The SEPsoniX FM processor contains several basic blocks to perform the necessary functions. Refering to
the block diagram above it includes:
1) An input conditioning block, which consists of an RFI-protected audio input amplifier, adjustable to the
nominal input signal. This is simplified by observing a front-panel LED Vu meter, and setting the levels
for nominal readings. The input is then passed through a 25Hz, high-pass filter, with an 18dB/octave
slope. This will reduce any audio subsonics that typically cause undesired results in audio processing,
and on-air anomalies in some transmitters. Audio is then passed through an all-pass phase shift
network, which helps compensate for asymmetrical audio peaks.
2) A broadband automatic gain control stage automatically adjusts for long range gain changes, as much
as +/-15dB. The detection of the gain is dependent on the peak limiter stage content, and derives the
control voltage from the output of the peak limiters. This allows for smoother gain-riding with varying
program content.
3) A dual-band peak compressor, with factory-set time constants, which processes audio independently
above and below 200Hz. Time constants are factory-adjusted so that the audio compression attack time
is fast enough to respond to progam content, and a responsive release time providing a non-fatuiging
audio quality. Front-panel dual-colored LED's show the operation of the compressor stages, changing
from green (normal) to high (red). Additionally, the use of compressor 'freeze' circuitry will cause any
gain changes to be suspended whenever program content suddenly stops. This prevents the
compressors from gain-riding during silent program content, reducing the 'breating' effect of
compression release times, which results in noise build-up, and un-natural audio quality. The
independent bands are then summed together, and passed through a safety low-pass filter, which
eliminates any ailiasing distortions that may occur into the pre-emphaisis limiter, as well as any
remaining switching products from the PWM stages.
4) The next block is an ultra-fast PWM-controlled pre-emphasis limiter, which responds to instantaneous
high-frequency audio that exceeds the internal limits along the 75µS (50µS and 100µS internally
programmable) pre-emphasis curve. The limiter is only affected by high frequencies (rather than the
peak level of the pre-emphasized audio content), eliminating the possibility of modulation of high
frequency content caused by low-frequency program content. The limit threshold is set internally, so that
the limiting can occur with the correct balance between high and low-frequency program content. The
user can adjust the input gain to the high-frequency limiter, if additional emphasis is desited on the
program content, resulting in an increase in the total audio drive to the stereo generator, while
increasing the amount of pre-emphasis limiting simultaneously. The amount of high-frequency limiting is
indicated with front-panel LED's.
5) Processed audio is then routed through a 6-pole active 15kHz low-pass filter, which eliminates any
frequency material that may be outside the desired audio passband. In addition, the use of an active
19kHz notch filter reduces the chances of any audio components interfering with the stereo 19kHz pilot
signal.
6) Filtered audio is passed on to the stereo generator, which produces the stereo baseband signal. Front
panel controls can enable the stereo pilot signal, indicated by an LED. When switched to MONO, the
19kHz pilot/38kHz carrier are fully turned off, and both Left and Right channels are summed into the
main channel. This eliminates the need to combine both program channels to a single input,
occasionaly required with some conventional processors. The stereo generator utilizes the 'matrix'
technique, as opposed to the 'switching' method. This eliminates the need for a costly baseband filter,
and only needs to modulate the L-R component. When in Mono, L+R component is passed directly as a
mono composite signal. The 38kHz L-R digital balanced modulator is superior to analog modulators,
and is essentially drift-free with regard to carrier suppression and temperature variations. The stereo
generator is internally factory aligned for optimum performance, and does not require re-calibration on a
normal basis. Pilot phase and injection are adjustable with multi-turn front-panel controls.
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