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Specification · Release 1.0

ATB Open Biosignal Profile (ATB-OBP) 1.0

Open, vendor-neutral, royalty-free Bluetooth LE GATT profile for multi-channel EEG and biosignal devices.

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Status: Release 1.0 · Date: 4 November 2026 · Owner: HMS Health Mobile Software / Auto Train Brain Inc. · Contact: info@healthmobilesoftware.com

ATB-OBP is an open, vendor-neutral Bluetooth Low Energy (BLE) GATT profile for multi-channel EEG and related biosignal devices. A device that implements this profile connects to ATB Desktop (Windows and macOS) — and later to the ATB mobile apps — without any vendor driver or SDK. The application discovers the device, reads a self-describing JSON record, starts the stream and immediately uses the device for live EEG monitoring, recordings, QEEG measurements and EEG-based brain exercise sessions in clinical EEG software.

The key words MUST, MUST NOT, SHOULD, SHOULD NOT and MAY are used as in RFC 2119.

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1. Scope and design goals

• One profile, any hardware: 1–32 channels, 128–4096 Hz, dry / dry-active / gel / saline electrodes.
• Self-describing: channel labels, sample rates, scaling and features are read from the device; the app needs no per-model code.
• Simple firmware: fixed little-endian binary packets; one notification per data packet; no compression.
• Robust: every packet carries a sequence number and a sample index, so the app detects and repairs loss exactly.
• Extensible: unknown JSON keys are ignored; vendor features use the x- prefix; optional characteristics may be absent.

Out of scope for 1.0: on-device signal processing results, stimulation output, firmware update (use the vendor's own DFU service), more than 32 channels (planned for 2.0).

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2. Conformance levels

A device is "ATB-OBP 1.0 Core compatible" when it passes every Core check of the in-app conformance test (§11).

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3. GATT layout

3.1 UUIDs

All ATB-OBP UUIDs share one 128-bit base. Replace xxxx with the 16-bit short code:

a7b1xxxx-0b1e-4e0e-8a7b-41544200b1e0        ("415442" = ASCII "ATB")

Standard SIG services that MAY be added alongside: Battery Service 0x180F (Battery Level 0x2A19), Device Information Service 0x180A.

3.2 Advertising

• The device MUST include the ATB-OBP Service UUID (128-bit) in its advertising data or scan response, so the app can filter devices by service.
• The device SHOULD advertise a human-readable Complete Local Name (e.g. AcmeEEG-8 1234; the last 4 characters of the serial help identify one of several units).
• Advertising interval while waiting for a connection: 100–250 ms recommended.

3.3 Byte order

All multi-byte integer and float fields are little-endian. Signed integers are two's complement.

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4. Connection sequence (what the app does)

1. Scan with filter services = [ATB-OBP Service]; the user picks the device in the app.
2. Connect; exchange MTU (the app's BLE stack requests the maximum; see §9.1).
3. Read Device Info (§5), parse and validate it. Validation errors abort the connection and are shown in the conformance test.
4. Enable indications on Control Point; enable notifications on Status, Impedance and Events if present. Read Status once.
5. Enable notifications on EEG Data.
6. If sync_time is listed in features: send SYNC_TIME.
7. Send START_STREAM(defaultRate, mask = all eeg+eog channels, format); wait for the response indication (timeout 3 s).
8. Stream. On user disconnect: send STOP_STREAM, wait ≤ 1.5 s for the response, then disconnect.

Firmware MUST tolerate steps 4–5 in any order and MUST NOT start streaming before START_STREAM.

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5. Device Info characteristic (0001)

A UTF-8 encoded JSON object (no BOM; trailing NUL bytes allowed). The value MAY be longer than the ATT MTU — the client uses ATT Read Blob (long read) automatically. Maximum length 2048 bytes. The value MUST NOT change while connected.

5.1 Fields

Unknown keys MUST be ignored by the app; vendors MAY add keys prefixed x- (e.g. "x-acme-gain": 24).

5.2 Channel entries

{ "label": "Fp1", "type": "eeg" }

• label: electrode name in the international 10-20 / 10-10 system (e.g. Fp1, F3, Cz, O2, TP9). Legacy names T3/T4/T5/T6 are accepted and mapped to T7/T8/P7/P8. Labels MUST be unique.
• type: eeg (default) · ref · eog · emg · ecg · aux. The app streams eeg and eog channels by default; at least one eeg channel is required.
• The channel's index (0-based position in the array) is its bit in the channel mask and its column in data packets.

