The threshold control governs the amount of gain reduction. The compressor initially has a "soft- knee" compression curve but is increased by the shape control.
The peak threshold control sets the level at which the peak limiting takes place. The
slope is greater than 10:1 and is fast enough to prevent overload of a transmitter or
digital recorder. When a signal's peak is greater than the average gain reduction, the
peak circuit will take control until the peak has passed, and then smoothly transfer
regulation back to the compressor. The green PK LED will light when the peak circuit is
active.
When the Pk Threshold Control is set to +20 dBm, the limiter will prevent any increase in
output level above that setting. The output will remain constant even if the gain of the
STC-8 is in- creased. The limiter circuit is post gain control. The dial calibration
relates to the maximum output level.
The gain control will adjust the compressor gain from O to 14 dB of gain. When in stereo link mode the gain controls operate independently to allow balancing of the stereo program source.
The side chain consists of two control circuits, one for peak control and one for compression control. The side chain insert only affects the average or compressor circuits. Due to the design of the stereo link, separate equalizers (with identical settings) must be used for both channels to achieve channel matched operation. This scheme allows reliable signal averaging regardless of the phase relationship between the audio channels. In a conventional stereo link circuit (one EQ for both channels) out of phase material is ignored by the compressor. This is also beneficial for stereo FM broadcast where phase relationships can affect the stereo generator.
The preset rotary switch allows fast and predictable set-up of the STC-8. At first
glance this knob may look a bit forbidding, but in operation, this switch is the key to
the flexibility and foolproof operation of the compressor-limiter.
The STC-8 features two advanced functions: 1)Program Dependent Release (PDR) and 2)
Dynamic Attack Modification (A-MOD). The four groups of presets simply give every
combination of these settings. Further, each group has three presets with predetermined,
optimized attack, release and shape settings and one variable setting (which activates the
front panel controls).
In the left half of the settings the Program Dependent Release is enabled. This function
is especially well suited for leveling the volume of the audio without bringing up
background noise. It is especially helpful controlling voice over, lead vocal and any
other complex program material where short release times tend to bring out room noise and
headphone leakage. The right half of the switch has constant release times, useful for
enhancing the sustain of guitars or room ambiance on percussion.
The Dynamic Attack Modification groups are located at the upper right and lower left
sections of the preset matrix. With either A-MOD group enabled, the peak limiter will
dynamically modify the attack time of the compressor function. When a slower attack time
(3 or higher) is selected on the compressor, any signal triggering the peak limiter
automatically shortens the attack time of the compressor. This gives the STC-8 the ability
to average program when employing long attack times while still maintaining overload
protection. More notably, the compressor will quickly control the audio after a period of
silence.
When in the link mode the gain control, bypass switches, and meter switches still
operate independently. All other functions are operated by the Channel 1 controls.
The bypass switch selects the output signal source. When the unit is bypassed all of
the compressor circuitry is removed from the audio path. However, the compressor-limiter
continues to operate in a monitoring mode. The unit defaults to the bypass condition when
the power is off.
The attack control determines the time it takes the compressor to respond to the input
signal and have gain reduction take place. Zero on the control is fast; 10 is slow.
The release control sets the time for the compressor's gain to return to the point of
no gain reduction in the absence of program. Zero on the control is fast; 10 is slow.
The attack and release times depend on the shape setting, amount of gain change, settings
on the mode and preset switches, and the program material. They all interact and allow the
creation of a wide variety of sounds that are not possible with any other gain control
device available.
The mode switch affects both channels identically. In the HARA position the audio path
is transparent. The KI position adds warmth and color and works very well with bass or
voice source material. The audio path and the side chain are modified to get this color
change and the effect is also dependent on the attack, release, and shape control
settings.
The shape control changes the gain curve, the relationship between amount of gain
reduction taking place and the signal level. The input to output ratio changes as a result
of this. With the control set to 0 the curve and slope are gentle. With the control set to
10 the curve is more aggressive and the slope is higher.
The meter has three modes of operation. In the output mode the
meter appears as a bat graph. O VU is referenced to +4dBm. In the PK mode the meter
appears as a dot and shows the amount of head room by showing peak level below clipping.
The third mode shows gain reduction. This also appears as a dot type display. The red OL
LED lights with clipping in the output stage and the green PK LED lights when the PK side
chain takes control from the average side chain. Only the left channel PK LED is
operational in stereo link mode. The meter switch selects what is being viewed and the
mode that the meter operates in.