Specifications

Setting Up The Processing
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To force gate a band you need to set the gate threshold control for that band to 'ON'. You then need to adjust
the RTR level to set the gain reduction in that band. The gain through the stage will equal ((BAND DRIVE in dB
- 12dB) - RTR LEVEL in dB). For example to create unity gain you must make sure that the drive control is 12dB
more positive than the RTR level IE: -12dB RTR with a drive of 0dB.
Version 2 adds the facility to control how the multi-band AGC gating works. Previous versions of DSPX software
had a seperate gate for each band with that gate only being able to control that band. Now you have the option
to gate all bands if any of the gate circuits become active. This control is called ‘gating type’ and we suggest
you leave it at its default setting of ‘individual’ unless you know what you are doing. By selecting the ‘combined’
option any of the gates will gate all bands. We have included this option as some processing veterans like to
freeze all bands when gating occurs, preserving the frequency balance. If you do decide to set up the gating this
way we suggest you turn the Band 1 and 4 gate thresholds to very low numbers or even OFF. If you don’t then
any program material that has passages with little low or high frequency energy will cause triggering of the gat-
ing system which could cause unwanted gating of the program material. Our experiments have found that bands
2 and 3 provide a good indication of valid program material and you can be more confident that the gating is
happening correctly. At BW we prefer to use the individual option but you are free to experiment with the com-
bined option if you know what you are doing.
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 DSPX are of the dual time constant variety. There is an attack and decay to han-
dle 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.
The average attack time is perhaps the most important control in the dual time constant detector as it sets the