2. FIXED PITCH NOISE
3. IMPULSIVE NOISE
4. RANDOM NOISE (HISS)
5. HISS REDUCTION
6. NOISE REDUCTION EXAMPLES
Random noise (hiss)
Rumble can also
be regarded as random noise, and much the same considerations apply to
its removal as hiss, which is what will be considered here.
distortion, wow and flutter are almost impossible to remove, it's
possible to effect a considerable improvement in random noise levels.
However, any such process tends to affect the original sound, so there
is always a balance of judgement to be made - reducing the noise
against taking the top off the recording. Unfortunately many transfer
engineers seem intent on removing almost all the noise, apparently
unconcerned about the woolly sound which results.
simplest way - and for many years the only way - to remove hiss was to
filter it. Of course this had serious effects on the actual recording,
and it became normal to expect 78rpm records to sound like this. With
the development of more sophisticated methods it became possible to
have far less effect on the original sound. There were a few analogue
systems, most notably the Packburn
(now available in a new version), which plays
78s with a stereo
pickup and continuously compares the two groove walls, switching
rapidly between them to choose the quieter to remove clicks and plops,
as well as providing hiss reduction and equalization. The first digital
was developed in 1988 after several years research. Initially it
took all night to process one 78rpm side, but now it's available both
as software and as hardware, which can process a record as it's played.
However, it's extremely expensive. Other systems include Sonic
at a professional level (now superceded), and in more
inexpensive systems such as Bias
SoundSoap (no longer available) and Adobe
become available and work on home computers: I use ClickRepair
by Brian Davies.
The basic process for
dealing with hiss
involves splitting the audio spectrum into a large number of very
narrow frequency bands - from 256 up to some thousands - and working on
each one separately. A sample of the recording where only the noise is
audible can be used, or a pre-determined template, and for each of the
frequency bands a level is noted which represents the noise.
the case of each narrow band, any audio which is lower than the
determined threshold is successively reduced in level as the level
drops - so that a 3 dB drop in the original might be replaced by a 6dB
drop: this is done until a level of reduction is reached which can be
set in the program. Above the threshold the audio is unaffected, in the
hope that this is the original recording and not noise. (Many 78s have
more noise than recording at higher frequencies and this presents a
considerable challenge to hiss-reduction programs.)
are a number of parameters which can be either manually set or more
usually are preset in the program: the point at which as the level
drops the reduction ceases: the speed with which the reduction is
applied when the level drops and removed when the level increases: and
the amount by which adjacent frequency bands affect the behaviour of
each band (large differences in the levels between bands can cause
unpleasant effect similar to a very low-bitrate mp3). There are other
more sophisticated considerations in the design of these programs,
which is why they all behave differently.
The next page
some examples of hiss reduction.