2. FIXED PITCH NOISE
3. IMPULSIVE NOISE
4. RANDOM NOISE (HISS)
5. HISS REDUCTION
6. NOISE REDUCTION EXAMPLES
sound recordings can suffer from faults, though modern recording
practice minimises most of them to the point where they aren't a
problem. However in the restoration of old (mostly analogue
there are a number of faults which are normal in recordings: the
restoration process involves trying to eliminate or at least reduce
them. This article deals with analogue recordings: uncompressed digital
by their nature tend to either be full quality or not work at all,
and other digitally compressed
recordings introduce their own
set of interesting artefacts. The
article is spread across six pages and includes some brief examples:
these are best listened to on wide-range speakers - the effects may not
be so obvious on small computer speakers.
principal problems which can affect analogue sound recordings are
distortion, incorrect frequency response, pitch errors, and noise.
Since this article is concerned with noise the first three are
discussed only briefly below: a more detailed article specifically on
the reproduction of 78rpm records is posted
most familiar form of distortion
is common in amateur recordings:
overloading the recording system so that loud parts of the audio are
clipped. Once this has happened there is little that can be done about
it. This fault is rarely present in professional recordings, but more
general harmonic distortion can occur in amplifying systems, leading to
additional frequencies being added to the sound - usually an octave or
an octave and-a-half above the original. The former is not too serious
if it is at a low level, the latter far more so: again, little can be
done about it.
occurring in the form of too little of the higher frequencies, this can
be caused by deterioration of the recorded media, or by incorrect
equalization - gramophone records in particular were not recorded
'flat' for technical reasons (see this
for a detailed explanation) and a wide range of
different equalization curves
were used over
the period they were in
common use. Adjusting this is a relatively simple matter, but if it's
necessary to boost the top, the noise usually comes up with it.
can be continuous, where a recording is simply being played at the
wrong speed; or cyclical - 'wow' (slow regular variations, such as a
gramophone record with on off-centre hole) or 'flutter' (rapid
variations, usually a fault on tape recordings). The former is,
obviously, simple to correct - simply adjust the playback speed: but
wow and flutter are almost impossible to eradicate (where the wow is
caused by an off-centre hole in a gramophone record the usual approach
is to enlarge the hole with a reamer and then align the record manually
until it's centred... not easy).
is the main bugbear of sound recording, and over the years various
methods have been applied to combat it - most famously the various
systems for tape and film
soundtracks, where the signal is
processed during recording, and de-processed during playback, in a way
designed to reduce noise.
Noise takes three main forms:
impulsive, fixed frequency, and random. Impulsive noise is clicks and
plops, most commonly caused by damage to gramophone records, but also
by electrical interference when tape recordings are being made (though
the latter is not usually a problem in professional studios). 78rpm
records suffer not only from clicks - usually scratches - but crackle:
in effect many continuous closely-spaced clicks usually caused by the
abrasive filler placed in the 78rpm shellac during manufacture to grind
the needle down to fit the groove shape.
Fixed frequency noise
takes the form of hum - usually induction from the mains supply - or,
if at a higher frequency, whistles: this is common on recordings made
, but 78rpm records
sometimes suffer from 'swarf whistle'
as the off-cut from the blank disk scrapes past the cutting stylus. As
this reduces in frequency towards the centre of the disk (where the
linear speed is slower) it is extremely difficult to eradicate.
noise usually takes the form of hiss: tape hiss is caused by the random
placement of the magnetic particles in the recording tape, and was a
problem which became less severe as years progressed and both the media
and the methods for recording on it were improved. Microphone
amplifiers can also produce audible hiss, though again modern systems
don't suffer audibly from this. In the case of 78s efficient crackle
removal still leaves residual hiss, usually at quite a high level.
Low-frequency noise - rumble - can be
caused by poor bearings on gramophone turntables (or the original
disk-cutter), or by external sounds such as ventilation, traffic, or
passing underground trains - the latter affected several famous
recording venues, but in many cases only modern wide-range playback
systems show it up.
The next page
following deal with the various types of noise and its reduction.