Orban OPTIMOD-FM 8600, Operating Manual

Page: 49 / 397

OPTIMOD-FM DIGITAL INTRODUCTION
1-21
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 1-2 on page
1-22 shows the relative difference between the absolute peak level and the indica-
tions of a VU meter and a PPM for a few seconds of music program.
Studio Line-up Levels and Headroom
The studio engineer is primarily concerned with calibrating the equipment to pro-
vide the required input level for proper operation of each device so that all devices
operate with the same input and output levels. This facilitates patching devices in
and out without recalibration.
For line-up, the studio engineer uses a calibration tone at a studio standard level,
commonly called line-up level, reference level, or operating level. Metering at the
studio is by a VU meter or PPM (Peak Program Meter). As discussed above, the VU or
PPM indication under-indicates the true peak level. Most modern studio audio de-
vices have a clipping level of no less than +21dBu and often +24dBu or more. The
studio standardizes on a maximum program indication on the meter that is lower
than the clipping level, so peaks that the meter does not indicate will not be
clipped. Line-up level is usually at this same maximum meter indication. In facilities
that use VU meters, this level is usually at 0VU, which corresponds to the studio
standard level, typ4 or +8dBu.
For facilities using +4dBu standard level, instantaneous peaks can reach +18dBu or
higher (particularly if the operator overdrives the console or desk). Older facilities
with +8dBu standard level and equipment that clips at +18 or +21dBu will experi-
ence noticeable clipping on some program material.
In facilities that use the BBC-standard PPM, maximum program level is usually PPM4
for music, PPM6 for speech. Line-up level is usually PPM4, which corresponds to
+4dBu. Instantaneous peaks will reach +17dBu or more on voice.