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.
Whereas 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.
Obviously, the 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 system, CEDAR
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 Solutions' 'No-Noise'
professional level (now superceded), and in more recent
times inexpensive systems such as Bias SoundSoap (no longer
available) and Adobe Audition
have become available
and work on home computers: I use ClickRepair and DeNoise by
Brian Davies (unfortunately now unavailable).
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.
In 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
There 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
The next page
examples of hiss reduction.