phase 0 spike · beta soon

Sync your old iPhones at millisecond accuracy. Apple Music, Spotify, your own library — all supported. Local files get studio-grade <5ms drift.

price$79/year
platformiOS 16+
scope32 devices
live sync demo
1080² · 15s loop
remotion · react-renderedphase 0 metaphor

Rivals drift
200ms+.
The human ear hears 20ms.

sync drift scalelog ms
choir
< 5ms
20ms
ear threshold
soundseeder
~80ms
ampme
~250ms
ampme max
500ms

When two audio sources drift more than 20 milliseconds apart, the brain reads it as echo or comb filtering. The signal richness collapses, everything sounds wet.

AmpMe and similar apps use acoustic matching; typical drift 200-500ms. Choir runs NTP-style UDP time sync targeting {'<'} 5ms.

ampme
~0ms
soundseeder
~0ms
choir target
<0ms
hear it yourselftakes 20 seconds

Drifting vs
in sync.

Tap one of the two buttons below. "Drifting" — the standard drift of the rivals; "In sync" — what Choir holds. The same pad plays on two channels, one through your left speaker, the other through your right. Measure the gap with your own ears.

Speakers with distinct L/R channels make this clearer. On Bluetooth headphones the gap is immediately obvious.

two devices · L + Roff
08s loop

Three modes. Honest roadmap.

01MVP

Amplify

All devices play the same full-band signal. Sample-accurate sync, total sound pressure scales proportionally. Behaves like one concert speaker.

Free tier 2 devices, Pro tier 32 devices. Practical ceiling is your WiFi router capacity. Use: picnic, house party, small concert, cafe loop. The flattest mode, direct showcase of the sync theory.

Pro 32 devices · full-band
02MVP

Stereo

Devices get L/R channels assigned. Real stereo imaging where one phone stays mono.

2 devices · L/R
03v1.2

Frequency band

Bass / mid / treble split across devices. DSP crossover, phase-preserved.

3-4 devices · DSP crossover

v1.2 and beyond is on the roadmap, not in MVP. Roadmap = intent, not a promise. v2.0 spatial/surround, v2.1 Android, v2.2 Spotify Cast (sync disabled, why?).

Where?
From 8 phones
to 32 devices.

Choir isn't locked to one scenario. It scales with device count, adapts to context. Here's where it fits in the real world.

Free2 devices

Street talk

Two people in the park. One phone in your left ear, one in your right. Real stereo, no Bluetooth speaker needed.

Mode: Stereo · Free tier
Pro8 devices

Beach night

8 friends' old iPhones placed around the blanket. They behave like one concert speaker. Sample-accurate sync; no cacophony, no comb filtering.

Mode: Amplify · Pro tier
Pro16 devices

Mini festival

Backyard party, 30 people, 16 devices spread around. Bass dense on one side, treble bright on the other. Total sound pressure already past a single speaker.

Mode: Amplify · Pro tier
Pro32 devices

Cafe loop system

Cafe owner buried 32 old demo iPhones under the counter. Same playlist all day, no Spotify Connect mess. $79/yr — a quarter of a Spotify Family plan.

Mode: Amplify · Pro tier · WiFi 6

Pro tier tested to 32 devices. Practical ceiling is your WiFi router capacity; v1.3 brings hierarchical sync for 50+. The architecture isn't fundamentally capped.

Detailed setup + practical tip for each scenario

Use honesty
as a marketing
weapon.

Rivals promise everything, deliver nothing. We promise less, deliver all of it. Below: 4 honest decisions — reasons and consequences.

deliberate decisions
  1. 01

    No <5ms on Spotify.

    DRM

    Spotify Connect works, Apple Music Sing works. But DRM physically blocks sample-buffer access — <5ms requires local files. Two modes in the app, clearly labeled: Stream Mode ~300ms (AmpMe-class), Tight Sync <5ms (studio-grade).

    You pay Spotify Premium yourself; we take nothing from it. Choir's subscription pays only for the sync engine.

