8 — Odd Forward, Even Reversed

Process an octophonic (8‑channel) sound such that odd‑indexed channels play forward while even‑indexed channels are time‑reversed — with options for indexing scheme, crossfades, alignment policy, and channel remapping.

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

Contents:

What this does Quick start Channel policy Parameters (Praat-style) Per-channel options Presets Workflow & Checks Edge cases Math

What this does

Takes an 8‑channel Sound and reverses either the even or odd subset while leaving the other subset forward. Useful for antiphonal textures, palindromic call‑and‑response, and spatial retrogrades. Options ensure consistent indexing (0‑based vs 1‑based), gapless crossfades at the boundaries, and preserved start/end alignment.

I/O: 8 in / 8 out, original order.
Indexing: Choose whether channels are labeled 0–7 or 1–8 for “odd/even”.
Alignment: Keep start/end anchors so forward and reversed subsets end simultaneously.

Quick start

Select an 8‑channel Sound in Praat.
Run script… → 8 - Odd forward Even reversed.praat.
Set Index base (0 or 1) and choose Reverse subset = Even (default) or Odd.
Adjust Crossfade at boundaries (ms) if you hear clicks.
Pick an Alignment policy (End‑locked recommended). Press OK.

Channel policy

Index base	Odd channels	Even channels	Example
0‑based (0..7)	1,3,5,7	0,2,4,6	“Even” includes channel 0
1‑based (1..8)	1,3,5,7	2,4,6,8	Matches typical speaker labels

If your audio interface/DAW labels start at 1, choose Index base = 1 to keep “odd/even” intuitive.

Parameters (Praat-style)

Label	Type	Default	Description
Index base	option	1	0 \| 1 — how odd/even are computed
Reverse subset	option	Even	Even \| Odd
Preserve alignment	option	End‑locked	Start‑locked \| End‑locked — which anchor remains fixed after reversing
Crossfade at boundaries (ms)	real	10	Fade‑in/out at clip edges of reversed channels to avoid clicks
Trim leading/trailing silence	boolean	no	Remove silence before/after processing
Channel map (CSV)	text	""	Optional remap (e.g., 0,1,2,3,4,5,6,7 or 1..8 for 1‑based UIs)
Normalize	boolean	yes	Peak normalize output to target
Target peak	real	0.99	Normalization target
Report	boolean	no	Print applied subset, index base, and alignment

Per‑channel options

Label	Type	Description
Exclude channels (CSV)	text	Channels that should bypass processing
Per‑channel offset (ms)	CSV	Delay/advance after reversal to create flams
Per‑channel gain (dB)	CSV	Level trims post‑process

Presets

🎭 Antiphonal mirror

Index base=1; Reverse subset=Even; Crossfade=15 ms; End‑locked; Normalize=yes

🪞 Palindrome pairs

Reverse subset=Even; Channel map=0,1,3,2,4,5,7,6; Crossfade=12 ms

⏱️ Flam spread

Per‑channel offset=0,8,0,8,0,8,0,8 ms; Crossfade=8 ms

Workflow & Checks

Confirm indexing: Compare with your DAW/driver. If channels are labeled 1–8, set Index base=1.
Pick anchor: End‑locked keeps all channels ending together (good for synced cues). Start‑locked preserves the onset alignment.
De‑click: Increase Crossfade if you hear clicks at the reversal boundaries.
Gain staging: Enable Normalize, especially if per‑channel offsets create overlaps that increase peaks.
Downmix sanity: If folding to stereo later, audition mono compatibility (reversals can change phase relations).

Edge cases

Short clips: With very short material, use small (≤5 ms) crossfades to avoid smearing transients.
Silence handling: Trimming silence can shift perceived entry times; prefer End‑locked when trimming.
Non‑octo inputs: The script expects 8 channels; if your source differs, route or pad to 8 first.

Math

Time reversal

Given channel i with samples x_i[0..N−1], the reversed signal is x_i^R[n] = x_i[N−1−n].
With end‑lock, the reversed channel is time‑shifted so that its last sample aligns with the original end.

Crossfade at boundaries

Apply fade‑in/out windows w(t) at the first/last T_f ms of each reversed channel: y = w_in·y + (1−w_in)·ε near start, and similarly at end; Hann windows avoid clicks.