Specifications

Setting Up The Processing
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and distortion that your listeners will find uncomfortable to listen to especially for extended periods of time. We
also suggest that you make final clipper drive adjustments in tandem with the multi-band clipper drive as what is
taken from or added from one can usually be made up for with the other.
The final clipper now has an additional control to help reduce IMD distortion. This clipper finesse control is an
additional program dependent mechansim that helps to reduce distortion by analysing the amount the level
of IMD distortion and attempting to lower it by controlling how much the low frequencies can push the higher
frequencies into the clipper. The control is very subtle and its range has been limited to restrict the amount of
control preventing pumping and loss of loudness which would undo what we want to use the clipper for, gaining
loudness.
You may not notice the effect of this control on all program material. When adjusting the clipper finesse control
we recommend that you turn the final clipper drive up past the point that you have it set at. This will make the
effect of the finesse control much more obvious and allow you to find the setting that sounds best for your for-
mat. once the clipper finesse control is set you can back down the final clipper drive to the point that sounds
best knowing that the clipper finesse control has been set correctly to help keep the distortion down on difficult
program material.
Lookahead limiter
A look-ahead limiter is used in the DSPX to provide peak control for the DR path of processing. This is not just a
simple single band peak limiter. The DSPX look-ahed limiter works in three bands to maximise the quality of the
processing while minimalising audible artifacts.
Each of the three bands has its own length delay line optimised to match that of the dual time constant gain
control circuit controlling that bands level. By realignment of delays the look-ahead limiter like all of the DSPX
processing maintains linear phase. You have access to the secondary (average responding) time constants for
each band and these controls allow an element of control over the texture of the look-ahead limiters processing.
The look-ahead limiter has its own fixed internal time constants which 'RIDE' piggyback on the secondary time
constants that you have access to. These fixed peak time constants are optimised for the band in question to
preserve transparency and peak control and are matched to the delay lines.
The look-ahead limiter has its own drive control adjustable over a +/- 6dB range. Like the multi-band clippers the
0dB reference point for the drive was chosen as a compromise between loudness and quality. As you increase
the drive more loudness will be obtained but at the expense of IM distortion which will start to make the audio
sound busy and packed.
The adjustable time constants in the look-ahead limiter also play a big role in affecting on air loudness and these
need to be adjusted carefully to prevent pumping when driving the look-ahead limiter more aggressively. Like
the dual time constants in the multi-band limiters you can adjust so that the peak time constants have most of
the control or the secondary ones have most of the control, or a balance, which works best. To lessen the effect
of the secondary time constants set the attack to 10 and the decay to 1. This minimalises energy distributed into
the secondary time constant circuit and the peak time constant circuit will dominate and control the audio. If we
were to do this with each of the bands of the look-ahead limiter we would significantly increase the loudness but
more business and IM distortion would be introduced to the audio especially under higher levels of drive. If we
were to flip the settings of the secondary time constants around to distribute most of the energy into the second-
ary time constant circuit we would see an improvement of audio quality at the expense of reduced volume due
to the longer release time constants being used. As previously pointed out a balance is best and that balance
will vary with each band of the look-ahead limiter. The solo mode may help you to achieve this.
One of the most important features of the look-ahead limiter is the shelf filter control. This is effectively an
adjustable low pass filter that is used to tame high frequencies. The shelf is needed when the DSPX has its
pre-emphasis control engaged, for several reasons. The first being that the DR processing path usually serves
a broadcast medium that does not have de-emphasis so outputting pre-emphasised audio will not sound very
nice. Secondly, the pre-emphasised audio will dominate the peak control of the look-ahead limiter and create
spectral modulation pumping. The shelf allows us to compensate for the effect of this increased high frequency
energy restoring a more natural tonal balance to the DR processing path. The shelf controls range is 0 to -17dB
with negative numbers equating to more cut. The numbers are the gain reduction being applied at 15 KHz. As
an example a processor setup for DUAL use that had its pre-emphasis set to 75uS would require a shelf cut
of between -17dB and -14dB to restore a more natural tonal balance and 50uS pre-emphasis would probably
require -15dB to -12dB. Even if the processor does not have pre-emphasis enabled you may find that you may
want to experiment with small cut figures of between -4dB to -1dB to tame the high frequency energy that may
have built up in the multi-band processing.
We need to draw your attention to one other control that can have an effect on the look-ahead limiter and which