Packburn 325 Owner's Manual - page 3
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AUDIO NOISE SUPPRESSOR MODEL 325
A single-ended noise suppressor designed to eliminate or reduce noises due
to imperfections in and damage to sound recordings in all analog audio media
and their digital reproduction, including cylinder, disk, film wire and tape
recordings, both monophonic and stereophonic
Model 325 contains three processors designed to suppress
transient noises (ticks, pops, clicks, crackle, scratch)
encountered in some 100 years of phonograph recordings,
wherever and however made, as well as the audible hiss
familiar in all audio media prior to the development of
successful encode/ decode noise suppression systems and,
more recently, digital audio.
Model 325 also incorporates a number of
necessities and conveniences for the optimum playback of
disk and cylinder recordings for best quality of sound.
The Switcher:
The first noise reduction processor in the Audio
Noise Suppressor is the Switcher. This is designed specifically
for the reduction of noises from monophonic disk and cylinder
recordings, taking advantage of the redundancy that exists in
these media: the same signal is engraved on each of the two
side walls of the groove. However, the distribution of
particulate matter in the record material (one cause of noise) is
random. Also, dirt, mildew and scratches do not affect each
side wall identically.
Prior to the development of the Packburn Switcher,
a monophonic disk or cylinder, if played back with a
stereophonic reproducing system, was best played by
summing (in the appropriate polarity) the signals from the left
and right channels. The Switcher does this when in the rest
position. However, at any moment when the reproduction from
the left or right channel is quieter (more noise-free) than the
sum signal, the Switcher can elect to reproduce just the quieter
groove wall. The Switcher switches among these three
possibilities (left, right, or sum) at a very rapid rate with
astonishing results. At lower frequencies (from 300 Hz down)
where switching would not accomplish anything, the two
channels are mixed to minimize rumble.
The idea of using the Switcher on vertical-cut
recordings may seem strange at first since there is only one
signal engraved in the bottom of the groove. However, the
vertically modulated signal is affected by disturbances in the
side walls of the groove. The Switcher is not as effective for
vertical recordings but it has a part to play.
The switching process is generally not applicable for
noise reduction of stereophonic records or of monophonic tape
recordings or broadcast. For these, transient noise
suppression must be achieved by the Blanker alone.
The Blanker:
The second noise reduction processor is the
Blanker. This is designed to cope with transient noises from
any source: whether from the output of the Switcher, where
noises remain that were common to both side walls of the
groove, or from a stereophonic disk recording. Also, it is
applicable to a monophonic tape, a broadcast or a CD record
of a disk recording that has transient noises. Note that the
Packburn Blanker works from any program source, in contrast
to some devices that require the vertical component of a
stereophonic disk to trigger their action.
The Blanker clips the amplitude of each individual
positive-going or negative-going pulsation of the noise
transient whenever it exceeds a threshold value determined by
the peak program level in the vicinity of the transient.
The Switcher plus the Blanker comprise the
Transient Noise Suppressor of the Audio Noise Suppressor.
The Continuous Noise Suppressor:
The third noise reduction processor reduces audible
hiss-or white noise, whatever one prefers to call it. It is a
constant annoyance in just about every cylinder, disk, wire,
analog tape or film recording made prior to the development of
successful encode/ decode systems of noise suppression,
followed by digital mastering.
This noise is perceived by the human ear as being
of a fairly continuous nature in contrast to the pops, ticks, and
clicks of transient noise and therefore, we call our third
processor the Continuous Noise Suppressor. This processor
comes third because it is expeditious to suppress the transient