BW Broadcast DSPXtra-FM, Руководство по эксплуатации

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Setting Up The Processing
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Hold
The multi-band limiters have an extra mechanism called HOLD which works in the same way as the GATE con-
trol in the AGC stages. Unlike the GATE control the HOLD control has no extra associated controls like return
to rest levels and speeds. The hold feature when triggered will HOLD the average platform level at its last value
before the hold was triggered. This feature can help reduce IM distortion, reduce pumping effects and avoid
suck up of noise. The HOLD feature can be adjusted over a level of -20 dB to -40dB with an option for 'OFF'.
Unless you have a specific reason to turn it off we recommend a setting of around -32dB but this may need to
be increased or decreased depending on drive levels into the limiter and how much effect you want the hold
control to have on the audio. If set too low you will rob yourself off some loudness, if set too high you will lose
some of the benefits of the mechanism by letting the limiter release too far before gating.
Delay
The delay control sets the amount of time the processor will hold the gain reduction for before releasing at the
peak decay rate. Careful setting of this control allows us to speed up the peak decay time without introducing
the audible effects of faster decay times. Setting this control to 1 is a negligible amount of delay before decay
which is equivalent to the delay being switched out of circuit. As you go up through the bands you will need
lower delay numbers to avoid causing audible problems being introduced by the delay circuit. We have found
that bands 3 and 4 work well with settings of 2-4, band 2 settings for 3-6 and band 1 settings of 5-8. Setting the
control to 10 introduces almost half a second of delay before decay so make sure you don’t use the higher set-
tings of this control on anything other than band 1.
If you are unsure about this control we suggest you set it to 1 to turn it OFF.
The mixer
The post limiters mixer in the DSPXtra-FM is not strictly a mixer but a band output level control where small EQ
changes 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 do 3 and 4 and also 5
and 6 so I suppose you could call it a half-mixer. The six 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 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
Without hold With hold