Orban Optimod-AM 9400, Operating Manual, Page: 42 / 295

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INTRODUCTION ORBAN MODEL 9400
may be perceived as distortion, “gurgling,” or other interference. Psychoacoustic
calculations are used to ensure that the added noise is masked by the desired pro-
gram material and not heard. Cascading several stages of such processing can raise
the added quantization noise above the threshold of masking into audibility. In ad-
dition, at least one other mechanism can cause the noise to become audible at the
radio. OPTIMOD-AM’s multiband limiter performs an “automatic equalization”
function that can radically change the frequency balance of the program. This can
cause noise that would otherwise have been masked to become unmasked because
the psychoacoustic masking conditions under which the masking thresholds were
originally computed have changed.
Accordingly, if you use lossy data reduction in the studio, you should use the highest
data rate possible. This maximizes the headroom between the added noise and the
threshold where it will be heard. Also, you should minimize the number of encode
and decode cycles, because each cycle moves the added noise closer to the threshold
where the added noise is heard.
About Transmission Levels and Metering
Meters
Studio engineers and transmission engineers consider audio levels and their meas-
urements differently, so they typically use different methods of metering to monitor
these levels. The VU meter is an average-responding meter (measuring the approxi-
mate RMS level) with a 300ms rise time and decay time; the VU indication usually
under-indicates the true peak level by 8 to 14dB. The Peak Program Meter (PPM) in-
dicates a level between RMS and the actual peak. The PPM has an attack time of
10ms, slow enough to cause the meter to ignore narrow peaks and under-indicate
the true peak level by 5 dB or more. The absolute peak-sensing meter or LED indica-
tor shows the true peak level. It has an instantaneous attack time, and a release
time slow enough to allow the engineer to read the peak level easily.
Figure 1-1
shows the relative difference between the absolute peak level, and the indications
of a VU meter and a PPM for a few seconds of music program.