A Low Frequency Oscillator. It is similar to a normal oscillator except that it outputs very low frequencies (very slow waves) and is used as a control input into another part of the synth. Examples of its use are to create vibrato or tremolo effects....
Poly-61
"...Back in 1983, the Poly-1 was a workhorse: dependable, with strong, easy-to-achieve
sounds. It was Korg's first instrument to abandon knobs and switches, yet all the main
parameter headings and value ranges are etched onto the casing in Korg's '80s-style,
pseudo-computer writing. Getting lost on the Korg Poly-61 is, well, unlikely, even for a
novice.
"The digital controlled oscillators are streamlined. DCO1 offers sawtooth, pulse, and
square waveforms; DCO-2 the same minus the pulse option. The oscillators can be
tuned separately, but not set at different volumes. DCO-2 can only be switched off. An
equally simple filter section sees lowpass, resonant filtering that has to be shaped by
the same ADSR envelope generator available to the amplifier. The filter can track the
keyboard, but the shared envelope generator is a definite limiting factor. The LFO can
affect the oscillators or the filter, but sine wave is the only option. So smooth vibrato is
okay, but hard-edged trills are out.
"This modest set of controls makes the Poly-61 a breeze to use, however. Most anyone
will be encouraged to start programming from day one; this makes an excellent synth on
which to learn about analog synthesis.
"And indeed, people are encouraged, because the Poly-61 sounds clean and purposful.
It specializes in harder, Clavi-type analog sounds, but the extra thickness afforded by
the dual-oscillator design makes thick, full string patches work well, in a synthy, as
opposed to lifelike, style. Unfortunately, Korg ditched chorus (found on the earlier Korg
Polysix), which can add greater swirl.
"A further loss from the Polysix is the unison keyboard mode. On the Poly-61, you must
employ the chord memory to stack up the oscillators, and though this is an undeniably
handy feature for instantly creating fat bass sounds, the results are not programmable.
Overall volumes is non-programmable as well.
"This list of losses and ommissions must be weighed not only against a fine arpegiator,
which can be clocked externally from similar-vintage, Korg drum machines and against
the luxury of being able to step througn the presets from a footswitch, a feature that is
an unrivaled plus for live playing.
"Along with what was the then called 'digital access control' design (i.e., no knobs and
switches -- the dawning of the era of programming as typing), Korg inaugurated the
joystick controller for pitch bend and modulation effects. It was and remains a bit dinky,
though Korg stalwarts are, by now, well used to the device.
"MIDI was right around the corner as the Poly-61 appeared, and Korg was quick to offer
a MIDI retrofit. A year later, the Poly-61M came with a retrofit as standard. These were
early MIDI days, so anomalies, weirdnesses and limitations on the MIDI side abounded.
MIDI also sounded the death knell for the dreaded (and temperamental) cassette
interface method of external storage. Ironically, though, the Poly-61's cassette interface
is both quick and stable."