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Don Cox wrote:

> I think an engineer who is a competent musician and is 
> careful and patient can improve the sound considerably, but 
> it is an art rather than a science. 

How very true! Even if one could get an exact transfer function for the
original recording apparatus, equalizing an acoustical recording according
to the inverse function would result in amplification of surface hiss and
rumble, which happen to lie, unfortunately, in the audible range. That's
why, at least with current technology, the best way to improve the sound of
an acoustical recording is to do it manually. In many cases, a considerable
amount of low-frequency information which otherwise is inaudible is visible
in spectral-analysis displays. The information alone can be digitally
amplified while keeping the out-of-band (i.e. non-musical) noise at a low
level. The same is true with high frequencies, although this is quite a bit
more difficult to do by hand. There are filtering programs such as Har-Bal
which can indeed improve the upper frequency response without increasing the
hiss unreasonably. In all cases, improvements must be modified according to
the ears of the restorer. There's no black box which will do all of the
work.

AZS