TwoUp

(formerly BTPhone — bundle id com.wahdany.twoup, service _twoup._udp; only the Xcode target and this repo keep the old working name)

A rider/passenger intercom for two iPhones. The phones talk directly to each other over Wi-Fi Aware (iOS 26's peer-to-peer Wi-Fi framework) — no internet, no SIM, no router. The link is paired, authenticated, and encrypted by the system, runs in Wi-Fi Aware's real-time performance mode, and — unlike the AirDrop-style AWDL transport this app used originally — keeps working with the phone locked in your pocket, because it only requires the app to be running (which the audio background mode guarantees), not frontmost.

Audio is uncompressed 16 kHz voice PCM over UDP with a small self-trimming jitter buffer, tuned for low latency. Voice processing (echo cancellation, noise suppression, auto gain) comes from Apple's voice-processing audio unit.

• Two iPhones on iOS 26 or later (Wi-Fi Aware hardware support; the app checks at runtime and tells you if a phone can't do it).
• A paid Apple Developer account for building (the Wi-Fi Aware entitlement is self-service, but not available to free personal teams).

# Generate the Xcode project (once, and after editing project.yml)
nix-shell -p xcodegen --run "xcodegen generate"
open BTPhone.xcodeproj

The team ID is set in project.yml; plug in each iPhone (Developer Mode enabled) and Run once per phone. Approve the microphone prompt on first launch.

1. Pair each phone with its own helmet headset before starting the app. TwoUp requests HFP (hands-free) routing so the helmet's boom mic is used.
2. One-time pairing: open TwoUp on both phones; on one tap Be discoverable, on the other tap Find other phone and select it. The pairing persists (manage it in iOS Settings if you ever want to remove it).
3. Wait for "Connected" + green. Tap the big button to mute/unmute. While muted, the other phone shows "The other phone is muted" instead of a connection warning.
4. Lock the phone and pocket it — the link and audio keep running.

Sessions are unlimited. On the free version a short reminder — a two-note chime and one on-device-synthesized sentence — plays into this phone's own earpiece every 15 minutes of a session; it never goes over the link, and the peer's voice is ducked, not cut, underneath it. A one-time lifetime unlock (StoreKit 2 non-consumable, Family Sharing) removes the reminders. The gate fails open: until the product exists in App Store Connect — including all dev builds — no reminders play. The local Configuration.storekit file feeds Xcode's StoreKit test environment when running via the scheme. UI ships localized in English and German (Localizable.xcstrings).

First real ride (July 2026, rider + passenger, phones locked in pockets, Bluetooth helmet headsets): a session of ~350,000 received packets — just under two hours of continuous audio at 50 packets/s — with battery drain low enough that neither phone noticed it. Precise %/hour and open-road range numbers still to be collected.

20 ms frames: 4-byte big-endian sequence number + 320 samples of 16 kHz mono Int16 PCM per UDP datagram (~50 packets/s). An empty payload is a mute keepalive (2/s) so the peer can distinguish "muted" from "gone". A 60 ms jitter buffer smooths playback and trims itself back down after bursts and clock drift; mouth-to-ear latency is roughly 100–150 ms phone-to-phone plus whatever the Bluetooth helmet headsets add (HFP typically 50–100 ms per side).

• "Pair with the other phone to start" won't go away: the phones aren't paired yet — run the one-time pairing (step 2 above). Both phones need Wi-Fi (and preferably Bluetooth) radios on.
• Choppy audio: check the loss % in the footer (it's a 10-second window). Sustained loss above a few percent means radio trouble — distance or heavy 2.4/5 GHz congestion.
• Wrong mic in use: iOS picks the route; reconnecting the headset while TwoUp is open makes it re-route — the app rebuilds its audio pipeline automatically on route changes, and a watchdog revives audio if anything kills it (the big button shows AUDIO OFF whenever audio isn't actually running).
• New passenger or new phone: tap Pair a different phone in the status card — the pairing UI is always reachable. Old pairings can only be removed in iOS Settings (there is no unpair API).
• Music: playback from other apps pauses while a session runs and resumes automatically when you end it. Mixing music into the session isn't offered: over a Bluetooth helmet headset the intercom holds the HFP voice link, which blocks high-quality A2DP — music would be call-quality mono.
• Battery: real-time Wi-Fi Aware mode plus continuous audio costs some battery, but you can ride with the screen locked, which more than makes up for it.