Beds and stems

The last craft chapter faces the direction we've ignored since Chapter 2: backwards, toward the channel-based world. Your collaborators live there. Sessions arrive as 5.1 stems; sample libraries ship quad ambiences; the sound designer's handoff is a 7.1.4 print. None of it is ambisonic, all of it is useful, and one object annexes it.

Companion patch: patchers/booklet/12-beds.maxpat.

The move

A channel-based bed is speaker feeds for a known layout — which means every channel has a defined direction (L = +30°, RS = −110°, …). So encode each channel at its canonical direction and sum:

[your 5-channel stem]              (one mc cord, L R C LS RS order)
    |
[ambitap.bed2hoa~ 3 surround_5_1]
    |
   bus — an ordinary ambisonic scene now

ambitap.bed2hoa~ takes the same <order> <layout> arguments and the same layout list as decode~ — it is, conceptually, that object's mirror image (decode: scene → feeds; bed2hoa: feeds → scene). The matrix is static (directions don't move), so it's among the cheapest objects in the package. Once through it, the material is scene: rotatable, mirrorable, re-decodable to any rig, binauralizable — the whole of Part III applies.

What the move is worth

Concretely, with the 5.1 stem session that just landed:

  • Monitor it on headphones, properly. bed2hoa~ 3 surround_5_1 → binaural~ 3 is an instant, correct virtual 5.1 room — each stem channel rendered from its true angle. For anyone who's mixed surround on a laptop, this alone justifies the object.
  • Re-deliver it anywhere. The 5.1 print, through the scene, decodes to your quad rig, an octagon, or a dome (Chapter 12 rules apply — allrad for the gappy targets). This is up/down/cross-mixing done by geometry instead of by folklore fold-down coefficients.
  • Use it as a bed in the Atmos sense: channel-based ambience layer underneath object-like foreground — here, encoded foreground sources (Chapter 9) summed onto the same bus as the imported bed. One cord, both paradigms, everything rotates together.
  • Rescue legacy work. Quad tape-music realizations, 5.1-era installations — re-staged into scenes, they stop being hostage to one speaker count.

And the disclaimer, since this object is where wishful thinking congregates: this is not upmixing magic. A bed encodes as five (or eleven) virtual speakers on the sphere — phantom images between them stay phantom (a source panned L↔C in the original is two correlated virtual speakers, not a true 15° source), and the scene inherits the bed's spatial resolution forever. bed2hoa~ relocates a channel-based mix faithfully; it cannot un-bake the panning decisions inside it.

The LFE, finally

Every layout in this package has been "no LFE" since Chapter 5, with promises. Here's the whole policy in one place.

The LFE channel (the ".1") is a delivery convention — a separate low-frequency effects track for a subwoofer — not a direction. An ambisonic scene describes directional sound, and sub-bass direction is barely perceptible (which is why one subwoofer suffices in the first place). So the bus simply does not carry it, and the routing is:

  • Importing a 5.1-with-LFE stem: peel channel 4 (the LFE) off before bed2hoa~ — mc.unpack~, feed the five mains to the object, and route the LFE directly to your subwoofer output (or, on a sub-less monitor rig, mix it into the render at −∞ to −10 dB per taste; it's effects seasoning by definition).
  • Delivering to an LFE-expecting format: decode the scene to the mains (Chapter 12), and derive the LFE as a low-passed mono sum (W, low-passed, is the classical choice) — or better, keep true LFE-type material (the explosion sweetener) on its own mono track through the whole project, outside the bus, and print it directly.
  • Bass management (rerouting mains' low end to the sub) is the monitor controller's job or a crossover after the decoder — playback plumbing, not scene content. Keep it downstream and out of the master.

The companion patch builds a synthetic 5.1 bed (five distinct sources mc.combined in canonical order — front trio, rear pair), folds it in, and renders binaurally, with the Chapter 14 heatmap showing five tidy blobs at the canonical angles: the bed, visibly staged on the sphere. Rotate it — five blobs turn as one — and the point of the whole exercise lands.

Checkpoint — and the end of the craft

Beds fold in by encoding each channel at its canonical angle (bed2hoa~, decode~'s mirror); the result is honest scene material with the bed's resolution baked in; the LFE routes around the bus in both directions.

Take stock: sources, distance, rooms, decoding, binaural, instruments, scene-mixing, XTC, beds. That is the complete AmbiTap toolkit and — by this book's argument — a complete spatial audio education in one package. Part IV takes exactly this skill set and shows you the same ideas wearing other uniforms: Pd, microphones, DAWs, game engines, and the A-word.