OPTIMOD SURROUND PROCESSOR OPERATION
3-77
ahead peak limiter is a powerful tool for achieving this with minimum
distortion or other side effects. Nevertheless, be aware that this function
is not like some familiar “look-ahead” limiters. The release time is in the
order of a few milliseconds and is not user adjustable. The purpose of the
limiter is only to limit peaks that pass through the earlier compressors be-
cause of their finite attack times. Functionally it is used like a peak clip-
per, but it has vastly reduced modulation distortion by comparison to a
clipper, whether “soft knee” or “hard knee.”
The main potential side effects of the look-ahead limiter are gain breath-
ing and a “warbling” sound in the midrange when heavy bass is simulta-
neously present. Listen carefully for this intermodulation effect (particu-
larly on vocals) when you are adjusting the F
INAL LIMIT control.
N) Adjust the Bass Clip Threshold and Bass Clip Shape controls to complement
the amount of final limiting.
For most mastering applications, you can set the BASS CLIP THRESHOLD to
O
FF. However, if you hear pumping or “warbling” distortion in the look-
ahead limiter caused by heavy bass transients, you can reduce this effect
by setting the B
ASS CLIP to a lower level. (The BASS CLIP control is cali-
brated in “dB below the look-ahead limiter threshold.”) It is most effec-
tive when the B1/B2
CROSSOVER control is set to 100 Hz, as this setting
prevents intermodulation between vocals and instrumental bass in the
clipper.
O) If you wish to compare your processed sound to the unprocessed original, re-
call the L
OOK AHEAD LIMITER preset and toggle between it and your processing
preset. If there is a gross loudness disparity, you may wish to edit the gain
within the L
OOK AHEAD LIMITER preset and save this as a user preset.
P) Save your preset using File/Save Preset.
Once you have created one “mastering” preset, you can edit it to create
others and save them under different names.
Q) For a 44.1 or 88.2 kHz output sample rate, set the digital output level (step 9
on page 2-27) to approximately –0.5dBfs; this will prevent overshoots caused
by sample rate conversion. For a 48 or 96 kHz output
sample rate, set the digi-
tal output level to –0.1dBfs.
At 44.1 kHz, the output samples are not exactly the same ones that the
look-ahead limiter controlled at the internal 48 kHz sample rate, so slight
overshoot can occur. At 48 kHz output sample rate, overshoot will be less
than 0.1dB.
If your mastered audio is intended for transmission via a lossy codec like
AAC, MP3 or WMA, be aware that the codec’s decoder may overshoot
and cause audible clipping distortion. See Setting Output/Modulation
Levels on page 1-21. If the 8685’s output is applied directly to a lossy co-
dec
, decrease the setting of the 8685’s output level control to allow the
necessary headroom. If the 8685’s output is applied to a linear PCM stor-
age medium (like a CD), it is better to use the entire dynamic range of
the linear medium by setting the 8685’s output level close to 0 dBfs. Then
compensate for codec overshoots by reducing gain when you transcode
from the linear PCM recording to the codec.
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