THE WAY THE 5043 WORKS
In order to control gain, a V.C.A. or Voltage Controlled Amplifier (or Attenuator) is used. There are many types of V.C.A., including the use of tubes, discrete and integrated solid state circuits and naturally non-linear devices, each one having its characteristic behavior that reflects sonically on the final performance, and gives it a character or signature that can be musically attractive or – not! The Portico 5043 makes use of a very accurate, low noise, low distortion V.C.A. having, essentially, no "signature" of its own.

A part of the audio signal is rectified and smoothed to produce a suitable control voltage for the V.C.A. which has to respond very quickly and have low distortion. If the response is too fast, low frequency signals will themselves, be “gain controlled”! If the response is too slow, the signal will overshoot and the first few cycles will not get compressed. The speed and accuracy of the response, known as the "attack", and the time frame that gain remains under the initial control, known as "release" or "recovery" and plays a large part in the way a compressor sounds.

FEED FORWARD or FEED BACK
The Portico 5043 provides a choice of feed-forward or feed-back compression modes. The FB Button allows the user to switch between the two modes.

If the V.C.A. Control voltage is taken from the 5043 input, (i.e. before the V.C.A.) the V.C.A. "knows" right away that a gain change is required and there is almost immediate response. This is known, logically, as a "Feed-Forward" compressor.

If the V.C.A. Control voltage is taken from the 5043 output, (i.e. after the V.C.A.) it cannot act immediately on the V.C.A. because it has already been modified by settings of the V.C.A. and circuits through which it has passed. This is known as a "Feed-Back" compressor. The two compression characteristics are quite different; there is more "Overshoot" and both the attack and recovery ramps are changed, providing the user with powerful choices.

COMPRESSION
For signals below the "threshold" level that has been set, a compressor provides a linear path allowing signals to be amplified without the gain being adjusted in any way. When signals exceed the "threshold" level, the gain is reduced in a controlled manner that depends on the Ratio that has been set.

RATIO
Range 1:1 to LIMIT (i.e. 40:1)
Above a given "THRESHOLD" signals are reduced by an adjustable amount ranging from 1:1, (which is linear, or no reduction at all), to more than 40:1 which is a very high ratio, equivalent to that of a Limiter. RATIO is sometimes referred to as "Slope" because when depicted on a graph, the slope of the graph representing Output versus Input, is what changes.

RATIO and THRESHOLD are closely inter-dependent. If a RATIO as high as 40:1 has been set, then if the THRESHOLD is set at 0 dBu, even when a massive signal of +40 dBu (unlikely!) is presented to the input, the output signal will only be +1 dBu. RATIOS as high as this would normally be set somewhere above 0 dBu - say at +14 dBu, in order to prevent the output signal level exceeding just over +14 dBu to protect, for example, a digital recorder. Similarly, if a RATIO of 5:1 has been set, an input signal which is 10dB above THRESHOLD will only rise by 2dB above that THRESHOLD at the output.

THRESHOLD control covers the Range from below -30dB to +22dBu. When THRESHOLD set at a low level, with a fairly high RATIO the amount of gain reduction will be considerable and it may be necessary to use some GAIN after the compressor to restore the apparent signal level.

RELEASE (or RECOVERY) and ATTACK TIME
Range of RELEASE time is 100 mS to 2.5 Seconds. Range of ATTACK time is 20 mS to 75 mS. The notes above explain how the 5043 handles signals of constant amplitude such as pure tones. Real program signals, however, are continually changing in level. The way in which a compressor deals with actual program material depends upon the magnitude and duration of peaks in the program level.

If the RELEASE TIME is set to be very short, a short duration signal will be compressed but the gain will return to normal very quickly, giving a fluctuating and un-natural sound known as "Pumping" when the background, or other signals, are forced up and down. The gain will also tend to follow the wave form of low frequency signals. RELEASE TIME should be set long enough for the gain to remain reasonably constant between each bass note or between speech syllables.

The ATTACK time is the time taken for the compression circuits to start compressing. A long ATTACK time allows short duration peaks to "escape" and go through uncompressed. This may cause overload on subsequent digital circuits. A very short attack time sounds un-natural and robs the signal of "life" by removing transients. Some transients are extremely fast and have little effect on the sound quality. Setting a long attack time often means that almost no gain reduction occurs because the transient is history (!) before compression has had time to operate. However, even the fastest circuits take time to operate which means that there is always some "Overshoot". Small amounts of "Overshoot" are musically desirable – there are exceptions, of course.

Setting the right values of RELEASE and ATTACK is what compression is all about! Once the principles are understood a Compressor-Limiter such as the 5043 provides a powerful tool that actually appears to enhance the dynamic range of a recording and so provide greater musical enjoyment.

GAIN
GAIN range provided is from -6dB to +20 dB. As already noted, when compression has taken place, it may be necessary to increase the overall gain to restore the apparent program level

COMPOSITE OPERATION
Cascading the two sections, A and B, of the Portico™ 5043 Compressor- Limiter provides extremely powerful and comprehensive control of Dynamic Range.

STEREO
When the LINK Push-Button is engaged and the two channels, A and B, set to approximately the same values, GAIN control on both channels will be the same to preserve Stereo balance, levels normally being controlled by the channel with the higher signal level.

DUCKING
When the LINK Push-Button is engaged, the signal passing through, channel A, may be used to control the amplitude of channel B. For example, the level of music through channel A can be controlled by speech on channel B, i.e. reducing the music level to make a "speech-over" announcement.

METERS
Two LED METERS are provided, OUTPUT LEVEL and GAIN REDUCTION. These are switched between Channel A and Channel B on the central METER SELECT Push Button.

OUTPUT LEVEL Indicates the actual output level in dBu. The GREEN segments cover the range from -10 to +12 dBu. The YELLOW segments cover the range from +16 to +18 dBu and the RED segment, +22 dBu and above which is regarded as overload.

REDUCTION is calibrated in dB covering the range 1 to 22 dB, reading backwards. This meter indicates the amount of actual reduction taking place and therefore is a valuable point of reference indicating the operation point of the 5043.

All Portico units are 1RU and half a rack wide.

There are mounting kits for a single Portico unit or two Portico units.

Back to top