MANUAL OF ANALOGUE SOUND RESTORATION ... - British Library

software. The senses of touch and instant responses are very important to an analogue
operator.
It is vital to plan a way around the noise difficulty. Kits are available which allow
the “system box” to be out of the room, while the keyboard screen and mouse remain on
the desktop. Alternatively, we might do trial sections on noise-excluding headphones, and
leave the computer to crunch through long recordings during the night-time. In section
1.4 I expressed the view that the restoration operator should not be twiddling knobs
subjectively. A computer running “out of real-time” forces the operator to plan his
processing logically, and actually prevents subjective intervention.
This brings us to the fact that desktop computers are only just beginning to cope
with real-time digital signal processing (DSP), although this sometimes implies dedicated
accelerator-boards or special “bus architectures” (both of which imply special software).
On the other hand, the desktop PC is an ideal tool for solving a rare technical problem.
Sound archivists do not have much cash, and there aren’t enough to provide a user-base
for the designers of special hardware or the writers of special software. But once we can
get a digitised recording into a PC and out again, it is relatively cheap to develop a tailormade
solution to a rare problem, which may be needed only once or twice in a decade.
Even so, archivists often have specialist requirements which are needed more often
than that. This writer considers an acceptable compromise is to purchase a special board
which will write digital audio into a DOS file on the hard disk of a PC (I am told Turtle
Beach Electronics makes such a board, although it is only 16-bit capable, and requires its
own software). Then special software can be loaded from floppy disk to perform special
signal processing.
3.10 Processes better handled in the analogue domain
The present state-of-the-art means that all digital recordings will be subject to difficulties
if we want to alter their speeds. To be pedantic, the difficulties occur when we want to
change the speed of a digital recording, rather than its playback into the analogue
domain. In principle a digital recording can be varispeeded while converting it back into
analogue simply by running it at a different sampling-frequency, and there are a few
compact disc players, R-DAT machines, and multitrack digital audio workstations which
permit a small amount of such adjustment. But vari-speeding a digital recording can only
be done on expensive specialist equipment, often by a constant fixed percentage, not
adjustable while you are actually listening to it. Furthermore, the process results in
fractional rounding-errors, as we saw earlier.
So it is vital to make every effort to get the playing-speed of an analogue medium
right before converting it to digital. The subject is dealt with in Chapter 4; but I mention it
now because it clearly forms an important part of the overall strategy. Discographical or
musical experts may be needed during the copying session to select the appropriate
playing-speed; it should not be done after digitisation.
With other processes (notably noise-reduction, Chapter 3, and equalisation,
Chapters 5, 6 and 11) it may be necessary to do a close study of the relationship between
the transfer and processing stages. The analogue transfer stage cannot always be
considered independently of digital processing stage(s), because correct processing may
be impossible in one of the domains. For readers who need to know the gory details, most
digital processes are impotent to handle the relative phases introduced by analogue
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