Adaptive Physical Communication System (APCS)

User Guide

This guide explains, step by step, how to use the Adaptive Physical Communication app to send a message, a link, a photo or a short video from one phone to another without any internet, Wi-Fi, Bluetooth or mobile data. The two phones talk to each other using only light (a moving QR code on the screen, read by the other phone’s camera), sound (tones from the speaker, heard by the other phone’s microphone) or vibration (buzzes felt by the other phone’s motion sensor). You do not need any technical knowledge to follow it. Every button name in this guide is written exactly as it appears in the app.

Back to the documentation index.

Contents

  1. What the app does
  2. Installing the app
  3. Permissions the app asks for
  4. The Home screen
  5. Writing a message (Compose)
  6. Choosing how to send: Light, Sound or Vibrate
  7. The send screen and its options
  8. Receiving a message
  9. The received message card
  10. Getting ready for the next message
  11. Tips for best results
  12. What to do if…

1. What the app does

You need two phones with the app installed. One phone is the sender and the other is the receiver. (Light and Sound can also send to several receivers at the same time.)

Channel How the message travels Distance Good for
Light The sender’s screen shows a fast-changing QR code. The receiver points its camera at it. 15–25 cm Photos, videos, longer text
Sound The sender’s speaker plays musical-sounding tones, or with Silent tones too high for most people to hear. The receiver listens with its microphone. Across a table (Silent: within arm’s reach) Short text, links
Vibrate (experimental) The sender’s phone buzzes in a pattern. The receiver, pressed against it, feels the buzzes. Phones touching Trying it out only: it doesn’t work reliably yet

Nothing leaves the two phones. Anyone nearby could see the QR code or hear the tones, so do not send secrets.

Vibrate is experimental. On real phones, vibration messages often don’t arrive, even short ones. Use Light or Sound for anything you need to get through.

Note: The screenshots in this guide were taken from the web build of the app in a phone-sized browser window. On a phone, the screens look the same, with two differences: the Home screen’s bottom line names your phone’s channels, and the Choose transmission mode panel also offers Vibrate.

2. Installing the app

Android

  1. Get the file app-release.apk from the person who built the app. Both phones must have the same version of the app, otherwise the Light channel will not work between them.
  2. Copy the APK to the phone (for example with a USB cable), or open it from wherever you received it.
  3. Tap the APK file. Android will warn that the app comes from an unknown source.
  4. Tap Settings in the warning, turn on Allow from this source (the wording differs a little between phone brands), then go back and tap Install.
  5. When installation finishes, tap Open. On your home screen the app is called Adaptive Comm.

iPhone

iPhones cannot install APK files. The app has to be installed from a Mac with Xcode by the person who built it. After that, it appears on your home screen as Adaptive Physical Communication.

Laptop (Chrome)

The app can also run in the Chrome browser on a laptop. In Chrome you can use Light (with the webcam) and Sound, but not Vibrate.

3. Permissions the app asks for

The app asks for permissions the first time it needs them. Please tap Allow each time. The app never uses the internet, so these permissions are only used for the physical channels.

Permission When it is asked Why it is needed
Camera The first time you open Receive on a phone To read the QR code on the other phone
Microphone The first time you open Receive, and the first time you start sending To hear Sound messages. The app sets up the microphone for any channel, so this prompt can appear even when you picked Light or Vibrate.
Storage (Android 9 and older only) The first time a received photo or video is saved To put the file in your Gallery. Android 10 and newer do not ask.
Photos (iPhone only) The first time a received photo or video is saved To put the file in the Adaptive Comm album
Motion (iPhone only) May be asked when you use Vibrate To feel the vibration pulses

If you tapped Don’t allow by mistake, open your phone’s Settings → Apps → Adaptive Comm → Permissions (or Settings → Adaptive Physical Communication on iPhone) and turn the permission on.

4. The Home screen

When you open the app you see the Home screen.

Home screen: the app logo, the title Adaptive Physical Communication, the line Send messages without internet, and two large buttons, Send and Receive

Figure 1. The Home screen.

From top to bottom it shows:

Before you start: decide which phone sends and which receives. On the receiving phone, tap Receive first so it is ready and listening. Then use Send on the other phone.

