Image Processing Reference
In-Depth Information
Uncompressed waveform
Compressed waveform
Figure 37-7 Analog compression.
37.7
Level Adjustment
Find the loudest passage in the audio and then adjust the overall volume of the tracks to
make sure that the peak volume does not exceed an acceptable loudness level.
The overall level of the audio should be consistent and not go up and down signifi-
cantly. In the quiet passages, there is fine detail that must be accentuated. Applying that
adjustment across the entire piece makes loud portions too noisy and causes clipping. This
is analogous to setting the brightness and contrast of a picture.
Some kind of automatically adjustable compensation must be applied. That
includes some gain adjustment and a compressor-limiter to prevent the sound volume
from fluctuating. The result will be a more even and consistent track. This ought to
have been done as part of the production process if you are encoding commercial mate-
rial. Your source may be some home-movie footage or something that has been recorded
live. Some audio tracks are spliced together from a variety of different sources. It is useful
to have the necessary tools already available to cope with audio processing when the need
arises.
The optimum level to set is to ensure that the peaks in your audio don't exceed a
level that is 3 dB down from the 0 dB reference on the VU or PPM meter on your audio
monitor.
Setting the gain so that the loudest passage reaches a level commonly known as 0 dB
is too high for some compression software. The recommended value of 3 dB below the ref-
erence allows the compression tools some headroom to cope with transients. The 0 dB
level will be indicated by a mark of some kind on your audio-level meter, or if you use an
LED monitor, it may turn red when this level is reached. Figure 37-8 shows how the audio
level is adjusted.
Be careful that you adjust the levels consistently across all of the tracks you are
encoding. This is important because otherwise you will inadvertently pan the audio
“image” off center and the sound will not be delivered as intended.
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