OPTIMOD-FM INTRODUCTION
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Using Lossy Data Reduction in the Studio
Many stations are now using lossy data reduction algorithms like MPEG-1 Layer 2 to
increase the storage time of digital playback media. In addition, source material is often
supplied through a lossy data reduction algorithm, whether from satellite or over land-
lines. Sometimes, several encode/decode cycles will be cascaded before the material is
finally presented to OPTIMOD-FM’s input.
All such algorithms operate by increasing the quantization noise in discrete frequency
bands. If not psychoacoustically masked by the program material, this noise may be per-
ceived as distortion, “gurgling,” or other interference. Psychoacoustic calculations are
used to ensure that the added noise is masked by the desired program material and not
heard. Cascading several stages of such processing can raise the added quantization noise
above the threshold of masking, such that it is heard. In addition, at least one other
mechanism can cause the noise to become audible at the radio. OPTIMOD-FM’s multi-
band 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 cy-
cles, 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 measure-
ments differently, so they typically use different methods of metering to monitor these
levels. The VU meter is an average-responding meter (measuring the approximate RMS
level) with a 300ms rise time and decay time; the VU indication usually under-indicates
the true peak level by 8 to 14 dB. The Peak Program Meter (PPM) indicates a level be-
tween 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 indicator shows the true peak level. It has
an instantaneous attack time, and a release time slow enough to allow the engineer to eas-
ily read the peak level. Fig. 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.
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