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Restoring Audio Quality > Introduction
Article last updated March 14, 2009
Audio restoration
is not as complex or time-consuming as video. And unlike
video, it can typically be restored completely in software.
This guide will discuss several of the most common audio
problems, as well as solutions and software options. Because
audio is not very CPU intensive, most audio editing software
comes with real-time previews. Restoring audio takes
experimentation without requiring vast amounts of time.
Error
Glossary
Much like the video restoration error glossary, this one
does not use technical terms. Tech terms simply confuse
people, so these are descriptive terms. Most audio
corrections are actually frequency carving. Restoring audio
primarily involves cutting away the bad portions of the
audio spectrum and then compensating the remaining parts to
sound good.
(1) Mild audio hiss: Almost all analog formats have
some sort of hiss. SoundForge, under the Process menu, in
the Paragraphic EQ, comes with a default Hiss Removal
filter. Audacity can use the High Pass filter found under
the Effect menu. In GoldWave, the Hiss Removal filter is
found under the Effect -> Filter -> Noise Reduction
filter, and using the Hiss Removal preset. Dart XP can work,
but it tends to carve out too much, so is not optimal. Hiss
reduction is accomplished by lowering the decibels (dB) of
the 10 to 25 kHz range by about 5dB. Dart XP can be used
here too, but it tends to carve out too much.
(2) Harsh audio hiss: Older tapes and copies of
copies normally have the source hiss compounded with it’s
own hiss. Mono audio does this as well. This is essentially
the same as the Mild audio hiss filter, but it is
accomplished by carving out a large chunk of the decibel
range. The heavier noise reduction filters chop out as much
as 10dB from the 3 to 25 kHz range. Dart XP excels at
removing excessive hiss, using the default DeHiss settings.
Sound Forge Paragraphic EQ using custom hiss removal
filter.
(3) Buzzing and Humming: Much like the ghosting
images error for video (discussed in video restoration
guides), humming is often power related. If the signal is
coax, be sure it is grounded properly. Check to make sure
the video equipment and computer is getting clean power.
Check the PC speakers and be sure it is really the file, not
the speakers. If this is on RCA audio cables, try another
set, preferably high grade gold plated shielded ones.
Sound Forge Paragraphic EQ using custom buzz removal
filter.
(4) Cracking distortions: This is typically caused by
having the recording volume or playing volume too high. The
sound has been clipped, as it reach above peak allowable
values. The GoldWave Smoother filter and Pop/Click filter
are good at fixing this error. The standard Dart XP DeHiss
will work. In SoundForge, use the Paragraphic EQ and drop
all dB after 2.5k, as well as 12dB at 20 kHz.
(5) Muffled/Low audio: The easy way to restore
muffled audio and mono audio is to restore the high pitches.
In SoundForge, slightly raise the dB in the Paragraphic EQ,
near the 5.1k mark. Even as few as 5dB high can make a huge
difference in the quality of the audio. In GoldWave, use the
Equalizer and raise the 6000Hz and 15kHz ranges several
points.
(6) High pitch: The opposite of high, pitchy audio is
muffled audio. The same filters used for harsh audio hiss
will typically correct this problem.
(7) Bass grumble: Excess bass can be removed with any
number of filters or equalizers. SoundForge comes with a
Remove Low Grumble preset for the Paragraphic EQ. It
essentially drops the 80k and below range of all dB.
GoldWave also comes with similar filters, and Audacity can
have the EQ manually adjusted.
GoldWave filter to remove bass grumble. SoundForge has
some too.
Conclusion
Restoring audio is mostly about playing with filters. This
is because no one audio filter will be able to correct all
possible audio problems. The seven most common errors have
been listed, along with possible fixes. An audio file may
need several filters run over it before being restored.
Always be sure the speakers on the computer will reflect the
same kind of audio that will be heard on the television.
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