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FM-531HD / May 2013
F M - 5 3 1 H D G U R U G U I
Audio Input and Output Metering
The FM‑531HD provides high resolution level metering for all input and output
signals.
The topmost pair of horizontal bargraph meters show the applied input levels of
the selected audio source, after that source’s Gain control. Like many meters in the
FM‑531HD the input level meters are calibrated in dBFS, for decibels Full Scale. The
right hand “0dB” marking represents the absolute level where there are no more digital
bits left for defining the input signal – any signal above this will cause severe distor
-
tion. The left hand “‑40dB” marking represents the audio level that many of the silence
sense/audio failover circuits use for making their logic decisions.
So, what are normal input levels? Because we are working in the digital domain
great care must be taken to ensure that audio levels aren’t run so high as to cause them
to run out of headroom. Good engineering practice dictates that levels arriving from
the studio or STL should be peaking between ‑20dB and ‑12dB on the input meters and
no higher. This allows headroom for the occasional “hot” audio level.
Operating the Input meters at lower indications won’t bring up noise or other artifacts
that we might be familiar with in analog circuits. With FM‑531HD’s 144dB internal
dynamic range there is virtually no chance of increasing noise by running with “light”
audio input levels, though users would likely find that they may need to increase the
setting of the AGC Drive control to compensate.
The FM‑531HD’s FM Analog, FM Digital and HD output levels are also shown
on the bargraph level meter display and their scales match that of the Input meter dis-
play. The big difference in the output meters is that because the output levels they are
displaying are “post processing” peak levels are not only known, they are very well
controlled. Therefore there is no need to leave headroom for output signals as we must
do for input signals.
Summary of Contents for FM-531 HD
Page 2: ...Technical Manual Wheatstone Corporation May 2013 FM HD Digital Audio Processor FM 531hd...
Page 122: ...page A 2 FM 531HD May 2013 A P P E N D I C E S Appendix A General Specifications A 3 Contents...
Page 128: ...page A 8 FM 531HD May 2013 A P P E N D I C E S Appendix B FM 531HD GPI Interface A 9 Contents...