5. Writing a message (Compose)

Tap Send on the Home screen. The Compose message screen opens.

Compose message screen: an empty text box, the Attach chips Image, Video, Link and Demo samples, an attached file cat_sketch_2kb.jpg of 2.0 KB with a remove button, and the Continue to send button at the bottom

Figure 2. The Compose message screen with a demo photo attached.

5.1 Sending text

  1. Tap the large box that says Type a message, paste a link… and type your message.
  2. Tap Continue to send at the bottom.

If you tap Continue to send with nothing typed and nothing attached, a message at the bottom says Enter a message or attach content.

There are two ways:

Note: because of this automatic recognition, a short message that happens to look like a web address (for example see.you) is sent as a link.

5.3 Sending a photo

  1. Tap the Image chip under Attach.
  2. Pick a photo from your phone.
  3. The app automatically shrinks the photo so that it is at most about 120 KB, because the physical channels are slow. PNG, WebP and GIF pictures are turned into JPEG.
  4. A small card appears showing the file name and size, with an ✕ button (label Remove attachment) if you change your mind.
  5. Tap Continue to send.

If the picture cannot be read, a message at the bottom explains why and nothing is attached.

5.4 Sending a video

  1. Tap the Video chip and pick a video.
  2. Videos are not shrunk by the app, so choose a very short, small clip. The built-in demo videos (next section) are a good size.
  3. If the file is larger than 512 KB, a window called Large file warns: “This file is … KB. Physical channels work best under 512 KB. Transfer may be slow or fail. Continue?” Tap Use anyway to keep it or Cancel to choose another.
  4. Tap Continue to send.

Important: if you type text and attach a photo or video, only the text is sent. To send the photo or video, leave the message box empty.

5.5 Sending other files

The Compose screen only has chips for Image, Video, Link and Demo samples. It has no general “file” option. Documents and other file types can only be sent from the Legacy Messages screen under Developer tools, which is meant for testing.

5.6 Using the built-in Demo samples

The app contains ready-made photos and videos, so you can try it without copying any files to the phone.

  1. Tap the Demo samples chip.
  2. A panel slides up from the bottom. Drag it up to see more. At the top it says Demo samples and “Built-in test files. Anything here works over Light; keep Sound to about 2 KB.”
  3. The panel has two lists:
    • Photos: four pictures (Cat sketch, Future city, Night sky, Robot lab), each in four sizes of about 2, 5, 10 and 20 KB. Each row shows a small preview.
    • Videos with sound: Binary numbers, Countdown beeps, How QR codes work, How sound travels, Morse code, Photosynthesis, Speed of light, The water cycle and Why we have seasons. Each row shows a play icon.
  4. Under each title you see its size and type, for example 9.8 KB · JPG or 78.1 KB · WEBM.
  5. Tap a row to attach it, then tap Continue to send.

Demo samples panel: the heading Demo samples, the line Built-in test files, and a Photos list with thumbnails, for example Cat sketch 2.0 KB JPG and Future city 5.0 KB JPG

Figure 3. The Demo samples panel. Scroll down for Videos with sound.

Good first choices: a 5 KB photo over Light finishes in a few seconds. Speed of light (about 78 KB) takes about 40 seconds over Light. The iPhone cannot play or save the Speed of light video because it is in WebM format; all other videos are MP4 and work everywhere.

6. Choosing how to send: Light, Sound or Vibrate

After you tap Continue to send, a panel titled Choose transmission mode slides up, with the line How should this message travel to nearby devices?

Choose transmission mode panel: the question How should this message travel to nearby devices, with a Light option (Animated QR, hold 15 to 25 cm) and a Sound option (Speaker tones, text and small files)

Figure 4. The Choose transmission mode panel (web build; phones also show Vibrate).

Option Description shown in the app
Light Animated QR — camera to screen, no Wi‑Fi (hold 15–25 cm)
Sound Speaker tones — text and small files, across a room
Vibrate Contact-only — short text, phones pressed together. It also shows 1:1 only — phones must touch in orange.

At the bottom is the tip Tip: Light uses fountain QR — hold phones 15–25 cm apart. Vibrate is not shown when the app runs in Chrome.

