Setting Up The Processing
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level for the bass (mix of bands 1 and 2) and leave the remaining half for the sum mix of the mid-clipper and the
HF-clipper. If these two clippers' clip thresholds were set so that they didn't add up to more than 50% modula-
tion, say -12dB and -12dB then the main clipper would have no work to do as all of the peak control would be
done with the multi-band clippers. By having defined peak clipper outputs we know that even with summation
our peak level can only be the sum of each of those peak clipper outputs. In practice it is best to let the main
output clipper do some of the work as a greater level of HF energy can be maintained. The best use of the multi-
band clippers is to control the bass energy fully and to keep the mid-range and HF energy from causing exces-
sive clipping distortion in the final clipper. We will discuss the bass-clipper next but before we do we would like
to recommend mid and HF-clipper clip thresholds of between -8 and -3dB. Higher numbers produce more bright-
ness but at the expense of greater distortion in the final clipper. A balance of multi-band clipping and final clipper
clipping produces the best results.
Bass clipping
Most competent processors have a bass-clipper prior to the final clipper. The purpose of the bass-clipper is to
keep low frequency energy to a pre-determined level to allow for the summation of the other bands. Without the
bass-clipper the bass signal can push the mid and HF audio waveforms into the final clipper creating audible IM
distortion, the worst type of distortion. By restricting the bass to a certain level the mid and HF energy has its
own reserved space in the summated waveform and we reduce the likelihood of bass generated IM distortion.
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 distortion generated. 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. We have been discussing a conventional bass-
clipper configuration and this is referred to as the hard bass-clipper option in the DSPX.
There is one more bass clipping option and that is known as the ‘SOFT’ option and uses look-ahead limiting.
Look-ahead limiting produces a soft-clipping function on the bass and this significantly reduces distortion in the
bass-clipper. There are a couple of down sides to this option. The first is latency (delay) as the DSPX needs
extra time to look-ahead to make the decisions to control the waveform before it arrives. The second is we don't
get the bass punch feature we spoke of earlier as there is less harmonic distortion generated. What we do get
is cleaner bass and the ability to use the look-ahead calculation time to modify the bass clip level dynamically to
let the bass fill in to 100% modulation when there is no mid or HF content taking up waveform real estate. When
you set the bass clip level in soft mode you are not setting the maximum level of bass as in hard mode but actu-
ally setting the maximum level it can be turned down to in the presence of mid and HF energy. For example, if
the bass clip level was set to -4dB and the audio waveform only contained bass, the bass-clipper level would
raise to 0dB and you would obtain 100% modulation with the bass. If the audio waveform had mid or HF content
the bass-clipper level would dynamically reduce to make room for the mid and HF content but could only reduce
by an amount equal to that of having a fixed bass-clipper threshold -4dB. This maximum amount of reduction
feature stops mid and HF energy from over controlling the bass.
You can think of the bass-clip threshold in the same way regardless of what mode you use. Just think of it as
creating space for the other bands.
What mode should i use? If you can live with the delay then try both and see what sound you prefer. Talk radio
and softer formats such as easy listening usually sound better with the soft option. Dance and urban formats
can benefit from the added punch generated by the controlled distortion with the hard bass-clipper. If you prefer
the soft option but the extra delay makes it uncomfortable for your DJ's then you can consider using the lower
delay monitor output from the DSPX as a studio feed.
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 15 kHz low-pass filter. This section of processing
is the last line of defence in the processing and is also the most critical part as regarding 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
