Borrowed rooms

Every room you have ever heard is a filter: clap once, and what comes back — the impulse response — is everything the room will ever do to any sound. Convolution reverb plays your signal through that recording. tap.convolve~ does it in true stereo against an impulse response held in a buffer~, using the standard engine of the genre (uniformly-partitioned overlap-save FFT convolution), and its defining property is worth stating up front: it is exact. Not "high quality" — exact. This chapter is what that buys, what it costs, and how to drive the two knobs that actually matter.

Companion material: the reference page and help patcher in the TapTools-Max package, and the verification notebook — the first notebook in this repo, and the template for all the others.

Exact, measured

The engine splits the IR into blocksize-sample partitions, transforms each once, and multiply-accumulates in the frequency domain over a delay line of past input spectra. The bookkeeping is intricate; the result is not. Measured against a direct time-domain convolution of the same float32 IR: maximum difference 3×10⁻¹² — double-precision noise. Change the block size and the output doesn't change either (64/256/1024 agree within 2×10⁻¹², latency-removed). An impulse through the engine reconstructs the IR to 5.5×10⁻¹⁴, and a synthetic 0.60 s-RT60 reverb measures back at 0.599 s. There is no "character" in this engine to audition; the character is entirely in the IR you load.

The wiring, and the one real cost

Stereo in, stereo out, IR from a named buffer~. The cost: latency of exactly blocksize samples — verified for every block size — on top of your I/O latency. That is the entire quality/latency dial:

• blocksize small (64–128): tight enough for live input; more CPU per sample (more, smaller FFT batches).
• blocksize large (512–2048): cheapest; latency grows to match. For a send/return reverb on a mix bus, nobody hears 21 ms of pre-delay you didn't ask for — except you, so use predelay deliberately instead.

maxsize reserves capacity (both are locked in while DSP runs and apply on restart). The rest of the surface mirrors tap.verb~ so the two reverbs read as siblings: mix, gain, predelay, normalize (energy-based, so quiet and hot IRs land at comparable levels), bypass, mute.

The wiring: one FFT in, one IFFT out, and a multiply-accumulate that is the only cost growing with IR length.

True stereo, by channel count

A stereo room isn't two mono rooms: sound from the left source arrives at the right ear too. The engine runs the full 2×2 matrix — LL, LR, RL, RR — and the buffer~'s channel count selects the topology:

• 4+ channels: true stereo, all four paths (measured: a signal sent only left emerges on the right at exactly the cross-feed path's gain, 0.600 expected, 0.600 measured, with zero leakage where paths are silent).
• 2 channels: dual mono — L and R convolved separately, no cross-feed.
• 1 channel: the same mono room on both sides.

Loading rooms while the music plays

IRs are analysed off the audio thread and published atomically into a double-buffered slot: swapping IRs mid-performance neither clicks nor drops (measured RMS across the swap instant: 21.9 before, 22.1 just after), and one block later the output is bit-identical to an engine that had the new IR from the start. Load rooms like presets; the engine doesn't flinch.

Recipes

• The honest room: a measured IR (church, plate, spring — the internet is full of them), @mix 25 @normalize 1, and resist the urge to EQ the IR itself before trying predelay — 10–30 ms of it buys clarity for free.
• Not a reverb at all: an IR is any filter. A single click is a delay; a strummed guitar body is a body simulator; a vowel is a formant filter. Convolution doesn't know it's supposed to make reverb.
• True-stereo width: record or synthesize the four paths with a genuinely different LR/RL from LL/RR — the cross-feed is where "being in the room" lives.

When it is not the right tool

• You want to design the reverb — decay knobs, damping, modulation, gated tails. A static IR can't do any of that; tap.verb~ (the algorithmic Moorer reverb, with its own oversampling and limiter) can.
• Zero-latency insert on a live path. The engine costs blocksize samples, full stop. At 64 that's 1.3 ms — small, not zero.
• Time-varying convolution. Swaps are click-free but discrete; the engine doesn't interpolate between rooms.

Checkpoint

Partitioned convolution is exact linear convolution — measured to 10⁻¹² — with one honest cost, blocksize samples of latency, and one honest dial, block size against CPU. True stereo comes from the buffer's channel count; IR swaps are atomic and dropout-free; and everything the effect sounds like is the impulse response you feed it. Borrow better rooms.