Specifications

Setting Up The Processing
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The mixer
The post limiters mixer in the DSPX is not strictly a mixer but a band output level control where small EQ chang-
es can be made. It has been called the mixer as most other processors have a mixer at this position and our
VIRTUAL mixer does the same job. Bands 1 and 2 do mix together at this point so I suppose you could call it a
half-mixer. The mix of band 1 and 2 is then fed off with the other two bands to the peak limiting paths for DR and
FM. The four bands have become three.
Be careful when making large EQ changes at this stage because there is no peak control prior to the clipping
system. It is easy to overload the clipping stages by setting these controls all to large positive values.
The control range for each band of +/- 3dB is purposely restricted for the above reasons.
The solo mode allows you to mute three of the four mixer inputs so you can listen to one band of processing
at a time. This is a useful feature for processing experts to fine tune the settings of the DSPX. We would like to
point out that you may here distortions when listening in solo mode that unnerve you and you may try to readjust
the processing parameters to process them out. When striving to be competitively loud you will need to accept
a certain level of distortion, all be it controlled distortion. Most of the time this distortion is psycho acoustically
masked in one way or another by the rest of the program material. When you use solo mode you remove the
content from the other bands that may be masking the distortion you here when listening in solo mode to one
band alone. With this in mind, we do not recommend you use solo mode to make clipper adjustments, for two
reasons. Firstly, the level going into the clipper will be different once the other bands are summed in. Secondly,
the masking effects are not evident when listening in solo mode.
In short, the solo mode is a useful tool, understand its limitations and don't let it fool you into making unneces-
sary processing adjustments. Your listeners will never listen to your station with the processor in solo mode so
don't rely on it as the main way of setting up your processor. I suppose we have made it sound like its not worth
using but in fact its quite useful for setting up the bass clipper and adjusting limiter time constants.
The multi-band clipper
While the outputs of the multi-band limiters are peak limited they suffer from overshoot. The audio waveform
can pass through before the limiter has time to 'attack' the signal. This is not a design flaw in the limiter but a
required response. If you remember from our earlier discussions about audio processing you will recall that the
processing can sound more natural and dynamic if a limiter lets sharp transients through rather than clamping
down the whole audio signal whenever a transient occurs. These small transients can audibly dominate the limit-
ing if the attack time of the limiter is too fast. It is much better to let these small transients through and deal with
them in the next stage of processing.
The multi-band clipper that follows the multi-band limiters is designed to deal with these limiter overshoots and
clip them to a pre-defined level. You may also recall from our earlier discussions on processing that clipping
transients and overshoots is pretty much inaudible if done in moderation. The combination of the multi-band lim-
iters and multi-band clippers provides us with the perfect answer. The limiters control the peaks of the audio but
suffer from overshoot, the clippers then provide us with a true defined peak ceiling. Because the limiters precede
the clippers we won't suffer from clipper overload induced distortion because the clippers are being fed with
audio that has a pretty constant peak level. We also gain the advantage of more dynamic natural sound from
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 DSPX multi-
band clipper has three clippers and three post clipper filters. bands 1 and 2 sum to serve the bass-clipper. Band
3 serves the mid-clipper and band 4 serves the HF-clipper. Low pass, band pass and high pass filters are used
respectively.
The DSPX 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