  2. 02

    No Android.

    HAL

    OEM audio HAL variance destroys the calibration profile. With five thousand Android models we can't honor sample-accurate sync.

    Phase 5 (v2.1). Prove on iOS first, then Oboe-based Android port.

  3. 03

    No account / no sign-up.

    P2P

    P2P flow. No server, no account. Anonymous device UUID is enough.

    Pro receipt validation only via Apple App Store receipt; cloud account database never.

  4. 04

    Fair subscription.

    $79/yr

    $7.99/mo, $79/yr. A quarter of AmpMe, half of Roon, a third of Spotify Family. No hidden fees, price never rises — annual locks you in.

    30-day free annual trial, cancel anytime in one tap. Beta supporters lock Founder Pricing $63.99/yr, forever at that price.

Sync drift telemetry.

Phase 0a sample data. Real TestFlight measurements will update here. The numbers are mock, the format is real.

phase 0a · clock_sync_drift
150s window
t (s)offsetskewrttconf
30+3.21ms-12.4ppm4.8msHIGH
60+3.18ms-12.6ppm5.1msHIGH
90+3.24ms-12.5ppm4.9msHIGH
120+3.19ms-12.6ppm5.0msHIGH
150+3.22ms-12.5ppm5.2msHIGH
...............
RMS deviation3.21ms
Target< 5ms
VerdictGO
Sync
  • NTP-style UDP, 250ms ping interval, 20-sample rolling window
  • Lowest-RTT-10 linear fit (offset + skew)
  • 2s refresh; chunked playback (drift correction)
Calibration
  • Round-robin self-loopback (cross-device underdetermined)
  • Swept sine 10ms (200Hz → 8kHz), cross-correlation peak
  • Symmetry assumption: input ≈ output latency
Offset model
  • Three separate offsets: clock_offset, output_latency, ToF
  • T_local = master_to_local(T_master) − output_latency_self
github.com/choir

Codex code-reviewed sync engine. Open source, MIT.

discovery mode

Pick by distance. Same sync, different scale.

Same sync engine, two transports for your use case. Devices in one room? AirDrop-style P2P. Larger venue? WiFi-wide. One pick, the right physics.

~10 m · awdl p2p
Proximity

AirDrop-like, no router needed.

Devices discover each other over AWDL P2P. No infrastructure. Fewer hops, tighter sync.

cap2–8 devices
drift< 2 ms
netAWDL P2P
for
  • Studio: 4 phones as true L/R/Mid/Side reference
  • Living room: 3 devices fill the space
  • Mobile set: pull out of the bag, connect instantly, no code
~30 m · lan udp
WiFi

One router, wide coverage.

Every device on the same wireless network joins the session. Up to 32; venue size is bound by the router.

cap2–32 devices
drift< 5 ms
netLAN UDP
for
  • Event: 12 phones cover a 200 m² space
  • Venue: office, store, restaurant background
  • Party: guests link every phone in the room

Both modes share the same sync core: three-offset model (clock + output latency + ToF), swept-sine cross-correlation calibration, 2-second refresh. Only the transport differs. Free tier can try both modes; the cap stays at 2 devices.

Choir, AmpMe,
SoundSeeder
and the rest.

Most categories are trade-offs. AmpMe supports more platforms but doesn't keep its sync promise (200ms+ typical). SoundSeeder honestly plays local files but iOS is weak. Bose is locked to its speakers. Choir deliberately stays iPhone-only: niche but keeps its word.