5.3 Features

5.4 Example

{
  "obp": "1.0",
  "vendor": "Acme Neuro",
  "model": "AN-8",
  "serial": "AN8-000123",
  "firmware": "1.2.0",
  "hardware": "rev C",
  "electrodes": "dry",
  "rates": [250, 500],
  "defaultRate": 250,
  "formats": ["int24"],
  "format": "int24",
  "lsb_uV": 0.02235,
  "range_uV": 187500,
  "reference": "linked-ears",
  "ground": "Fpz",
  "channels": [
    { "label": "Fp1", "type": "eeg" }, { "label": "Fp2", "type": "eeg" },
    { "label": "C3",  "type": "eeg" }, { "label": "C4",  "type": "eeg" },
    { "label": "P3",  "type": "eeg" }, { "label": "P4",  "type": "eeg" },
    { "label": "O1",  "type": "eeg" }, { "label": "O2",  "type": "eeg" }
  ],
  "maxFramesPerPacket": 9,
  "features": ["contact_quality", "status", "sync_time", "identify", "battery_service"],
  "filters": { "hp_Hz": 0.5, "lp_Hz": 100, "notch_Hz": null }
}

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6. Control Point characteristic (0002)

The app writes a command (Write with response). The device answers every command with one indication on the same characteristic.

6.1 Command format

channelMask: bit i selects channels[i]. Bits ≥ channel count MUST be ignored. A mask of 0 is INVALID_PARAM.

6.2 Response indication

Response data:

• SYNC_TIME → sampleIndex u32 of the device at reception (lets the app align device sample clock to wall time).
• GET_STATUS → the 8-byte Status payload (§8.2).
• START_STREAM → MAY return rate_Hz u16 · channelMask u32 · format u8 actually applied.

Timing: the response SHOULD be indicated within 200 ms (MUST within 3 s). START_STREAM while already streaming with identical parameters SHOULD return SUCCESS (idempotent). After START_STREAM the first data packet MUST start with firstSampleIndex = 0.

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7. EEG Data characteristic (0003)

One notification = one packet = a 10-byte header + nFrames frames. A frame is one sample of every channel in the active mask, in ascending channel-index order.

7.1 Packet layout

bytesPerSample: int16 = 2, int24 = 3 (little-endian, sign-extended from bit 23), float32 = 4 (IEEE-754).

Scaling: µV = raw × lsb_uV for int formats; float32 values are µV directly.

7.2 Sizing packets

The ATT notification payload is MTU − 3 bytes. The firmware MUST choose nFrames so that 10 + nFrames × nChannels × bytesPerSample ≤ MTU − 3.

Firmware SHOULD send a packet at least every 40 ms even if it is not full (latency, §9.2).

7.3 Golden packets (for firmware unit tests)

int16, seq = 0x0102, firstSampleIndex = 1000, 2 frames × 2 channels, samples [100, −100], [32767, −32768]:

01 02 01 E8 03 00 00 02 02 00 64 00 9C FF FF 7F 00 80

int24, seq = 0, index 0, LEAD_OFF set, 1 frame × 3 channels, samples [1, −1, −8388608]:

02 00 00 00 00 00 00 01 03 01 01 00 00 FF FF FF 00 00 80

7.4 Loss handling (app side, informative)

• seq gap → packets lost; firstSampleIndex gap → exact number of frames lost.
• Gaps ≤ 1 s are bridged by linear interpolation so filters and spectra stay continuous; the loss is counted and reported (loss %).
• Gaps > 1 s break the stream (discontinuity); recordings mark the break.
• Packets with an older firstSampleIndex (duplicates / reordered) are dropped.
• The device MUST NOT renumber samples after a buffer overrun: if frames are dropped on the device, skip their indices (so the app sees the gap) and increment the Status droppedPackets counter.

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8. Optional characteristics

8.1 Impedance / Contact (0004, Notify)

Sent while impedance mode is active (IMPEDANCE_START … IMPEDANCE_STOP), at least once per second.