Tap the one you want. Swipe the panel down to go back to your message without choosing.

Which one should I pick?

7. The send screen and its options

The next screen is titled Send via Light, Send via Sound or Send via Vibrate. At the top left is an ✕ button (label Cancel) that takes you back.

7.1 Before you start (the “ready” screen)

The screen shows a large icon and a heading:

Channel Heading
Light Ready to transmit via QR
Sound Ready to play
Vibrate Ready to vibrate

Below it is a card with a preview of your message (the text, or the file name).

Send via Light screen: Ready to transmit via QR, a card for cat_sketch_2kb.jpg reading 2.0 KB, Fountain QR, 160 B per frame, about 2 s, a Tips for a smooth transfer card, the QR density buttons Auto, Safe, Standard and Fast with Auto selected, and the Show QR and send button

Figure 5. The Send via Light screen, ready to send.

Light options

The message card has an extra line, for example:

80.0 KB · Fountain QR · 330 B/frame · ~38s

This means: the message is 80.0 KB; it is sent as a moving QR code; each QR picture carries 330 bytes; and a receiver that is aimed well should finish in about 38 seconds. (See 7.4 What the time estimate means.)

A card titled Tips for a smooth transfer lists:

Under QR density are four buttons. The app selects Auto every time you open this screen.

Button What it does When to use it
Auto Chooses the QR size for you: small, easy-to-read codes for small messages, and slightly denser codes only for bigger files. 12 pictures per second. Almost always.
Safe The simplest, easiest-to-read code (160 bytes per picture), shown more slowly (8 per second). The receiver is far away, shaky, or the screen looks washed out.
Standard A denser code (330 bytes per picture), 12 per second. Both phones are held very steady.
Fast The densest code (600 bytes per picture), 12 per second. Only when the phones are close, steady (for example propped up) and the light is good.

When you change the density, the size, bytes-per-frame and time estimate in the line above update.

Sound options

A card titled Tips for a smooth transfer lists, for Audible:

and for Silent:

Sound band has two buttons:

Under Speed are the buttons for the chosen band. Each shows its speed in bytes per second. The app starts on Standard; switching band picks Standard or Silent. Your choice is remembered until you close the app.

Band Button Best for (hint shown in the app)
Audible Fast · 36 B/s Quiet room, phones touching
Audible Standard · 27 B/s Normal room, across a table
Audible Safe · 18 B/s Background noise or chatter
Audible Rugged · 11 B/s Loud room, phones metres apart
Silent Silent · 5.0 B/s Inaudible 18–20 kHz, phones within arm’s reach
Silent Silent Robust · 3.4 B/s Inaudible, weak speaker or a loud crowd

Under the buttons the app repeats the hint for the selected speed, followed by “The receiving phone detects the band and speed by itself.” You do not need to choose anything on the receiving phone.

Next comes FRAME TO BE SENT: a coloured bar showing how each sound frame’s time is spent (Marker, the start signal; Header; Message, your data; CRC, a check; and Parity, the repair data that lets a damaged frame be fixed). Under it are the TONES range in kHz, the length of one FRAME in seconds, and how many FRAMES your message needs at minimum. A one-line text fits in one frame: 2.4 s on Standard, 4.8 s on Silent.

Below that is a line such as 24 B on air · ~10s typical: the size of your message as sent, and roughly how long it takes at the chosen speed in a normal room. When every frame is heard cleanly it is quicker, down to the time shown under FRAMES.

Vibrate options

There are no options. An orange label reads Contact required — 1:1 only.

7.2 Starting to send

Tap the big button at the bottom:

Channel Button
Light Show QR & send
Sound Play & send
Vibrate Start vibration

While sending over Light

The whole screen turns white and shows a large black-and-white QR code that changes about 12 times per second. Your screen brightness goes to maximum automatically and returns to normal afterwards.

Light sending screen: a white screen with a large black-and-white QR code, the line Streaming, tap Stop when the receiver shows DONE, the details Auto, 160 B per frame, 3.3 KB, K=21, frame 38, and a Stop button at the bottom

Figure 6. Sending over Light. The QR code changes about 12 times per second.

