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Quick and Dirty System Setup
Some basic guidelines can be used when setting up the sound system
using these indicators. These simple steps are for “on the
fly” situations, and are not intended to be a substitute for
an actual system calibration.
1. Use a program source that is typical of what is being reproduced
by the system. This can be music or pink noise. Remember that
the peaks present during a live show will often be much higher
than the peaks of most pink noise or compact disc sources.
2. Feed the source to a mixer channel and set the controls to
produce a “zero VU” output level.
3. Check the next device in the signal chain for clip ping. Most
processors will have a clip LED for this purpose. Increase the
mixer’s output level until the clip LED of this device begins
to flash intermittently. Note the reading on the mixer’s
meter and do not exceed it.
4. Check the remaining devices in the signal chain for clipping,
using their respective clip LEDs. If clipping occurs, reduce the
output level of the device ahead of the one that is clipping.
5. Assuming that the amplifier has been properly selected for
the loudspeaker (that’s a whole other story!) advance the
amplifier’s input level control until the sound level is
loud enough or the clip light begins to flash.
The level indicators on audio devices are there for a reason. There
are two conditions that indicate a problem. These are “no reading”
and “overload.” In either case, the sound from the system
will not be the best that it can be. Pb
The Best of Both Worlds. . .
The “complete” picture of an audio waveform must include
peak information and an indication of the energy content of the
signal. Peak LEDs provide the former, while the traditional VI provides
the latter. The “best of both worlds” would be a meter
that allowed both of these parameters to be monitored simultaneously.
The Dorrough Loudness Monitor is such a meter. It allows system
operators to see the peak and average levels of complex signals
in real-time. A few minutes spent with this meter demonstrates the
inadequacy of meters that only display one parameter or the other.
Applications for this meter abound. It comes factory calibrated
to +4 dBu at meter zero. A trim pot is included to allow the calibration
to be changed. Two inputs are provided, and each can be calibrated
to a different zero reference. I left one input calibrated to the
factory set ting for monitoring mixer and signal processor outputs.
I calibrated the other input to read zero on an input volt age of
2.83 volts when driven through a 10 dB pad. The meter is now a dBW
meter and reads in dB ref. 1 Watt for an 8 Ohm load. This allows
the output of a small power amplifier to be monitored. My Crown
D75 clips at + 18 dBW (60 watts) but both peak and average can be
monitored. Most music program sources cannot produce more than about
6 Watts (-10 dB ref. 60 watts) into the loudspeaker prior to clipping.
I’ve always said that a good audio demonstration is one that
illustrates a complicated concept in a simple and obvious manner.
We’ve watched the “lights come on” for many seminar
attendees by using the Dorrough meter to demonstrate peak levels,
continuous levels and crest factor. This once confusing subject
becomes self- explanatory once one is able to see it happen. We
can now easily answer some perplexing questions such as:
“What is headroom and how much do I need?
“Why must my amplifier be rated higher than my loudspeaker’s
continuous power rating?”
“Does my peak limiter use peak or RMS detection?”
These are all very relevant questions, but the answers can be quite
obscure without a good picture of the concept.
I would recommend some quality time with a Dorrough meter to anyone
that works with sound systems. The enlightenment that I gained is
on par with what I experienced with my original TEF analyzer and
the EASE room modeling program. These valuable tools provide pictures
of the not-so-obvious, and allow many questions to answer themselves.
The factory setting puts meter zero at 1.23 VRMS (+4 dBu). When
various program sources drive the device to meter zero, the peaks
can be clearly observed. The effect of compressors and limiters
is visually obvious. In this configuration, the maximum level that
can be displayed is 12.5 V (+24 dBu).
To make a “power meter” I calibrated meter zero to be
2.83 volts just ahead of a 10 dB pad driving the meter The pad is
to prevent the meter from being over driven. The reading is now
in dBW and meter zero is 1 Watt into 8 Ohms. Maximum displayable
power is 100 Watts (+20 dBW).
For more information on the Dorrough meter, visit www.dorrough.com.
Pb
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