Large channel counts MAY be split across several notifications using firstChannel. Unless impedance_during_stream is listed, the device MAY pause EEG data during impedance mode and MUST resume it on IMPEDANCE_STOP (continuing seq/firstSampleIndex numbering, which the app treats as a gap).

App colour coding: kΩ ≤ 20 green, ≤ 50 amber, > 50 red; quality ≥ 70 green, ≥ 40 amber.

8.2 Status (0005, Read + Notify)

8 bytes. Notify on change and at least every 10 s while connected.

Safety recommendation: devices SHOULD NOT stream EEG while charging from mains (mains noise and electrical safety); report CHARGING so the app can warn.

8.3 Events / Markers (0007, Notify)

7 bytes per event.

8.4 Aux — IMU / PPG (0006, Notify)

ATB Desktop 1.11 ignores Aux data; it is reserved for motion-artifact rejection and HRV in a later release.

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9. Timing and performance

9.1 MTU and connection parameters

• Support ATT MTU exchange up to 247 (BLE 4.2 Data Length Extension). The device MUST still work at the default MTU 23 by sending smaller packets.
• Connection interval 7.5–30 ms recommended (15 ms typical); slave latency 0 while streaming; supervision timeout 2–4 s.
• 2M PHY SHOULD be supported for ≥ 16 channels or ≥ 500 Hz.

9.2 Sampling and latency

9.3 Signal quality (recommendations)

• Input-referred noise ≤ 1 µV RMS (0.5–40 Hz); CMRR ≥ 100 dB; resolution ≤ 0.1 µV/LSB.
• Do not apply a mains notch filter on the device unless reported in filters; the app applies 50/60 Hz notch by region.
• High-pass ≤ 0.5 Hz; anti-alias low-pass < 0.45 × sample rate.

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10. Security and privacy

• LE Secure Connections pairing (Just Works or Passkey) is RECOMMENDED; the profile also works unpaired for evaluation units.
• The device MUST NOT store or transmit personal or health information about the wearer. serial identifies the hardware only.
• The Control Point MUST reject malformed commands with INVALID_PARAM and never crash or reboot.
• Firmware updates are out of scope; use the vendor's own DFU service (it MUST NOT be reachable through ATB-OBP characteristics).

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11. Conformance checklist (in-app test)

ATB Desktop → Clinic → device card "ATB-OBP Bluetooth device" → 🧪 Conformance test runs these checks live and exports a JSON report (Save report). Send the report to info@healthmobilesoftware.com to be listed as compatible.

Firmware-side self test: reproduce the golden packets of §7.3 byte-for-byte.

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12. Versioning

• obp = "major.minor". Minor versions only add optional fields, opcodes, characteristics or flag bits; a 1.x app accepts any 1.y device (unknown parts ignored, a newer minor is reported as a warning).
• A major version change (2.0) may change packet layouts; it will use a new service UUID short code so 1.x and 2.x can coexist on one device.
• Reserved bits and bytes MUST be sent as 0 and ignored on receipt.

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13. Reference material

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Appendix A — Minimal firmware state machine

           ┌────────────── IMPEDANCE_STOP ───────────────┐
           ▼                                              │
  IDLE ──START_STREAM──► STREAMING ──IMPEDANCE_START──► IMPEDANCE
   ▲  ▲                     │                             │
   │  └──── STOP_STREAM ────┘                             │
   └────────────── STOP_STREAM / disconnect ──────────────┘

• On disconnect: stop streaming, keep advertising.
• On START_STREAM: reset seq and firstSampleIndex to 0, apply rate/mask/format, respond SUCCESS, then start notifications.

Appendix B — Example timeline

App  → Read 0001                         ← {"obp":"1.0",...}
App  → CCCD 0002 = indicate, 0003/0004/0005 = notify
App  → 0002: 07 <unix ms u64>            ← 80 07 00 <sampleIndex u32>        (SYNC_TIME)
App  → 0002: 01 FA 00 FF 00 00 00 02     ← 80 01 00                          (START 250 Hz, 8 ch, int24)
     ← 0003: 02 00 00 00 00 00 00 09 08 00 <216 bytes>   seq 0, idx 0
     ← 0003: 02 01 00 09 00 00 00 09 08 00 <216 bytes>   seq 1, idx 9
     …
App  → 0002: 02                          ← 80 02 00                          (STOP)