Orban OPTIMOD-FM 8700, Operating Manual

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OPTIMOD-FM DIGITAL OPERATION
3-89
The ear is far less sensitive to bass than to midrange sounds. You can see this for
yourself by examining the classic Fletcher-Munson "equal-loudness" curves. This
means that if you want robust bass, this will take up a great deal of room in your
modulation waveform. This room could otherwise be used for midrange, where far
smaller amounts of energy yield the same amount of loudness. Accordingly, there is
an important tradeoff between loudness and bass—if you want more bass, you will
have to accept either less loudness or noticeably more distortion, which occurs when
the bass waveforms push the midrange and high frequency material into the 8700i’s
final clipper.
There is one psychoacoustic trick you can use to increase the amount of perceived
bass while using modulation headroom efficiently. For hundreds of years, pipe or-
gan makers have tricked the ear into hearing non-existent fundamental tones
(which would require huge, expensive pipes) by replacing them with several, smaller
pipes tuned to the lower harmonics of the missing fundamental. In the 8700i, you
can use the bass clipper to make harmonic distortion for this purpose. As explained
above, the bass clipper has three settings—
S
OFT
, M
EDIUM
,
AND
H
ARD
—that determine
the amount of distortion the clipper makes when its clips bands 1 and 2. S
OFT
pro-
vides the purest sound, but M
EDIUM
and H
ARD
create progressively more distortion
on bass. Because H
ARD
can make noticeable voice distortion, the factory program-
mers prefer M
EDIUM
for most presets. However, if you are willing to trade off voice
distortion against bass punch, then you could also use H
ARD
. H
ARD
is particularly ef-
fective in increasing bass punch because it flattops bass transients and this allows
the waveform to accommodate fundamentals that have a larger peak level (by up to
2 dB) than the peak level of the flat-top. (The fundamental of a square wave has a
peak level 2.1 dB higher than the peak level of the square wave.) In essence, by do-
ing this, your bass fundamentals can exceed 100% modulation without having the
composite stereo waveform itself exceed this level.
The subharmonic synthesizer is particularly useful for older material, where it can
provide a punchy bottom end by extrapolating bass that does not exist on the origi-
nal recording. However, be aware that frequencies in the range of 25 to 45 Hz
(those generated by the synthesizer) take up a lot of peak modulation without con-
tributing significantly to loudness, so for a given amount of distortion, you must
trade off bass punch against loudness. See
Subharmonic Injection on page 3-36 for a
more detailed discussion.
The attack time of the band 1 compressor affects bass punch by determining the
amount of bass transient that is allowed to pass through the compressor before the
attack clamps down the rest of the waveform. Any transient that passes through the
band 1 compressor will hit the bass clipper, so slower attack times on band 1 will in-
crease bass punch at the expense of distortion (particularly on voice). The
B
AND
1
A
TTACK
T
IME
settings in the factory presets have been adjusted with this tradeoff in
mind, but you might prefer to make a different one.
The threshold of the band 1 compressor will also affect bass punch. We recommend
that you carefully study the setting of this control (and the
B
AND
1 A
TTACK
T
IME
con-
trol) in the various 8700i factory presets before making your own adjustments, so
you can get a feel for how we made the tradeoff between punch and distortion at
the factory. If you set the threshold much above –6 dB, you will typically get some