Nexo GEO D - Manual (Page 73)

S YSTEM ALIGNMENT GUIDELINES Page 73/97
7.3 Flown or Stacked combined GEO SUB’s and GEO D’s clusters
Recommended NX242 setup family: OVERLAP
When GEO SUB’s are flown or stacked within the GEO D array, GEO SUB delay should be set at the
same value than GEO D’s, ie there should be no delay difference between systems.
7.4 Separate GEO SUB and GEO D clusters
Delay must be set to compensate for horizontal distance offset between front edges.
When GEO SUB’s and GEO D’s are flown in separate clusters, distance between clusters should be:
NX242 GEOD setup family: OVERLAP
• 1.25m (4 ft) minimum center to center or 50cm (1.5 ft) side to side;
• 1.75m (6 ft) maximum center to center or 1m (3 ft) side to side.
NX242 GEOD setup family: X-OVER
• 1.25m (4 ft) minimum center to center or 50cm side to side;
• No
upper limit.
7.5 Driving the GEO SUB’s from the AUX send
It is quite common to use the AUX send of a mixing desk to drive the Sub section of a PA system. This
gives the mixing engineer more flexibility to set the level of the subbass relative to the main PA, apply
special effects, or to use a different EQ on the Sub. However, it also raises some serious issues for the
performance & safety of the system (mostly time alignment).
At NEXO, great care is taken to design optimum phase alignment from one octave above to one octave
below the crossover frequency point. By doing so, drivers are working perfectly together and providing
the best efficiency possible. It is then up to the user to adjust the delay on the NX242 to match the
physical path difference of the different systems. It is thus possible to get a well adjusted system, even
without measuring instruments.
If GEO SUB’s are driven form an AUX output, NX242 is fed with two signals coming from different
sources. If those two sources (MAIN output & AUX send) are not exactly in phase, delay is introduced
into the crossover between the GEO D array and the GEO SUB’s. It is then mandatory to use proper
measurement tool to optimize phase response.
Why is it unlikely that AUX and MAIN outputs have the same phase?
• Signal paths are likely to be different; any filter modifying the bandwidth and EQ of the
signal is also affecting the phase.
Example: a 24dB/oct high pass filter set at 15Hz is affecting amplitude of the signal by only
0.6dB at 30Hz, but the phase shift is 90°!! At 100Hz we can still measure 25° of phase shift.
• Limiting bandwidth with a low pass filter can introduce a phase difference of up to 180°
(completely out of phase) at the cross over point.
• If the signal is passing though any digital equipment, between 1.4ms and 2.2ms is being
added (around 70° phase shift at 100Hz) due solely to the converter delay! The additional
delay due to the processing itself (look ahead compressor, delay…) can be quite important
as well.
If both outputs are not measured in the actual configuration, it is very likely that phase alignment will not
be correct.