Specifications

Setting Up The Processing
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SETTING UP THE PROCESSING ON THE DSPX
This section has more detailed information on setting up the DSPX’s processing. The section is laid out in the
same order as the processing route through the DSPX.
High pass filter
The high-pass filter has five selectable cut off frequencies and a bypass option. Most users will bypass the high-
pass stage but there are several cases where enabling the high-pass has an advantage. The first of those is
stations that mainly play vinyl recordings. Vinyl recordings can suffer from low frequency rumble and the high
pass filter reduces the effect of these low frequency rumbles. Some processing experts believe that removing
the very low frequency content from the program material improves the rest of the bass sound from the proc-
essor. The theory is that most people can’t here or speakers can’t produce the very low frequency bass. They
believe that by removing this sub-sonic bass more room is made in the processed waveform for frequencies that
can be heard. Another reason that is given is that this very low frequency bass can dominate the band 1 AGC
and limiter, especially after bass enhancement has been carried out. The low frequency shelving filters used in
processors have much higher gains at 20Hz than say 50Hz where most people can here and speakers repro-
duce bass. The processing stages will respond to this amplified 20Hz content even though most people won’t
ever hear it when listening to your radio station.
Some radio transmitters suffer from AFC bounce and overshoot when driven with high levels of very low fre-
quency bass. If your transmitter suffers from this phenomenon you may need to turn your modulation down to
accommodate these overshoots. The high-pass filter in the DSPX can cure this problem by removing the very
low frequency content from the program material
Phase rotator
This parameter if enabled will help to reduce vocal distortion in aggressive presets by reducing asymmetry in the
voice which would otherwise put more workload on the clipping stages. Human speech (especially male) can
be very asymmetric compared to music and the phase rotator helps to bring symmetry to the audio waveforms.
We recommend enabling this option if you are after maximum loudness; Conservative formats such as classical
may prefer to leave it off as the phase rotation process does colour the sound slightly, although this coloration is
often used for artistic effect.
Pre-emphasis
This parameter selects the standard 50uS and 75uS pre-emphasis curves used in FM broadcasts. The param-
eter can be bypassed with the ‘OFF’ option if the DSPX is not processing audio for FM transmission.
The wideband AGC
The wideband AGC in the DSPX employs an RMS based level detector for superior performance. This enables
the DSPX to control input level variations based on the true loudness of the input waveform unlike other sim-
pler average responding peak detectors used in other digital audio processors. When you couple the advanced
detector with the user adjustable and hidden intelligent controls you really do have a powerful levelling tool.
Over the course of the next few pages we have included several scope shots clearly illustrating the input and
output of the AGC together with the AGC control signal. The effect of the control signal is clearly evident on the
output audio waveform. These scope shots help to visually illustrate the concepts under discussion.
The first control that most people reach for is the drive control, stop! As the wideband AGC is designed to
respond in a slow manner increasing the drive level won't help. The only time we recommend increasing or
decreasing the drive is if your program material is pre-processed and the resting level of the wideband AGC is
far off the midway resting level of -12dB gain reduction. The wideband AGC is designed so that a 0VU input
level to the processor (with input gain set correctly) will drive the wideband AGC to the midway resting level of -
12dB. If you are driving the processor correctly you should not need to adjust the drive level of the WB AGC and
doing so will bring minimal benefits.
The attack and decay times of the AGC have a range of 1-10 and this corresponds to time constants of 100mS
to 30S. We suggest an attack somewhere in the region of 3-4 and a decay setting of 1 or 2 positions higher than
that.
Like most competent audio processors the AGC stages in the DSPX are gated. This slows down the release
time of the AGC when the program material drops below a certain level. This prevents noise suck up and gain
hunting from occurring during quiet periods or lulls in the audio. The DSPX has three controls that affect gating.
The first is the gate level and this can be adjusted over a range of -20dB to -40dB. This is the level at which the
program material must fall below for the gate to become active. The gate level control has two more options,
OFF and ON. OFF is self explanatory and prevents the gate from having any effect. ON is often referred to in