BW Broadcast DSPXtra-FM, Operational Manual, Page: 35 / 63

Setting Up The Processing
35
the limiters because we are not worrying about overshoot and can set the attack times of the limiters higher than
what would have been possible without the multi-band clippers.
Radio stations have a desire to be competitively loud and clipping is the easiest and most effective way in gain-
ing loudness in a processor. While clipping is effective there is only so far you can push a clipper before notice-
able distortion occurs. We can push this boundary of distortion back further by filtering out some of the distortion
post clipper. By filtering after the clippers we are able to significantly reduce audible distortion.
The DSPXtra-FM multi-band clipper has three clippers and three post clipper filters. Bands 1 and 2 sum to serve
the bass-clipper. Band 3 and 4 serve the mid-clipper and band 5 and 6 serve the HF-clipper. Low pass, band
pass and high pass filters are used respectively.
The DSPXtra-FM has several controls that relate to the multi-band clippers. The first is a drive control and is self
explanatory. It is a ganged level control that works in conjunction with the mixer controls. The 0dB drive level
is a reference point that we choose that drives the multi-band clippers at a level that will produce a competitive
amount of loudness. You may want to increase this if your goal is maximum loudness, listening carefully for dis-
tortion at the same time or decrease it when you do not want to process so heavily. The mixer level controls and
multi-band limiters peak attack times and thresholds will have an effect in how much drive gets through to the
multi-band clippers so you may need to compensate with the multi-band clipper drive should you adjust these.
Each of the clippers has a threshold control and these thresholds are referenced against the main output clip-
pers clip level. For example, setting the bass-clipper at -6dB would allocate half of your available modulation
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 DSPXtra-FM.
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 DSPXtra-FM
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 dynami-
cally 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 actually 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