Crown CTs 4200USP/CN Operation Manual page 18

3 Operation
(continued)
Most amplifi er/load systems can be confi gured and supervised
by following these steps:
1. Confi gure your audio system using a known "good" load,
then enable the Load Supervision feature.
2. Provide typical program material at a level high enough to
light the "test" indicator.
3. Run the system at this level until the average impedance
stabi lizes. This may take seconds to minutes depending on
level, duty-cycle, etc.
4. Set the nominal impedance at the measured value average.
This optimizes the supervision algorithm for voltage and
cur rent levels versus the actual load. Note: a higher nominal
set ting will require higher output levels.
5. Set the high limit at twice average and the low limit at one-
fourth nominal. (These limits are somewhat arbitrary but
should be a good starting point.)
6. Let the system run for extended periods using any and all
typi cal program material.
7. Adjust the high/low limits, if necessary, to account for any
variance in average measured impedance.
8. Enable error reporting, if desired.
This procedure should work well for most applications. However,
some applications can be a little more diffi cult. Some very low-
level and/or low duty-cycle signals may not adequately "test" the
load. Lab and situation testing have shown output levels as small
40 dB below rated amplifi er output to be enough for most low-
impedance loads. Higher impedance loads such as those used in
"lightly-loaded" 70V distribution lines may require signal level
near 20 dB below rated output.
The "Nominal Load Impedance" control is used to optimize the
system for the most accurate calculation of load impedance. It
should be set to the expected nominal (or rated) impedance of the
"normal" load. The high limit should be set for at least 2 times the
expected nominal or actual measured load, while the low limit
should be set to ½ the expected nominal or actual mea sured load.
The following example calculates the SPL necessary for
supervi sion of a typical 8-ohm system. While the resulting 80-dB
SPL @ 1 meter is defi nitely above conversation level, it is not
uncom fortable.
page 18
An "8 ohm" example:
30 mA into 8 ohms = 0.007watt.
8-ohm driver sensitivity = 100dB for 1W @ 1 meter.
0.007W/1W = –20dB.
Required SPL for supervision test is 100dB – 20dB = 80dB SPL
@ 1 meter.
3.2.27 Typical Load Characteristics
It is well known that the typical loudspeaker impedance is not the
same for all frequencies. This variance is due to the effect of
elec trical properties such as the expected increase in impedance
at high frequencies due to driver voice-coil inductance, or the
peaks and valleys due to passive crossovers. Testing of various
passive boxes has shown peaks of 100 ohms or more! Low-
frequency impedance variation can come from the interaction of
the driver compliance with that of the box. The low frequency
variations are usually wide bandwidth and may vary from 6 to 30
ohms on an 8-ohm driver.
These anomalies are easily averaged out by the USP/CN
supervi sion algorithm in most systems. However, there may be
some extreme situations for very narrow bandwidth (i.e. single-
note) signals and/or very widely varying loads that the algorithm
sim ply cannot overcome. In these cases, widening the high and
low limits will help decrease the "sensitivity" of supervision and
decrease the chance of "nuisance" error reports.
3.2.28 Filters
Each audio channel has fi ve separate places where fi lters can be
placed in the system. There are 64 fi lters total and they can be
placed anywhere within the system. In addition to fi ltering, each
possesses up to ± 24 dB of gain. The fi lters will vary based on the
fi rmware and software being run. The following fi lters are
available:
• Lowpass: Bessel 2-4, Butterworth1-4, and Linkwitz-Riley 4
(Firmware 2.0 provides up to 8 Linkwitz-Riley fi lters)
• Highpass: Bessel 2-4, Butterworth1-4, and Linkwitz-Riley 4
(Firmware 2.0 provides up to 8 Linkwitz-Riley fi lters)
• Lowshelf: Low-frequency shelving EQ
• Highshelf: High-frequency shelving EQ
• Lowpass EQ: Variable Q from 0.1 to 35
• Highpass EQ: Variable Q from 0.1 to 35
• Parametric EQ: Variable Q from 0.1 to 35
• All-Pass: 1st and 2nd order
CTs Multi-Channel Power Amplifi ers
All-Pass fi lters provide no gain change to the output, but provide a
phase change at the selected frequency. This corrects the phase
relationship of the output without a gain reduction, such as is
found in other fi lters.
3.2.29 Delay
Due to the nature of DSP processing, there is some inherent delay
or latency within the system These delays are:
DSP processing: 1 ms or 1000 µs.
Digital-to-analog conversion: 250 µs.
Analog-to-digital conversion: 250 µs.
Amplifi er: 100 µs.
In addition to these unavoidable delays, additional delay can be
added to each channel. Each channel is capable of 2.0 seconds of
delay in 20.8 µs increments.
Overall delay = inherent delay + coarse delay + fi ne delay.
3.2.30 Noise Generator
A noise generator shared between channels allows noise to be
mixed into the audio signal. This is useful for noise masking
applications and testing. Each channel has the following controls:
• Noise On/Off: The channel's noise generator can be
indepen dently turned on.
• Noise Type: Full spectrum white noise or pink noise.
• Noise Level: A fader to allow the noise level to be controlled.
3.2.31 Sine-wave Generator
A sine-wave signal generator allows the mixing of a single tone
into the audio signal. Typical applications can be for the injection
of a high frequency tone (19 kHz) into the signal in paging type
system to continually drive the speaker, allowing continuous
mon itoring of the speaker load. The following controls exist:
• Sine On/Off: The sine-wave signal generator's function.
• Sine Frequency: Controllable from 20 Hz to 20 kHz.
• Sine Level: Each channel's sine-wave signal level can be
independently controlled.
3.2.32 Input Signal Router
Each channel of the module's signal processing has an Input
Sig nal Router that lets you choose the audio signal that will be
used by the channel. Choose one of the following confi gurations:
Operation Manual