The photo in Figure 6 is 2.0 KB but 3.3 KB on air, because the app re-encodes photos before sending (see Known Issues §4.1).

The QR code never stops on its own (except after a 10-minute safety limit), because the sending phone has no way of knowing when the receiver has finished. Watch the receiving phone. When it shows DONE (or the message appears), tap Stop.

While sending over Sound

The screen shows a pulsing sound icon and Playing acoustic tones…, with the reminder “Volume up, speaker facing the other phone. Keep playing until the other device says it has the message.”

Below the progress card, Sending now shows the frequency being played at this moment, in kHz, and a small 0–22 kHz strip with a line for each tone. Audible sends play several tones at once (for example 1.94–6.80 kHz · 8 tones at once). Silent sends play one tone between 18.3 and 19.9 kHz.

A card appears with:

The progress bar fills when the usual amount of sound has been played. Sound keeps playing after that, in case the receiver missed some parts, until you tap Stop or a built-in limit is reached. When the receiver shows the message, tap Stop on this card. The tones stop straight away.

At the very bottom there is also a Cancel transmission button. It stops the tones and leaves the send screen, as do the ✕ button and the back button.

While sending over Vibrate

The screen shows a pulsing vibration icon with Vibrating… (or Transmitting… between buzzes) and “Hold phones together — vibration is contact-only (1:1)”. Keep the phones pressed together until it finishes. Cancel transmission at the bottom stops it.

Vibration is very slow: even a two-letter message takes more than a minute. It is also experimental, so the message may not arrive at all.

7.3 When sending ends

Situation What you see Buttons
You tapped Stop on Light (or the 10-minute limit was reached) Streaming stopped, with a line such as “Streaming stopped (463 QR frames in 38s). If the receiver shows DONE, the file arrived; otherwise tap Resume — it keeps its progress.” Receiver shows DONE and Resume streaming
Sound finished or you tapped Stop; Vibrate finished successfully Sent successfully Done
Something went wrong Send failed, with the reason Back and Retry

Pressing your phone’s back button while sending cancels the send and leaves the screen.

7.4 What the time estimate means

The number after ~ (for example ~38s) is how long a well-aimed, steady receiver typically needs, not how long the sender plays.

Either way, keep sending until the receiver says it is done.

8. Receiving a message

  1. On the receiving phone, tap Receive on the Home screen.
  2. A panel titled Choose receive mode appears with the line How should this device listen for messages? It has the same three options as the sender. Pick the same channel as the sender. If you swipe the panel away, you go back to the Home screen.
  3. The screen title becomes Receive via Light, Receive via Sound or Receive via Vibrate. To switch channel later, tap the Change mode button (sliders icon) in the top-right corner.

Leaving the Receive screen (the back arrow) stops the camera, microphone or motion sensor.

At the top of the screen is a coloured status banner that tells you what the phone is doing. When a message arrives, the banner turns green and reads Message received — still listening.

8.1 Receiving over Light

The screen shows the camera view with four white corner brackets marking a square. Only the part inside the square is read.

How to hold the phone:

  1. Hold the receiving phone 15–25 cm from the sender’s screen. Closer is not better: many phone cameras cannot focus closer than that.
  2. Move until the whole QR code is inside the corner brackets. It should fill most of the square without its corners touching the brackets.
  3. Keep still. Rest your elbows on a table, or lean the phones against something.
  4. If the picture looks blurry, tap the camera view on the QR code. This makes the camera focus again at that spot.
  5. Tilt slightly if you see a bright reflection on the sender’s screen.

Zoom chips (top-right of the camera view): 1x, 1.5x, 2x, 3x. The camera starts at 1.5x. The selected chip is white. If the QR code looks small, tap 2x; this lets you stay at a comfortable distance while the code still fills the square. Only the zoom levels your camera supports are shown, and the chips are hidden if your camera has no zoom.

The status box (HUD) at the bottom of the camera view:

The first row shows a status badge:

Badge Meaning
SCAN (grey) Looking for a QR code; nothing read yet.
LOCK (blue) Found the sender’s code and collecting pieces.
DONE (green) The whole message has been received.

