572
10
572 Basics
Chapter 5
An SPL computer periodically samples the ambient sound in a room. Having this information, the
SPL computer uses a combination of historical data and information stored during the unit's initial
calibration to determine the optimum gain setting for the sound system.
How Does It Work?
The 572 uses the combination of analog circuitry and digital circuitry under the control of a
microprocessor. The microprocessor excels at following a set of instructions and making decisions.
Since really high-performance digital audio circuitry is still quite expensive, the 572 uses a record-
ing studio-quality voltage-controlled-amplifier (VCA) to control audio levels.
In operation, the 572 monitors its input signals for silence. Anytime that silence occurs (an option
switch allows changing the silence threshold), the 572 switches to sense mode. In sense mode, the
572 reduces the amplifier input by at least 30 dB, the loudspeakers are disconnected from the power
amplifier and connected to a high-gain differential amplifier (which cancels the hum and other junk
picked up by the unshielded speaker wires). The 572’s microprocessor monitors the output of the
sense preamp and compares that signal to the calibration conditions stored in the 572’s parameter
storage RAM, and at the last sample period. Based on this information and the front-panel set-
tings, the 572 decides how loud the sound system must operate to restore audibility.
The 572 also has an internal timer, controlled by the front-panel S
AMPLE
I
NTERVAL
control that
forces a sample if there haven’t been any silent periods in the input signal. Anytime that the input
signal causes a sample period, the timer resets. If the input material has sufficient silent periods,
the timer-generated sample periods never occur.
In CAL mode, the 572 acquires a history of the environment’s sonic behavior. The 572 takes three
measurements: the environment's minimum level, the sound system's minimum level, and the sound
system’s maximum level. By running the system for about 8 seconds at the desired minimum and
maximum sound pressure levels, the microprocessor remembers these three key performance
parameters, which are automatically stored in nonvolatile memory.
During calibration, the installer sets minimum and maximum levels-the 572 “reads” the controls as
they are changed, and stays in the appropriate calibration mode for about 8 seconds after the last
change is made. This allows the installer time to reset operating levels as necessary.
When the system switches itself back to operate mode, the acoustical performance history
acquired during calibration becomes the baseline for gain change decisions. In operate mode, the
system goes about measuring environmental noise levels and internal signal levels. Because the
computer has already committed to memory the parameters stored during calibration, it is able to
predict the gain setting required.
What About the Sensing Microphone?
Microphone? What microphone? The 572 doesn’t use sensing microphones. It does use the sound
system’s loudspeakers as microphones during sensing . There aren’t many constraints on your
choice of loudspeakers. Horn-type speakers are more directional as well as being highly efficient
(both as speakers and as microphones). We have found that planar-type speakers do not work well
as microphones. Internally, the sense signal drives a bandpass filter (A-weighting approximation)
before the 572 measures its level (strength).
Содержание 572 SPL
Страница 1: ...572 572 SPL Computer User s Guide...
