfant]

Have you got any way of finding out what kind of phase shift the filter will impose?

I'd look to filter at about 15Hz & frankly to do any good you'll need a multi pole filter of at least 18Db/octave. If it's doing it digitally (which i should imagine it must be thinking about it :scratch:) you should be fine & you may well end up with no extra phase shift in lower bass notes. Analogue filters will always shift the phase though & this can be problematic.

I suggest you try recording something that has plenty of deep bass with a filter & compare the results with the original using that oscilloscope of yours :)

[RobHolt]

I'd also go 18db/oct but start rolling at 20Hz.

Something to consider is that while the amplified warp signal at say 5-12Hz might look alarming on a reflex speaker design it is also quite detrimental to overall sound quality, particularly in a small-medium 2-way since mid range is effectively being sourced from a moving point. Try listening to solo piano while the bass/mid driver is doing (inaudible) summersaults and the pitch will be clearly modulated and cracked.
Ridding yourself of this problem is likely more important than any small phase shifts at the very bottom end IMO.

[AlexM]

I have been experimenting with the Audacity High Pass Filter, and it does seem to filter very low frequencies effectively, judging by the visual effects on the waveform.

I will experiment a little further using test tones and looking at the FFT histograms to see how the low frequencies are rolled off.

Very low frequency content is bad news for valve amps as it leads to early saturation of the output transformers, as well as causing useless and excessive excursion of the bass drivers.

Thanks for the tips - I'll update once I have had a chance to try some different processing methods, and post a brief how-to guide describing my software tools and workflow.

Regards,
Alex