Next to the badge, SID is a short code identifying this transfer (you can ignore it). On the right it shows the density name, or K= followed by the number of pieces once locked.

The second row shows live numbers:

Label What it means What a good value looks like
CAP How many camera pictures per second the phone is taking Around 15–30
DEC How many QR codes per second are successfully read The higher the better; several per second is good
DROP Camera pictures skipped because the phone was still busy reading the previous one Any number is fine; this is normal
GOOD Useful data received per second, in KB/s Rises while receiving
NEW/DUP/RED Useful new pieces / pieces already received / pieces that added nothing new NEW should keep climbing

When a transfer is in progress, a thin progress bar appears with a line such as 120 / 243 symbols · 330 B (pieces collected / pieces needed, and bytes per piece). The file is complete when the first number reaches the second.

If the camera is working but no QR code can be read for a few seconds, an amber hint appears: “No codes readable — whole QR inside the square at 15–25 cm, hold steady, tap to refocus, or try 2x zoom”. If some progress was already made, it starts with Progress kept., which means nothing is lost; just re-aim.

Below the camera view is a progress bar and one of these messages:

Message Meaning
Hold 15–25 cm away, whole QR inside the square · tap to refocus Waiting for a QR code
Reading QR… Something QR-like is being read
Decoding — 120 / 243 symbols · keep steady Collecting pieces
Progress kept (120 / 243) — re-aim: whole QR inside the square, hold steady Stalled; re-aim

The status banner at the top may also read Receiving fountain QR 120 / 243 symbols with Keep camera aimed at the sender QR.

If you move away, lose the code, or even leave and re-open the screen, the phone keeps the pieces it already has (for up to 3 different transfers). Aim again and it continues.

When the file is complete, a green Transfer complete card may briefly show the size, time and speed (for example 80.0 KB in 41.2s (1.9 KB/s)), and then the message itself appears.

8.2 Receiving over Sound

Receive via Sound screen: a blue banner Mic live, listening for tones, an ear icon, the status Mic live, waiting for audio, a Hearing now card showing No clear tone above a 0 to 20 kHz spectrum strip with the Audible and Silent bands shaded, the meters Mic input, Tone signal and Silent band 18 to 20 kHz at 0 percent, and the line Microphone streaming

Figure 7. Receiving over Sound, listening before the sender starts.

  1. Allow the microphone if asked.
  2. Place the receiving phone so its microphone (usually at the bottom edge) faces the sender’s speaker, across a table at most. A quiet room is faster.
  3. Ask the sender to tap Play & send.

The screen shows a pulsing ear icon (blue when listening, amber when it hears tones) and a status line such as:

Status Meaning
Starting microphone… Getting ready
Mic live — waiting for audio (speak or play tones nearby) Microphone works but hears almost nothing
Mic live — listening for sender tones Ready and listening
Tones detected — decoding message Hearing the sender
Receiving — 3 of 5 blocks Collecting pieces
Message decoded — ready for next Done
Microphone permission required The microphone is blocked

Receive progress card (appears while a message is arriving):

Meters at the bottom:

The status banner at the top shows messages such as Mic live — listening for tones (Hold within ~30 cm of sender in a quiet room), Tones detected — decoding… (Hold phones close until message appears), Microphone blocked (Tap Enable microphone below) and Sound message decoded.

8.3 Receiving over Vibrate

Vibrate is experimental: the message often doesn’t arrive, even if you do everything below.

  1. Choose Vibrate on both phones. Open Receive on this phone before the sender taps Start vibration.
  2. Place the phones touching, for example back to back or one on top of the other on a table, and press them together firmly.

The screen shows Detecting vibration… and “Press phones together firmly. Vibration is contact-only and best for short text.” Below it, Signal …% shows how strongly this phone feels movement. It jumps up when the other phone buzzes. Keep the phones still between buzzes.

9. The received message card

When a message arrives it replaces the camera view or meters and fills the screen. The card’s header shows the type (Text message, Link, Image, Video or File) and the size.

