Rupert Neve Designs Portico 543: 500 Series Mono Compressor

A fully controllable compressor/limiter with feed-forward/feedback modes, peak/RMS detection, and sidechain filter.




The Rupert Neve Designs Portico 543 Mono
Compressor delivers the unobtrusive, musical-sounding dynamic control
and brick-wall limiting made famous in the Portico 5043 to the 500
series format. The 543 features a fully controllable compressor-limiter
with feed-forward / feedback modes, Peak / RMS detection and a built-in
sidechain high pass filter. With an unrivaled heritage and a tremendous
feature set, the 543 yields a combination of rich warmth, flexibility
and precision that is sure to resonate in the ears of sound engineers
the world over.

Built around Mr. Rupert Neve’s custom transformer
designs, each 543 module encompasses individually controllable
threshold, attack, release, makeup gain, ratio, side chain HPF,
Feed-Forward / Feed-back selection and Peak/RMS detection modes. With
the compressor inactive, the 543 may be used as a transformer-coupled,
high-performance line amplifier, and two 543 may be linked for stereo
operation as well. The chassis is built to standard 500 series
specifications, with power and I/O provided by the rack.

How the Compressor 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. including the use of
tubes, discrete and integrated solid state circuits, and naturally
non-linear devices. Each one has 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 543
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
543 also has the ability to switch between feed-forward and feed-back
modes. If the V.C.A. Control voltage is taken from the 543 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.

In most of Mr. Rupert
Neve earliest designs, feed-back detection controlling the VCA with a
rectified voltage from the unit output was intrinsic to the musical
dynamic response. However, the very nature of a feedback compressor
limits the attack time of the compression circuit. To offer faster, more
technically accurate response times, feed-forward detection was
implemented on Mr. Rupert Neve’s more modern designs. With the FF / FB
switch, both classic and modern VCA responses are available.

RMS or Peak?
One
of the more unique features of the 543 is the new Peak / RMS mode also
found in the Portico II Channel. This switch allows the VCA (voltage
control amplifier) to respond to both RMS (Root Mean Square) and peak
levels. RMS circuits are considered to better mimic the way the ears
perceive apparent loudness, while peak circuits tend to directly respond
to the waveform voltage which may be more of a concern for prevention
of clipping and maximizing levels. In this case, peak mode uses a
combination of both methods to get the best of both worlds, and avoids
the drawbacks of each method on its own.

Ratio and Threshold

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, and if the THRESHOLD is set at 0dBu, 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 0dBu – say at +14dBu, 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.

Attack Time
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 543 provides a powerful tool that actually appears to enhance the
dynamic range of a recording and so provide greater musical enjoyment.

Release / Recovery
The
notes above explain how the 543 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.

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.

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

Peak - When engaged, the
543 uses a combination of peak and RMS (average) detection to determine
level. When disengaged, it follows just RMS.

Side Chain High-Pass Filter -
Introduces a HPF at 250Hz to the VCA side chain signal. This allows the
compressor to disregard sustained low frequency content.

Ratio - Range from 1:1 to LIMIT (i.e. 40:1)

Threshold - Range from -30dBu to +20dBU

Attack Time - Range from 20mS to 75mS.

Release / Recovery Time - Range of RELEASE time is 100mS to 2.5Seconds.

Stereo Operation -
When the LINK switch is engaged on two 543 modules and settings are
approximately the same values, GAIN reduction on both channels will be
the same to preserve stereo balance and position. This will work
properly in link bus-enabled 500 series racks, or by hardwiring two 543
modules link busses together in non-compatible racks.

Ducking -
When the LINK switch is engaged on two connected 543 modules, the
signal passing through the first module may be used to control the
amplitude of a second module. 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 "voice-over" announcement.

Meters -
Two Peak LED METERS are provided that show OUTPUT LEVEL and GAIN
REDUCTION. Output level meter covers the range from -10 to +22dBu and
above which is regarded as overload. REDUCTION is calibrated in dB
covering the range +1 to 22dBu, reading from right to left,


Features

Fully controllable compressor-limiter with feed-forward / feedback
modes, Peak / RMS detection and a built in side chain high pass filter.
Sidechain high-pass filter at 250Hz to the VCA sidechain signal - lets bass through without dragging down mix
8-segment LED meter for monitoring output level
8-segment LED meter for monitoring gain reduction
FF/FB button changes the detection from feed-forward to feedback
RMS/Peak button changes the VCA detection from RMS (average) to a Peak-RMS hybrid
Link switch engages link bus for stereo operation of two Portico 543s



Looking for the secret sauce that only highly musical compression can
provide? Put a little Neve up your sleeve - order a Portico 543 Mono
Compressor - or get two and let Neve drive your 2-bus.



Portico 543: 500 Series Mono Compressor Specifications:

Gain range: Continuously variable from 6dB to +20dB
Threshold range: Continuously variable from 36dB to +22dB
Ratio range: Continuously variable from 1.1:1 to Limit (40:1)
Attack range: Continuously variable from 20mS to 75mS
Release range: Continuously variable from 100mS to 2.5 Seconds
FF/FB: Feed-Forward or Feedback VCA control
Maximum output level (balanced and floating transformer output maximum output from 20Hz to 40kHz) +23 dbu
Total Harmonic Distortion and Noise:
- @ 1kHz, +20 dBu output level, no load
- Main Output, compressor bypassed: Better than 0.0006%
- Main Output, compressor engaged: Better than 0.02%
Noise
(measured at main output, un-weighted, 22Hz-22kHz, terminated 40 Ohm):
With gain at unity, compressor disengaged: better than 103dBu; with Gain
at Unity, Compressor engaged: better than 92dBu
Frequency Response: Main output, unity gain @ 150 kHz -3dB
Crosstalk (measured channel to channel): Better than 90dB @ 15kHz
METERS: Monitor INPUT LEVEL and GAIN REDUCTION
















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