Choir

Sync quality
< 5ms (local) · ~300ms (stream)
Device type
iPhone only
Music source
Local + Apple Music + Spotify + YouTube
Price
$79/yr ($64 beta) · $7.99/mo
Account required
No
Open source
Yes (sync engine)
Device limit
Free 2 · Pro 32

AmpMe

Sync quality
200-500ms
Device type
iOS + Android
Music source
Spotify, YouTube, SoundCloud
Price
$22/mo = $264/yr
Account required
Yes
Open source
No
Device limit
100+ claim

SoundSeeder

Sync quality
50-150ms
Device type
Android + weak iOS
Music source
Local file only
Price
$5 one-time
Account required
No
Open source
No
Device limit
Limited

Bose Connect

Sync quality
< 5ms (hardware)
Device type
Bose speakers only
Music source
App music sources
Price
Hardware included
Account required
No
Open source
No
Device limit
100+

Apple Audio Sharing

Sync quality
< 5ms (hardware)
Device type
Apple devices only
Music source
Apple Music + other
Price
OS built-in
Account required
Apple ID
Open source
No
Device limit
2 headphones

Who Choir is NOT for: Android owners, Spotify-centric users, anyone wanting many more devices (10+). For that, Sonos or a brand ecosystem makes more sense.

Full comparison + verdict for each competitor

AmpMe Premium $22/mo
Choir $79/yr.

Who saves how much? Slide the years, see your own number. Choir's subscription is a quarter of AmpMe.

How many years?
5yr
1 yr10 yr
AmpMe Premium
$1,320
$22/mo × 12 mo × 5
Choir Pro
$400
$79.99/yr × 5
Net savings
$920
And the sync is 40x tighter.

Source: AmpMe Premium App Store price (2026). Choir Pro $79.99/yr annual subscription. 30-day free trial, cancel anytime. Beta supporters lock in Founder Pricing $63.99/yr forever.

Roadmap,
open and votable.

Intent, not promise. Beta members can vote on priority. Mock numbers for now; real voting after Phase 0 GO.

  1. v1.0
    planned

    MVP launch (September 2026)

    • Amplify mode (full-band, 2-32 devices)
    • Stereo mode (L/R, 2 devices)
    • NTP-style UDP sync (< 5ms target)
    • Stream Mode: Apple Music + Spotify Connect (~300ms)
    • Session Flow: auto-advance + crossfade
    • $79.99/yr annual + $7.99/mo monthly subscription
  2. v1.1
    planned
    0votes

    Karaoke Mode (November 2026)

    • Apple Music Sing integration
    • LRC files for lyrics teleprompter
    • Multi-phone vocal routing (singer + backing)
    • Mic capture → backing phones over Bonjour
  3. v1.2
    planned
    47votes

    Frequency band

    • DSP crossover (bass / mid / treble split)
    • Phase-preserved split across 3-4 devices
    • Concert-style arrangement
  4. v1.3
    planned
    31votes

    Hierarchical sync

    • 50+ device support (UDP multicast)
    • Master-relay-slave topology
    • Large-event mode (festival, venue)
  5. v2.0
    research
    22votes

    Spatial / surround

    • Beyond stereo: 3D scene
    • HRTF-aware device placement
    • Dynamic mix based on listener position
  6. v2.1
    research
    18votes

    Android port

    • Oboe-based low-latency audio
    • HAL profile database
    • Cross-platform session
  7. v2.2
    future
    9votes

    Spotify Cast Mode

    • Spotify Connect orchestration
    • Sync promise disabled (DRM constraint)
    • Separate, labeled mode in UI

Roadmap = intent, not promise. Order or timing may shift; each release evaluated together.

Answers to
the 10 things
you'd ask first.

More in the full FAQ or related pages. Still can't find it? hello@choir.fm.

  1. No. Spotify's DRM blocks the sample-accurate buffer access Choir requires. We prefer to be honest about this. v2.2 will add it as “Cast Mode”, but sync is disabled, just Spotify Connect orchestration.

    Read the full guide

TestFlight beta
soon.

We'll take 50-100 testers. Sending invites once Phase 0 sync measurements are done. No spam, just the invite email. Beta supporters lock in Founder Pricing: $63.99/yr forever, never rises to the normal $79.99.

Hard requirements: iOS 16+, at least 2 devices, same WiFi. Founder Pricing $63.99/yr × first 30 days free.