Chorus: Multi-Speaker Sync

Chorus turns the phones around you into one set of speakers, all playing the
same song at the same moment.

Start a party on one phone, let your friends join from theirs, and the music
plays from every device at once — louder, wider, and genuinely in time.

WORKS WITH NO INTERNET AT ALL
Phones find each other directly over Bluetooth and Wi-Fi Direct. No account, no
signup, no signal. Chorus works on a beach, in a park, in a basement, or on a
plane — anywhere other music sync apps stop working because there is no
connection to a server.

REAL SYNC, NOT ROUGHLY TOGETHER
Two sounds more than about 10 milliseconds apart stop being one sound and start
being an echo. Chorus holds every speaker far inside that.

It does it the way a broadcast studio would. Each phone continuously measures
the difference between its clock and the host's, keeping only the fastest
exchanges — the only ones that carry reliable timing. It then tracks how fast
each phone's crystal actually runs, so a device stays aligned between
measurements instead of sliding. Any error left over is absorbed a few samples
at a time, far below what an ear can hear, so the music never stutters to
resync.

ACOUSTIC CALIBRATION
Bluetooth speakers lie about their delay. A speaker that reports 160
milliseconds is often really 250, and the true figure wanders while it plays.

So Chorus measures it instead of asking. Your phone plays a short sweep at
18–20 kHz — above what adults can hear — listens for it through its own
microphone, and works out exactly how long the sound really took to leave the
speaker. That measurement is what makes a Bluetooth speaker sit in time with a
phone instead of trailing it.

REAL STEREO ACROSS DEVICES
Assign one phone the left channel and another the right, and a pile of handsets
becomes an actual stereo pair. It is also the most convincing proof the sync is
real: a stereo image collapses the instant timing slips.

SEE THE NUMBERS
Every speaker shows its live error in milliseconds, its measured delay, and
whether it is locked. The claim this app makes is checkable, so it lets you
check it.

WHAT YOU NEED
Two or more Android phones or tablets in the same room, with Bluetooth and
Wi-Fi turned on. Any audio file on the host device. Nothing else.

FREE AND PRO
Chorus is free to use with one extra speaker, network sync, and one-hour
parties — a real party, not a demo.

Chorus Pro adds:
• Unlimited speakers
• Acoustic calibration for every device
• Stereo mode across devices
• Parties with no time limit
• Shared queue with no gap between songs

Pro is a subscription, billed weekly, monthly or yearly through Google Play.
Cancel any time in your Play subscriptions.

Music sync for parties, multi-room audio, Bluetooth speaker groups, and turning
a room full of phones into one sound system.