Digital audio cleanup
AUDIO CLEANUP, SOUND RECOVERY, RESTORING RECORDINGS
I am frequently sent recordings in poor
sound
quality by collectors and others. I restore the quality
as well as I can. Sometimes the recordings
are "boomy" (contain too much bass); sometimes they
have excessive hiss, and occasionally they are just indistinct or very
low in volume. They are frequently on aged cassettes or
reels. Sometimes
they're on surveillance tapes or mobile 'phones.
I usually concentrate on archive work, but have done jobs for solicitors,
programme makers, and others. Recently I cleaned up
the sound track for a television documentary. A couple of weeks before that I worked on an old
cassette recording for BBC radio 7. It's interesting work.
There are now a number of digital software packages which can improve the quality of poor recordings significantly. COOLEDIT and COOLEDIT PRO are particularly good.
EXAMPLES OF WHAT THESE PROGRAMS CAN DO
NORMALISATION
This consists of boosting the volume of a recording until it reaches a predefined level, usually -3 to 0 dB. This is very useful for recovering lost volume in a faint recording. When you do this, the background noise level comes up quite a lot so you need.....
FILTERS
There are filters on most software packages which can boost or reduce the amplification in definite frequency bands, rather like "graphic equaliser" controls on a hi-fi. Some filters are infinitely variable and can even mask out constant-frequency buzzes and hums. Parametric filters don't really use frequency "bands" but the way they work is similar.
NOISE REDUCTION
A bit of "silence" in the defective recording is sampled, to get an idea of the background noise. Then you tell the computer to subtract this noise - either very gently or more aggressively - from the whole recording, or just the section with the noise problem. The results are sometimes astonishing. Hisses are hardest to remove; they are essentially more random than, say, a 50Hz buzz, because they contain the sounds of many frequencies mixed together.
Some recordings clean up very easily; some are problematic and take longer. Occasionally one meets a recording which is so degraded that it's beyond recovery, but this is rare.
Sound recovery works best if you regard
it like carpentry. You cannot get a good finish at the
first attempt. Remove the unwanted noise gently, in
layers, so you don't damage the sound underneath - just
as you'd use fine glasspaper on a valuable antique.
If you need some audio cleanup doing, feel free to get in touch. Please note that
I do not do transfers of old music 78's to digital format. It's usually waste of time, unless it's
a historic recording.
Nigel Deacon / Diversity website
