Specifications

Setting Up The Processing
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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.
Xover
The Xover in the DSPXtreme has five selectable frequencies per band split.. The band 1/2 split can be set to 80,
100, 125, 160 or 200 Hz. The band 2/3 split can be set to 400, 500, 650, 800 or 1000 Hz and the band 3/4 split
can be set to 1100, 1400, 1800, 2250 or 2800 Hz.
The xover employs linear phase FIR filtering for audio transparency. Extra delay lines time align the audio bands
ensuring a flat response across the whole audio spectrum regardless of gain reduction levels. Unlike most other
audio processors the DSPXtreme maintains this linear phase time aligned property throughout all of its process-
ing stages. This can be verified by the application of a low frequency square wave to the DSPXtreme's inputs
and monitoring the flat top response on the DSPXtreme's outputs. You must make sure that all of the bass
enhancement and input conditioning filters such as the phase rotator are switched to off before conducting this
test. The bypass preset will do this for you (pre-emphasis and de-emphasis is not switched by a preset change).
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 DSPXtreme 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-
stant but the sustained energy will also charge the secondary slower circuit. When the audio energy falls away
and the circuit goes into release the peak decay will dominate until it reaches a point where it hands over to the
slower secondary time constant for a slower rate of decay. The illustrations show this to good effect, where tran-
sients have a fast release but multiple or sustained transients build up energy in the secondary circuit which acts
as a platform for the peak to release to. The secondary circuit's platform can be thought of as the average level
of limiting. Having this fast peak responding circuit ride on top of the average circuit creates many advantages,
limiter transparency, less chance of pumping and greater loudness. By setting the time constants appropriately
we can have the multiple time constant based detectors work as peak handling, average handling or the opti-
mum setting of a balance of the two.
The peak attack time should be set to the desired attack time required from that limiter. The range is 1-10 which
corresponds to 1 to 200mS on an exponential scale. The peak decay time should be set to the desired peak
decay time required for transients. The range is 1-10 which corresponds to a decay time of 10 to 1000mS.