Phonic PCX 4000 User Manual page 11

Runtime Correction
When using a multi-way system, if the drivers are not exactly
aligned on the vertical axis, the result can be apparent phase
errors and cancellations. Particularly for high-frequency bands,
which have shorter wavelengths, it is important that drivers be
positioned vertical to one another rather than horizontal. This
helps avoid unwanted reflections from the ceiling. In many cases,
even if the drivers are perfectly aligned, runtime errors may still
occur. This is mainly due to the different design and construction
of different speakers. For this reason, PCX4000 have included
a delay function on low bands that allows up to 2 milliseconds
of delay to be applied to your signal. This will allow you to push
specific cabinets as far back as 68.6 cm (as a 2ms delay will allow
for a 68.6 cm offset). Sound travels at approximately 343 meters
per second, or 34.3 cm a millisecond. Frequency is measured in
oscillations per second, the unit of which is called "Hertz." So, for
example, if you adjust a crossover frequency of 4 kHz between the
high and mid range systems, the wave length (typically represented
by the λ symbol) is calculated by dividing the speed of sound (c)
by the frequency (f). Therefore, the wavelength for 4 kHz is:
With a virtual distance of 68.6 cm, use of the delay control could
possibly ensure phase coincidence. If the offset of the drivers is
greater than 68.6 cm, however, none of these settings will be able
to correct the runtime. Correcting the runtime is important, as it
allows your system to correcting reproduce pulse peaks and will
maintain phase coincidence when the signal frequency deters
from the crossover frequency.
Correcting Runtime Differences
For all frequency ranges, the runtime and phasing can usually be
corrected quickly and easily by following these few short steps:
1. Measure the offset between the drivers in cm and move
cabinets (or adjust their corresponding delays) until you think
they are correctly aligned. Be sure that the distance between
the two drivers is not greater than 51.5 cm, as your attempts
to get the speakers in phase and without runtime errors will
be in vain.
2. Run pink-noise through your system and get your hand-held
analyzer ready.
3. Adjust the bands above and below your crossover frequency
so that each of them produces a 0 dB reading at the crossover
frequency (as read by your analyzer). Mute all bands other
than the two surrounding the crossover frequency. If the two
bands together produce a +3 dB reading, than they can be
considered in phase.
4. Next, raise the crossover frequency moderately (about 100 Hz
or so) and perform steps 2 and 3 again. The analyzer should
read +3 dB here for both bands too.
5. You can also reverse the phase of one of the bands here to
check for sound cancellation.
Keep in mind that only one speaker stack should be corrected
at any one time, and you should normally start from the highest
crossover frequency and work your way down.
PCX4000
Limiters
In the PCX4000, the various output controls should be the first
resort to helping combat excessive signals that could cause
problems for your speakers or other devices. Limiting the output
signals in a crossover should be avoided if at all possible, but is
ideal for ensuring your output signals don't become too excessive.
There is one limiter control that allows users to adjust the limiter
threshold between -6 dB and OFF, however each output channel
does have its own independent limiting circuit and LED.
To set the limiters, first run pink noise through your system,
driving your amps at full load (0 dB) – this is assuming your
amps and speakers are compatible in terms of power rating. Set
the threshold control to off, and engage the limiter button. Adjust
the threshold lower gradually, until a point where just a few of the
limiter LEDs flash.
Low Sum
When the Low Sum button is pushed, the low-band signals of
channels 1 and 2 are combined as a single channel and sent
through the low output of channel 1. This allows the single channel
to output the entire range of low frequency sounds in your audio,
thereby allowing a single sub to better handle your signal.
Constant Directivity Horn Switch
Constant-directivity horns have become increasingly popular in
recent years, as they produce a very regular dispersion throughout
their frequency range. Unfortunately, as the frequency gets
increasingly high, the constant directivity horn becomes less
efficient. The PCX4000 includes a CD Horn button to compensate
for this weakness, allowing high frequencies to be boosted slightly
(by 3 dB at 3.5 kHz, increasing gradually by 6 dB per octave up
to 22.5 kHz).