Mix Multi‑Channel Sound into Stereo — User Guide

Fold down mono, stereo, 5.1/7.1, or arbitrary N‑channel signals into a two‑channel mix with control over routing, coefficients, normalization, and phase.

Author: Shai Cohen Affiliation: Department of Music, Bar‑Ilan University, Israel Version: 0.1 (2025) Praat

Contents:

What this does Quick start Channel Order & Routing Recommended Downmix Coefficients Parameters & Options Presets Tips & Pitfalls Math

What this does

This tool mixes any selected Sound with N≥1 channels into stereo L/R. It supports common cinema/music layouts (mono, stereo, 5.1, 7.1) and arbitrary input by means of a configurable routing matrix. Optional normalization, bass/LFE handling, phase‑safe summing, and dB‑based coefficients are provided.

Automatic layout detection for common orders, with manual override.
Downmix laws (center/surround/LFE trims) aligned with common practice.
Arbitrary N×2 matrix routing when channels are unconventional.
Normalize to target peak, and protect mono to reduce cancellations.

Output: A two‑channel Sound named name_stereo is created alongside the original.

Quick start

In Praat, select a multi‑channel Sound.
Run script… → mix_selected_multi_channel_into_stereo.praat.
Choose a preset (e.g., “5.1 → Stereo (Film)”).
Optionally adjust center, surround, and LFE trims.
Enable normalize if desired and set target (e.g., 0.99).
Press OK. The stereo mix appears as a new object.

Channel Order & Routing

Common orders are shown below. If your input differs, use Manual Matrix.

Layout	Order (indices)	Notes
Mono	C (0)	Duplicated to L/R with trim
Stereo	L (0), R (1)	Pass‑through or rebalanced
3.0 (LCR)	L (0), R (1), C (2)	C trimmed into L/R
5.1	L (0), R (1), C (2), LFE (3), Ls (4), Rs (5)	Film order
7.1	L (0), R (1), C (2), LFE (3), Ls (4), Rs (5), Lrs (6), Rrs (7)	Surround & rears folded

Recommended Downmix Coefficients

The following trims are typical starting points for music/film fold‑downs. Adjust to taste.

Source	To L	To R	Comment
L	+0 dB	—	Direct
R	—	+0 dB	Direct
C	-3 to -6 dB	-3 to -6 dB	Shared to both sides
Ls	-3 dB	—	Decorrelated; sometimes -4.5 dB
Rs	—	-3 dB	Decorrelated; sometimes -4.5 dB
Lrs	-4.5 dB	—	Rear to same side
Rrs	—	-4.5 dB	Rear to same side
LFE	-10 dB	-10 dB	Optional; low‑passed
Mono (C only)	-3 dB	-3 dB	Duplicate then trim

Why trims? Summing multiple coherent sources increases level; trims avoid overload and maintain spectral balance.

Parameters & Options

Parameter	Type	Default	Description
preset	option	Auto	Auto, Stereo pass‑through, 3.0→st, 5.1→st (Music/Film), 7.1→st
center_trim_db	real (dB)	-3	Level for C when split to L/R
surround_trim_db	real (dB)	-3	Level for Ls/Rs
rear_trim_db	real (dB)	-4.5	Level for rear surrounds
lfe_trim_db	real (dB)	-10	Contribution of LFE to L/R
lfe_lowpass_hz	real	120	Low‑pass cutoff for LFE feed
matrix_manual	text	""	CSV of N×2 gains (linear). Overrides trims.
normalize	boolean	yes	Normalize output peak to norm_target
norm_target	real	0.99	Peak amplitude target
protect_mono	boolean	yes	Reduce opposite‑side bleed to mitigate cancellations
phase_safe_sum	boolean	yes	Energy‑sum approximation for partially correlated channels
report	boolean	no	Print effective matrix and headroom estimate

Presets

🎧 Stereo pass‑through

Leave L/R untouched; optional rebalance with small trims.

🎬 5.1 → Stereo (Film)

C = -6 dB to both, Ls/Rs = -3 dB to same side, LFE = -10 dB (optional LPF).

🎵 5.1 → Stereo (Music)

C = -3 dB to both, Ls/Rs = -3 dB, LFE muted or -10 dB low‑passed.

🏟️ 7.1 → Stereo

As above plus Lrs→L (-4.5 dB), Rrs→R (-4.5 dB).

🔧 Manual Matrix

Provide CSV rows (one per input channel): gainL,gainR (linear). Example for 3.0 LCR: 1,0 | 0,1 | 0.707,0.707.

Tips & Pitfalls

Headroom: When summing coherent signals (e.g., C), trims of -3 to -6 dB avoid overload.
LFE: In music, consider muting or filtering LFE; in film, keep it low‑passed and at -10 dB.
Phase: Decorrelated surrounds add width; if cancellations occur in mono, reduce opposite‑side feed.
Normalization: Normalize after downmixing, not before.
Verification: Always check mono fold‑down and correlation meters for translation.

Watch out: Incorrect channel order is the #1 cause of odd balances. If the result sounds shifted or hollow, verify the input order and routing matrix.

Math

Matrix Form

Let x ∈ ℝ^{N×T} be channels stacked by rows, and G ∈ ℝ^{N×2} gains.
Stereo y ∈ ℝ^{2×T}: y = Gᵀ x ⇒ y_L = Σ g_{iL} x_i; y_R = Σ g_{iR} x_i.

Coefficient Conversion

Linear gain from dB: g = 10^{(dB/20)}.
Example: -6 dB ⇒ 0.501; -3 dB ⇒ 0.708; -10 dB ⇒ 0.316.

Energy‑Preserving Approximation

When channels are partially correlated, approximate power sum:
y ≈ √( Σ (g_i x_i)^2 ). In practice, we apply a small safety factor to reduce overload risk.