Out of the headphones

Binaural rendering is a wonderful lie told to two ears. This chapter tells the truth to a room instead: the same scene you built in Chapters 3–4, decoded to actual loudspeakers, where the soundfield exists in the air and listeners can walk around inside it.

If you don't own four loudspeakers, read on anyway — the chapter ends with what to do about that, and the concepts here (layouts, decoders, channel order) are load-bearing for the whole rest of the book.

The last box

Open patchers/booklet/03-out-of-the-headphones.maxpat, or swap the renderer at the end of Chapter 4's patch:

[pink~]
   |
[ambitap.encode~ 3]
   |
[ambitap.rotate~ 3]
   |
[ambitap.decode~ 3 quad]      ← scene in, four speaker feeds out
   |
[mc.dac~ 1 2 3 4]

ambitap.decode~ takes two creation arguments: the order (match the bus, as always) and a layout name that fixes how many output channels it produces and where it assumes the speakers are. The built-in layouts:

The decoder's output is one mc cable, one channel per speaker, in the layout's canonical order — for quad that's front-left, back-left, back-right, front-right. mc.dac~ 1 2 3 4 then maps those onto your audio interface's outputs in that order, so the only setup job is knowing which interface output feeds which physical speaker (and passing mc.dac~ the right numbers if it isn't 1–4).

Setting up four speakers honestly

The decode assumes a listener at the center of a square of speakers. You don't need an anechoic chamber, but three disciplines repay you instantly:

1. Equal distances. The math assumes all speakers equidistant from the center. A tape measure is a spatial audio tool.
2. Equal levels. Play pink noise through each speaker in turn (sweep the encoder azimuth to 45°, 135°, −135°, −45° — at those exact angles, a quad decode hands one speaker most of the signal) and match by ear or SPL meter.
3. Angles as advertised. ±45° and ±135° from the listening position — the decoder is computing gains for those directions, not for wherever the furniture allowed. Close counts; 20° off doesn't.

Now run the orbit from Chapter 3. Walk around. Sit in the middle and sweep the rotator's yaw. Notice what headphones couldn't give you: the field is in the room — externalization for free, no HRTF required — and several people can hear it at once. Notice also what got worse: away from the center, images pull toward the nearest speaker (Chapter 1's precedence effect, back for revenge), and a four-speaker ring has no idea what "above" means — send elevation 1.0 and the circle just… flattens. Height needs height speakers (cube, surround_7_1_4) or headphones.

What the decoder actually did

Chapter 2 promised: a decoder is a small matrix computed once for your layout. Concretely, ambitap.decode~ 3 quad built a 4×16 matrix — four speakers, sixteen scene channels — and every sample of your speaker feeds is that matrix applied to the bus. All the intelligence lives in choosing the matrix; there are competing philosophies, and the object exposes them:

• the decoder_type attribute selects the construction — mode_match (the default), allrad, or epad;
• the max_re attribute (off by default) applies a per-order weighting that trades a little theoretical sharpness for cleaner energy concentration — usually a good trade in real rooms.

Chapter 12 is an entire chapter about that choice, with figures computed from the actual matrices, and a rule of thumb for which construction suits which array. For a regular ring like quad, the honest summary is: the defaults are fine, try max_re 1, and don't lose sleep.

One deliberate omission: there is no LFE channel anywhere in this. The surround_5_1 layout is five full-range speakers. An ambisonic scene describes direction, and sub-bass direction is barely perceptible — so LFE/bass management is a playback concern, handled downstream of the decode (Chapter 17 shows the routing when we meet channel-based beds).

No speakers? Two honest options

Option one: stereo decode. ambitap.decode~ 3 stereo produces feeds for a normal ±30° speaker pair. It works — the frontal stage is stable and CPU cost is nil — but understand what it is: a projection of your sphere onto stereo's narrow window. Rear and height content doesn't vanish (the decode folds it in at reduced level so energy isn't lost), but it no longer sounds behind you. It's the right tool for "my ambisonic piece needs a stereo bounce," not for monitoring spatial decisions.

Option two — the recommended one: keep monitoring binaurally. This isn't a consolation prize. ambitap.binaural~ is a decode — internally it renders the scene as if through a fine array of virtual loudspeakers, each convolved to your ears — and it's a truthful monitor for direction, which is what you're composing with. Professionals mixing for domes they visit twice a year work exactly this way: compose binaurally, decode on site, spend the precious room time on level calibration instead of composition. The companion patch is wired for the same discipline — the quad decode and a binaural monitor side by side, one mc gain to switch between them — so "check it on speakers" stays a five-second habit even when the speakers are hypothetical.

Checkpoint — and the end of the beginning

You can now take a scene from silence to sound three different ways: binaural for headphones, a quad ring, a stereo fold-down — same scene, one box swapped. That's the "mix once, decode anywhere" promise of Chapter 2, demonstrated rather than asserted.

You also, quietly, now hold the entire ambisonic signal chain: encode → transform → decode. Everything else in this book is a richer version of one of those three stages — fancier sources into the bus (rooms, recordings, beds), fancier transforms on it (mirrors, compressors, virtual microphones), fancier renders out of it (arrays, personalized HRTFs, crosstalk cancellation). Before the craft, though, one debt from Chapter 3 is still outstanding: what are those sixteen channels, actually? Part II opens the bus.