Type What you see and can do
Text message The text in large print. Press and hold to select and copy it.
Link The address, underlined (you can select and copy it), and an Open link button that opens it in your browser. If it cannot be opened, you see Invalid URL or Could not open link. (Opening a web page needs internet; receiving the link does not.)
Image The photo. Tap it to open it full screen on a black background; pinch with two fingers to zoom and drag to move around. Tap outside the picture or press back to close.
Video The video starts playing automatically, with sound, and repeats. Use the round play/pause button below it; the time shows as 00:12 / 00:24. If a video cannot be played you see Could not play video (… bytes).
File The file name and size. The app does not open or save other file types.

Under the card, a small line says Photo received — still listening for more, Playing received video — still listening for more or Still listening — send another message anytime.

Received photos and videos are saved to your Gallery automatically, in an album called Adaptive Comm. The status banner then reads Photo saved to Gallery (Adaptive Comm) (or Video saved…). Each message is only ever saved once.

The button under a photo or video shows the save state:

Button Meaning
Save to Gallery Not saved yet. Tap to save.
Saving to Gallery… (with a spinner) Saving now.
Saved to Gallery · Adaptive Comm (with a green tick, greyed out) Saved.
Retry save, with an orange reason underneath Saving failed. Fix the reason and tap to try again.

Possible reasons: Gallery permission denied, Not enough storage, Format not supported by the gallery or Could not save to gallery. The Gallery button does not appear when the app runs in Chrome.

To find saved files, open your phone’s Gallery or Photos app and look for the Adaptive Comm album.

10. Getting ready for the next message

The phone keeps listening while it shows a message. To clear the screen and wait for the next one, tap Clear & keep listening under the card. The camera view or meters come back.

If a message you already received appears as soon as you open the Receive screen, it is the last message from before; tap Clear & keep listening to hide it.

11. Tips for best results

Light

Sound

Vibrate

Vibrate is experimental and doesn’t work reliably yet. If you want to try it:

12. What to do if…

Problem What to do
The receiver stays on SCAN Hold 15–25 cm away with the whole QR inside the brackets. Tap the camera view to focus. Try 2x. Tilt to remove glare. Make sure both phones have the same version of the app.
DEC is low but CAP is fine Hold steadier (rest your elbows), switch the sender’s QR density to Safe, and wipe the camera lens.
Progress stops at “… / … symbols” Keep the sender streaming and re-aim. If the sender already stopped, tap Resume streaming on the sender. The receiver kept its progress.
The camera view just says “Starting camera…” The camera permission is probably off. Turn it on in the phone’s Settings, then leave and reopen Receive.
Sound: nothing happens Tap Enable microphone. Check that Mic input moves when you speak. Turn the sender’s volume up and check that Tone signal moves while it plays.
Sound: “too damaged” keeps rising Turn the volume up, point the speaker at the microphone, move closer, or choose Rugged on the sender.
Sound: Silent sends never arrive Look at the receiver’s Silent band 18–20 kHz meter while the sender plays. If it stays near 0%, turn the sender’s media volume to maximum and disconnect Bluetooth headphones; then try swapping which phone sends. If it still doesn’t move, one phone can’t play or hear 18–20 kHz: use Audible. If it moves but nothing decodes, move closer, hold still, or choose Silent Robust.
Sound: the two readouts disagree Compare the sender’s Sending now with the receiver’s Hearing now. If the receiver shows —, the sound isn’t reaching it: volume up, move closer, uncover the speaker and microphone. If it shows a different steady frequency, something nearby (a fan, a charger, another app) is louder than the sender. The sender’s readout may change a split second before the sound; that’s normal.
Vibrate: “Send failed” Make sure the other phone is on Receive → Vibrate before you start, press the phones together firmly, and tap Retry. Vibrate is experimental, so if it keeps failing, send over Light or Sound instead.
“Payload too large for Light fountain QR” The file is too big. Choose a smaller photo or video.
A photo is not in the Gallery Look in the Adaptive Comm album. Tap Retry save on the card. On Android 9 or older, allow storage access.
A video will not play on iPhone Use an MP4 video. iPhones cannot play or save WebM videos such as Speed of light.
An old message appears when I open Receive Tap Clear & keep listening.

For more help see the FAQ, the Troubleshooting guide and the Showcase guide for running a demonstration. For how each channel works, see Light, Sound and Vibration.

This site is open source. Improve this page.