Ambi Decoder Sub

s3g Ambi Decoder Sub 8 derives one to eight low-frequency feeds from an ACN/SN3D field. A single-sub setup low-passes the omnidirectional W component. Multi-sub setups can add horizontal first-order X/Y direction so low-frequency energy is distributed around an evenly spaced ring instead of being copied identically to every output.

Sub is a parallel low-frequency extractor, not a complete speaker decoder or crossover manager. It does not high-pass the main speaker feeds, align loudspeaker and subwoofer delay, apply room correction, or prevent overlap with another bass-management stage.

Ambi Decoder Sub interface showing the single-sub field, 3OA input setting, output gain, one active sub, 90 Hz cutoff, direction width, and bypass control — The default one-sub state extracts low-passed W; increasing Subs exposes the evenly spaced horizontal directional feeds described below.

Workflow

Insert s3g Ambi Decoder Sub 8 on a track carrying the encoded field, match ORDER to the source, and choose the active SUBS count from one to eight.
Route output lanes 1 through the selected sub count to the corresponding subwoofer feeds, keeping them separate from the full-range speaker decode.
Set CUTOFF for the intended low-frequency band and adjust WIDTH while listening at the system crossover region.
Trim OUT, verify polarity, delay, and combined level with the main speakers, and use BYPASS for a silent sub-path comparison.

Signal Flow

The decoder reads W and, for directional multi-sub operation, the horizontal first-order Y and X components.
One sub, 0OA, or WIDTH 0 produces an omnidirectional distribution. With multiple subs and nonzero width, each feed adds the X/Y projection for its ring azimuth.
The omnidirectional and directional terms are normalized as width increases.
Each active feed passes through an independent one-pole low-pass at CUTOFF and then through OUT.
Outputs above the active sub count are silent. BYPASS also silences the complete output bus.

Directional Sub Feeds

Sub 1 begins at -45°. Additional outputs proceed clockwise at equal angular intervals. For four subs, the nominal directions are -45°, -135°, +135°, and +45°. These are decoding directions used to weight X/Y; they are not editable measured subwoofer coordinates.

Width	Distribution
0	Every active output receives the same low-passed `W` signal.
1	Balances `W` with one unit of the horizontal directional projection, then normalizes by two.
2	Weights the horizontal projection more strongly and normalizes the combined term by three.

Vertical Z and all second- through seventh-order components are not used by the sub extractor. Higher ORDER settings preserve host compatibility with wider fields; they do not add higher-order directivity to the sub feeds.

Controls

Control	Meaning
ORDER	Selects `0OA` through `7OA`. `0OA` forces an omnidirectional `W`-only result.
SUBS	Activates one to eight leading output lanes.
CUTOFF	Sets the one-pole low-pass frequency from 20 to 240 Hz.
WIDTH	Scales horizontal `X/Y` direction from 0 to 2 for multi-sub output.
OUT	Sets final sub-path gain from -60 to +18 dB.
BYPASS	Silences all eight plugin outputs; it does not pass the ambisonic input through.

INIT restores 3OA, one sub, a 90 Hz cutoff, Width 1, and Bypass off while preserving the current OUT trim.

Order and Routing

Order	Nominal field width	Components used
0OA	1 channel	`W`
1OA	4 channels	`W/Y/X`; `Z` and higher-order components are ignored.
2OA	9 channels
3OA	16 channels
4OA	25 channels
5OA	36 channels
6OA	49 channels
7OA	64 channels

The CLAP plugin exposes one fixed 64-channel input bus and one fixed eight-channel output bus. Only the first SUBS output lanes are active. Because this is not an in-place decoder, place it on a parallel route or dedicated sub bus rather than expecting it to preserve the full-range ambisonic field.

Bass-Management Boundary

The low-pass is a compact one-pole filter, so CUTOFF is a musical extraction control rather than a complete calibrated crossover. Combine it with the main-system high-pass, polarity, delay, protection limiting, and room alignment required by the loudspeaker system. If another decoder or monitor controller already redirects bass, avoid running both paths without measuring the combined response.