OPTIMOD-FM OPERATION
3-3
AGC meters show the gain reduction of the slow AGC processing that precedes the mul-
tiband compressor. Full-scale is 25 dB gain reduction.
The AGC is a two-band unit with Orban’s patented bass coupling system.
The two meters indicate the gain reduction of the AGC Master and Bass
bands.
Gate indicators show gate activity. They light when the input audio falls below the
threshold set by the gate threshold controls. (There are two gating circuits—one for the
AGC and one for the multiband limiter—each with its own gate threshold control.) When
gating occurs, the AGC and compressor’s recovery times slow drastically to prevent
noise rush-up during low-level passages.
Multiband gain reduction meters show the gain reduction in the multiband compressor.
Full-scale is 25 dB gain reduction.
If the Five-Band structure is operational, all the meters display gain reduc-
tion (G/R) activity. If the Two-Band structure is operational, only the two
leftmost meters display G/R activity.
Multiplex Power meter indicates the action of the ITU Multiplex Power controller. It
shows how much the MPX Power Controller has reduced the clipper drive, thereby re-
ducing the average power in the processed audio.
This meter, labeled “P,” is displayed on the 8400’s screen. It always appears
when the Two-Band Structure is active. If the Five-Band Structure is active,
it only appears if the MPX Power Controller is turned on.
HD (High Definition Digital Radio) G/R meter shows the gain reductions in the left and
right HD look-ahead limiters.
This meter only appears when the HD option is installed and the Meter Sel.
Switch (on page 5 of the Input/Output menu) is set to HD GR.
Introduction to Processing
Some Audio Processing Concepts
Reducing the peak-to-average ratio of the audio increases loudness. If peaks are reduced,
the average level can be increased within the permitted modulation limits. The effective-
ness with which this can be accomplished without introducing objectionable side effects
(such as pumping or intermodulation distortion) is the single best measure of audio proc-
essing effectiveness.
Compression reduces the difference in level between the soft and loud sounds to make
more efficient use of permitted peak level limits, resulting in a subjective increase in the
loudness of soft sounds. It cannot make loud sounds seem louder. Compression reduces
dynamic range relatively slowly in a manner similar to riding the gain: Limiting and clip-
ping, on the other hand, reduce the short-term peak-to-average ratio of the audio.
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