DSPXmini-FM operational manual V1.2
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The downside to bass clipping is you are restricting the bass to a lesser level than what it would be without it.
The upside is that moderate levels of bass clipping won't cause a large loss of bass loudness and should have
minimal audible artefacts.
When bass-clipper is being driven more aggressively you will start to notice generated distortion. This distortion
can be used to actually give the illusion of more bass, especially on smaller radios that are incapable of produc-
ing the lower frequency fundamental bass waveform. This can be viewed as an upside of bass clipping. You
need to decide what level of bass clipping is acceptable to your format, both in creating room for summation
from the other bands and making the punch/distortion trade-off.
The final clipper
The final clipper, used in the FM processing path is a sophisticated highly over-sampled peak limiter that incor-
porates distortion controlling techniques and has an embedded 1 kHz low-pass filter. This section of processing
is the last line of defence in the processing and is also the most critical part in the loudness/quality trade-off.
While each of the proceeding processing stages play a part in reducing the peak to average ratio of the audio
waveform none has the same effect on the peak to average ratio as the final clipper.
Great care is needed in setting the final clipper drive control. This control needs to be adjusted carefully and
only you can make the decision on the balance between loudness and quality. As you increase the drive you will
obviously obtain more loudness but at the expense of distortion. There is a fine line between artistic distortion
and distortion that your listeners will find uncomfortable to listen to, especially for extended periods of time.
The final clipper now has an additional control to help reduce IMD distortion. This clipper hardness control is an
additional program dependent mechanism that helps to reduce distortion by analysing the the level of IMD dis-
tortion and dynamically adjsuting time constants. The control is subtle and its range has been limited to restrict
the amount of control, preventing pumping and a loss of loudness which would undo what we want to use the
clipper for which is gaining loudness.
You may not notice the effect of this control on all program material. When adjusting the clipper hardness 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 hardness control much more obvious and allow you to find the setting that sounds best for your for-
mat. Once the clipper hardness control is set you can back down the final clipper drive to the point that sounds
best knowing that the clipper hardness control has been set correctly to help keep the distortion down on difficult
program material.
There's also an overshoot control for the clipper. Setting this control to lower numbers allows more over-
shoots through to the composite clipper which trades off main clipper distortion for composite clipper distortion.
Composite clipper distortion can sound a touch nicer albeit at the expense of slight contamination of the output
spectrum.
Composite clipper
The composite clipper in the DSPXmini-FM's stereo encoder is highly over-sampled and allows you to gain an
extra dB or two of modulation loudness when using the multiplex output to drive your FM transmitter. The range
of the composite clipper is -0. to 2dB.
Here at BW we prefer to use the final clipper to generate the required amount of loudness because composite
clipping introduces stereo crosstalk. When we do use the composite clipper we prefer to use small amounts of
no more than 1dB.
Auditioning individual stages of the DSPXmini-FM processing
When setting up the DSPXmini-FM it is sometimes useful to hear the effects of the adjustments on that particu-
lar processing stage. By setting the other processing blocks up in a certain way you can make it easier to hear
the effects of the one you are adjusting.
For example, to listen to just the action of the multi-band limiters bypass the AGC stage and back down the
multi-band and final clipper drives. To hear just the clippers do the same back the drive down into the limiters to
-6dB. You can then up the drive into the clippers with the clipper drive controls and mixer controls to here just
the clippers working.
