OPTIMOD SURROUND PROCESSOR INSTALLATION
2-31
OUTPUT SOURCE is AGC or MB), then excessive clipping is occurring. Turn
down 2.0
LIMITER DRIVE control until you no longer see clipping on the
meter.
Occasional light clipping is usually inaudible. It is sometimes preferable to
allow occasional clipping in order to use the headroom available in the
downstream channel most efficiently. This depends on the headroom
available—for example, in a 24-bit channel, there is no excuse for clip-
ping at any time.
If the 2.0
OUTPUT LEVEL meter is indicating very low levels, you may wish
to turn up the 2.0
LIMITER DRIVE control to better use the downstream
headroom.
c) Once you have determined a good setting for this control, save the preset
as User preset.
11. Set the 2.0 processing’s output level if O
UTPUT PREEMPHASIS control is set
to 50μs or 75μs.
[Skip this step if the O
UTPUT PREEMPHASIS control is set FLAT.]
We recommend using 5-Band “TVA” presets (or User Presets derived from them)
when the 2.0
OUTPUT PREEMPHASIS control [step (9.H) on page 2-29] is set to 50μs
or 75μs. The 2.0 processing in the “TVA” presets is designed to feed an analog TV
aural channel using 50us or 75us preemphasis. The surround processing in these
presets is identical to the surround processing in the corresponding “TV” presets,
whose 2.0 processing is optimized to drive channels with no preemphasis. The
main purpose of the “TVA” presets is to allow the 8685 to process analog and
digital transmission channels simultaneously, which is useful for stations in coun-
tries that have not completed the transition to all-digital broadcasting.
To simplify setup, the “TVA” presets have local values of 2.0
DIALNORM (–17 dB),
2.0 L
OWPASS FILTER CUTOFF (15 kHz), and 2.0 HIGHPASS FILTER CUTOFF (20 Hz).
These values override the corresponding global values in the active Transmission
Preset.
The filter settings complement analog aural carriers, while the value of D
IALNORM
matches the loudness of the 8685’s 2.0 processing to the loudness of an Opti-
mod-TV designed for analog television (like Optimod-TV 8282 and 8382) when
the peak output levels of the two processors are the same and when the 2.0
O
UTPUT 100% control is set to 0.0 dBfs.
A) Set the 2.0 Output Source to MB+L
IMIT (step (9.G) on page 2-28).
The 8685’s look-ahead limiter will automatically control the peak modu-
lation of a transmitter that applies 50μs or 75μs to its input. Moreover,
the average level and loudness do not change when you adjust the 2.0
OUTPUT LEVEL (100%) control. The control only sets the maximum peak
level at the 8685’s output. For every dB that the 2.0 Output Source is
turned down, the 8685 automatically increases the limiter drive by 1 dB
to compensate. This is to ensure that the output loudness is always con-
sistent with the 8685’s active Dialnorm setting.
B) Make sure that preemphasis is set according to the standard used in your
country. See step (9.H) on page 2-29.
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