OPTIMOD-FM OPERATION
3-57
Speech is detected if (1) the input is mono, and (2) there are syllabic pauses at least once
every 1.5 seconds. Speech with a stereo music background will usually be detected as
“music,” or the detector may switch back and forth randomly if the stereo content is right
at the stereo/mono detector’s threshold. Mono music with a “speech-like” envelope may
be incorrectly detected as “speech.” Music incorrectly detected as “speech” will exhibit a
slight loss of loudness and punch, but misdetection will never cause objectionable side
effects.
Speech that is not located in the center of the stereo sound field will always be detected
as “music” because the detector always identifies stereo material as “music.” This can
increase clipping distortion on such speech.
If the
Bass Clip Mode is set to Hard, the speech/music detector will automatically set it to
Medium when speech is detected and Hard otherwise (unless Latency is Low, in which case
Medium bass clipping is unavailable and bass clipping will stay Hard).
Speech always sounds cleaner with
Medium bass clipping, and the increased bass
“punch” supplied by
Hard is irrelevant to speech.
MB Speech Threshold
(“Speech Thr,” located in the Multiband page of Advanced Con-
trol) lets you set the increment (in dB) by which the setting of the
MB Limit Thr control is
reduced when speech is detected. This control is particularly useful in minimizing speech
distortion when you use the
LLHard bass clipper—it allows the main clipping distortion
controller to work harder on speech while preserving punch in music.
ITU-R Multiplex Power Controller
The ITU-R recommends that the power in the composite baseband signal (including the
pilot tone), integrated over any 60-second interval, not exceed the power in a sinewave
that modulates the FM carrier to ±19 kHz (25.3% modulation). Many European countries
are now enforcing this recommendation. (See ITU-R 412 Compliance on page 3-11 for
more information.)
Multiplex Power Threshold
: The 8400 provides a means to limit the integrated multi-
plex power to the ITU standard by a patented closed-loop technique that allows you to
use any preset and to create customized presets freely. The multiplex power controller is
adjusted in the Input/Output: Utilities screen by the
Multiplex Power Threshold control.
Set it
Off if your country does not enforce the standard.
The control is located in the Input/Output: Utilities screen because the regu-
lation applies to all operation of the processor in a given installation.
If your country enforces the standard, you should set the control to complement the
amount of peak overshoot in the transmission system following the 8400. Setting the con-
trol at “0” will correctly control the multiplex power when there is no overshoot after the
8400. This will typically be true when you are using the 8400’s built-in multiplex en-
coder to drive the transmitter directly.
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