BW Broadcast DSPXtra-FM, Руководство по эксплуатации

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Setting Up The Processing
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re-create Left and Right at the Arianes outputs. Normally, this setting should be about -5 dB, but it can be
set higher for a bigger stereo effect, or lower for reduced multi-path interference. As you might guess, this
setting is somewhat subjective.
Stereo
Channel Coupling/Restriction The two channels can be coupled together, in an adjustable amount,
from full coupling (“0 dB”) to the relatively independent 15 dB , or “Off,” no coupling at all. The upside is
that you can control channel imbalances. The downside is that you risk occasionally shifting the centre
image if Release and/or IDR controls are set up aggressively.
Bass enhancement
The frequency contouring effect of multi-band audio processors often leaves the bass lacking a little. The sum-
mation of the bands tends to give a boost to the presence frequencies and leaves the bass sounding a little thin.
This effect can be compensated somewhat by enhancing the bass prior to multi-band processing.
The DSPXtra-FM has two types of bass enhancement filter. A low frequency shelving boost filter and a peaking
bass equaliser.
The shelving filter has a 12dB/octave slope and can be adjusted to provide between 0 and 12dB of bass boost.
Use this control with caution as too much low frequency boost can cause loss of mid-bass because the low
bass, often inaudible on many receivers dominates the gain reduction of the BAND 1 AGC and limiter. A setting
of 6dB is a good compromise and starting point.
The peaking bass equalizer is a pseudo parametric style bass equalizer control that allows you to sweet tune
the bass. Four frequencies, amplitudes and Q's are provided giving you 64 different bass curves to select from.
Frequencies selectable: 60Hz, 76Hz, 95Hz and 120Hz. Q's selectable: 0.4, 1, 2 and 4. Gains selectable: 0,
1.5dB, 3dB, 4.5dB, 6dB. A starting setting of 95Hz, Q of 1 and gain of 4.5dB warms the bass up quite nicely but
you are free to experiment to get the bass sound you're after.
Additionally, bass tune control allows control of the 'flavour' of the bass by adjusting various points in the bass
dynamic control system.
Multi-band limiters
The multi-band limiters drive can be adjusted over a +/- 12dB range. Increasing the drive will increase the level
of limiting and with it on air loudness, above a certain level of drive no more loudness will be obtained and all
that will happen is you will generate higher levels of IM distortion and the sound will take on a busy packed tex-
ture. You may also observe higher levels of high frequency noise when the band 3 and 4 drives are increased.
We don't usually find much use for drives above +6dB but more may be required if other settings are adjusted
to compensate. In any case, observe the peak limiter meters for a good indication of how much drive to use. We
don't recommend more than 12dB of gain reduction especially on bands 2, 3 and 4. Gain reductions of 4-8dB
are a good compromise between loudness and quality.
The multi-band limiters have a threshold control and care should be taken when adjusting it as distortion in the
following peak clipping stages can result if the threshold is set too high. The range is +/- 6dB.
The multi-band limiters in the DSPXtra-FM are of the dual time constant variety. There is an attack and decay to
handle the peaks and an attack and decay to handle the average level of limiting. Understanding how the two
time constants interact is imperative if you want to make major changes to how each bands limiter reacts. We
have included some scope screen captures to illustrate things a little clearer. The peak and average function can
clearly be seen in the images.
Traditionally audio limiters have two time constants, an attack, the time is takes the limiter to respond to a signal
above the threshold and a decay or release which is the time is takes to respond to a drop in level. In a tradi-
tional audio limiter the attack time is usually set to somewhere in the region of a few milliseconds and the decay
time considerably longer at somewhere in the hundreds of milliseconds. This is not the most optimum solution
because transients that last only a few milliseconds will reduce the level of the waveform for hundreds of mil-
liseconds, reducing loudness and creating audible pumping effects.
The solution is multiple time constants where one set of time constants can be set to handle the fast peaks and
another to handle the average level of limiting. Fast transients will release in a faster less noticeable way and
won't punch holes in the sound in a way that single time constant limiters can. The secondary slower time con-
stant circuit will not have much effect on the audio waveform when hit with a transient because the higher attack
time, generally in the hundreds of milliseconds will not allow a build up of energy. In the case of a sustained
envelope of audio above the threshold the multiple time constant will attack as normal with the peak time con-