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FM-531HD / May 2013
F M - 5 3 1 H D P R O G U I
Band Coupling
The Coupling
control has one specific purpose – to keep the gains of bands 1, 2, 4,
and 5 from exceeding the gain of band 3 when those “outer” bands are “ungated.” This
prevents unnatural gain rush‑up by those bands when such behavior would be undesir‑
able. Without the “Coupling” feature the gains of the bands at the frequency extremes
can increase their gain and cause a spectrally incorrect sonic mixture at the output of
the five band section.
Band 3 was chose as the “master” band for the high and low coupling control because
that band is in the approximate center of the audio spectrum.
The Coupling control may be adjusted over the range of “0dB” to “‑6dB” (High to
Mid) or “0dB” to “‑12dB” (Low to Mid). When each coupling setting is set for “0dB”
bands 1, 2, 4, and 5 can never have more gain than band 3. Such a setting is useful in
formats such as Classical and Jazz that typically would not wish the spectral “correc‑
tion” caused by the outer bands increasing their gain but still need those bands to reduce
their gains when appropriate.
In typical programming situations the best setting of the Coupling control will likely
be somewhere between ‑2.0dB and ‑4.0dB. ‑6.0dB (‑12dB Low to Mid) was chosen
as the maximum setting. Any further decoupling at the point allows the outer bands to
“wander” around too much, causing an unnatural spectral buildup on some material.
AGC Backoff
The AGC Backoff adjusts the processing balance between slower AGC action and
faster compression.
With the AGC Backoff control set at “0,” the audio gain control is primarily due to
slower AGC action, and therefore short term program dynamics are largely unaffected
unless they exceed the AGC gain reduction by at least 1dB. In that case momentary gain
reduction by the compressor will place the gain where it needs to be to return control
to the AGC.
With the AGC Backoff control at the other extreme, or ‑6dB, gain control is primarily
due to fast compression and short term dynamics will be aggressively controlled. The
setting of the AGC Backoff control allows the audio compression texture to be modi‑
fied from a gentle, almost unobtrusive control (
AGC Backoff between “0” and about
minus 2.0dB), to very obvious compression (AGC Backoff control at “‑3.0dB” or greater
negative numbers) creating a sound that is similar to vintage compressors and limiters
from the 1960’s.
Note that regardless of the setting of the AGC Backoff control, the AGC and Com‑
pressor stages operate with their algorithms linked mathematically and there is no other
method for changing the relative thresholds between the AGC and compressors. If for
some reason “less” AGC control signal is desired, it may be achieved by lengthening
the AGC attack times.
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...