Master Bus Compressor & Meters

The master strip carries a stereo bus compressor — inspired by the classic SSL 4000 G-series center-section compressor, the “glue” behind decades of mix buses — together with a dedicated metering section: two VU needle meters, a stereo peak meter, and a gain-reduction bar. This page explains every control and, just as importantly, exactly which levels each meter shows and how the section is calibrated.

Where it sits in the signal chain

The compressor is placed in front of the master fader:

all strips summed → master insert chain → master EQ → BUS COMPRESSOR → master fader → output

Consequences of that placement:

Riding the master fader never changes how hard the compressor works — the threshold always sees the summed mix at unit gain.
The compressor’s make-up gain feeds the fader, so the fader remains the final say over loudness.
A fade-out performed on the master fader stays clean: the compressor does not “breathe up” as the level falls, because it sits before the fader.

Controls

All controls follow the hardware’s exact ranges and switch positions.

Control	Type	Positions / Range	Default
IN	switch	in / out of circuit	out
THRESHOLD	continuous	−15 … +15 dB	0
MAKE-UP	continuous	0 … +15 dB	0
ATTACK	switched	0.1 / 0.3 / 1 / 3 / 10 / 30 ms	30 ms
RELEASE	switched	0.1 / 0.3 / 0.6 / 1.2 s / AUTO	AUTO
RATIO	switched	2:1 / 4:1 / 10:1	4:1

Notes:

IN is a true bypass — out of circuit, the audio path is bit-exact (the compressor is not merely turned down, it is not run at all). The make-up gain is part of the compressor, so it is bypassed too.
AUTO release is the program-dependent dual time constant that made the original famous: roughly half of the gain reduction recovers quickly (about 100 ms), the rest over several seconds. Short peaks release fast while sustained material breathes back gradually — the “glue” setting.
Detection is stereo-linked from the louder channel, driving one shared gain for both channels. Heavy compression can never shift the stereo image.
The detector is peak-sensing with a firm soft knee that tightens as the ratio rises, and there is no lookahead — the compressor adds zero latency and is safe in the live monitoring path.

A classic starting point: RATIO 4:1, ATTACK 30 ms, RELEASE AUTO, then bring THRESHOLD down until the gain-reduction bar taps 2–4 dB on the loud beats.

The metering section

Three meter systems sit side by side, and each answers a different question:

Meter	Question it answers	Signal it reads
VU needles (L / R)	How loud is the mix, on average?	300 ms RMS
Peak bars (L / R)	How much headroom is left before clipping?	True peak envelope
GR bar	How hard is the compressor working?	Compressor gain reduction

All three are measured inside the audio engine (in Rust, per audio quantum) and published to the UI — the interface only draws; it never computes audio math. The meters run even when the compressor is bypassed; only the gain-reduction bar sits at zero then, because no reduction is happening.

The IN / OUT switch

The VU needles and peak bars can read either of two tap points:

IN — before the compressor (and before the master fader). This is the level the THRESHOLD control is compared against: park the meter on IN to set your threshold, then punch the compressor in.
OUT — after the master fader: exactly what leaves the engine and what any recording of the master would capture. This matches the reference point of the main output peak meter.

VU meters — average level

The two needle meters are true VU instruments:

Ballistics: the level is integrated with a 300 ms mean-square average (the VU standard) in the engine, and the needle itself moves with the standard’s ~300 ms swing. VUs deliberately ignore short transients — they show the body of the mix, which tracks perceived loudness far better than any peak reading.
Scale law: needle deflection is linear in voltage, logarithmic in dB — the authentic meter law. That is why −20…−7 crowd the left of the face while −3…+3 spread wide through the working zone, with 0 VU at about ⅔ of the arc and the red 0…+3 section after it.
Calibration: 0 VU = −14 dBFS RMS. The historical console alignment is −18 dBFS (+4 dBu), but modern in-the-box mixes run hotter and would pin the needles permanently. −14 (the K-14 convention) keeps the needles in the useful arc at contemporary levels: a healthy mix rides around 0 VU on loud sections and kisses the red on the hottest ones.

Peak meters — headroom

The two vertical bars show the true peak envelope per channel (instant attack, ~20 dB/s fall-back) on a −60 … 0 dBFS scale.

Red means one thing only: digital clipping. The bar runs green into yellow, and the fill turns red exclusively when the peak reaches −0.3 dBFS or above — an actual digital over. Red on this meter is never decoration; if you see it, the output is clipping.

Gain-reduction bar

The horizontal bar under the VU pair shows the compressor’s current gain reduction on a 20 dB full scale (matching the hardware’s GR meter range), with a numeric dB readout beside it. 2–6 dB on peaks is gentle glue; past 10 dB you are audibly crushing — which is a legitimate choice, just an informed one.

Reading the section as a whole

The value of showing RMS and peak side by side is the gap between them — the crest factor of your mix:

Needles near 0 VU, peak bars comfortably below the top — a healthy, dynamic mix with headroom.
Needles pinned, peaks near the top — the whole bus is hot; pull the master content down rather than the fader (the fader is after the meters’ IN tap).
Needles low but peaks spiking high — a very transient-heavy mix; this is exactly what the bus compressor is for. Watch the gap close on the meters as you bring the threshold down.
Needles rising while peaks stay put (comparing IN vs OUT) — the compressor is doing its job: raising the average, containing the peaks.