Heart rate, straight into LabStreamingLayer.

What it is

HRBand-LSL scans for any BLE device exposing the standard Heart Rate Service (0x180D), connects on selection, parses the Heart Rate Measurement characteristic (0x2A37), and publishes two LSL outlets per session: one for instantaneous heart rate (bpm), one for the corresponding beat-to-beat intervals in milliseconds.

Bands are split into two protocol paths. ECG-based bands (Polar H10, Wahoo TICKR, Garmin HRM-Pro) advertise true R-R intervals directly on 0x2A37 — the bridge labels these as RR. PPG-based bands (Polar Verity Sense, OH1, OH1+) do not, so the bridge falls through to Polar's vendor PMD service and subscribes to peak-to-peak (PPI) frames; these are labelled as PP. Receivers can therefore tell at a glance whether a stream is suitable for clinical-grade HRV (RR) or only for an approximation of it (PP).

The bridge is a single static binary, has no GUI, runs from the shell or from a Nix flake, and produces streams that are bit-compatible with the Android sibling (RRStreamer) and the historical Python reference. LabRecorder, pylsl receivers, and LabStreamingLayer-aware acquisition software pick up both outlets with no special configuration.

Data shape

Each session produces exactly two LSL outlets. The interval outlet's name reflects its physiological provenance — RR from ECG, PP from PPG. Sample shape, channel count, and format are identical so a downstream receiver written against the Python or Android reference works without changes.

Stream contract

// Sample lines, as printed by the bridge during a Polar Verity Sense session

[ok] LSL outlets created (HR + PP)
PMD CP returned SUCCESS; PPI streaming committed
HR: 57  PP: [1051] ms
HR: 57  PP: [1034] ms
HR: 57  PP: [1070] ms
HR: 59  PP: [1009] ms
HR: 55  PP: [1083] ms
HR: 55  PP: [1094] ms
HR: 55  PP: [1080] ms

Quick start

Run with Nix (one-shot)

$ nix run github:abcsds/HRBand-LSL

Build from source with Cargo

$ git clone https://github.com/abcsds/HRBand-LSL
$ cd HRBand-LSL
$ nix develop --command cargo build --release
$ ./target/release/hrband-lsl

The application scans for 5 seconds, lists detected bands, and either auto-selects (when exactly one Polar device is in range) or prompts. Once subscribed it prints HR + interval samples to stdout and pushes them to LSL until interrupted by Ctrl-C or SIGTERM; on shutdown it writes STOP_PPI to Polar PMD bands so the band's battery isn't drained by an orphaned subscription.

Receive in Python (pylsl)

import pylsl

# Resolve every stream, print sample values from each HR/RR/PP outlet.
streams = pylsl.resolve_streams(wait_time=5.0)
inlets  = [pylsl.StreamInlet(s) for s in streams
           if s.name().startswith(('HR ', 'RR ', 'PP '))]

while True:
    for inlet in inlets:
        sample, ts = inlet.pull_sample(timeout=0.0)
        if sample is not None:
            print(f'{ts:.3f}  {inlet.info().name()}: {sample[0]}')

Hardware

Anything advertising the BLE Heart Rate Service should be discoverable; the protocol path the bridge takes is decided automatically from the first Heart Rate Measurement frame and (for PPG bands) the presence of Polar's vendor PMD service.

Architecture

Single-process Rust binary on top of btleplug (BLE) and a thin direct FFI to liblsl. Protocol selection is a small state machine; the LSL outlets are constructed lazily once the first frame settles which kind it is.

  +--------------+       +-----------+       +--------------+       +-------------+
  |  BLE band    |  -->  | btleplug  |  -->  | HeartRateClient|  --> | LslOutlet   |
  |  (HR / PMD)  |       | (Linux/   |       |  parse + state  |     | (HR + RR/PP)|
  |              |       |  macOS/   |       |  machine        |     |             |
  +--------------+       |  Win)     |       +-------+--------+      +------+------+
                         +-----------+               |                       |
                                                     v                       v
                                            +-----------------+      +----------------+
                                            | parse_hr (RR)   |      | LSL multicast  |
                                            | polar_pmd (PP)  |      | --> LabRecorder|
                                            +-----------------+      |    pylsl       |
                                                                     |    Lab.js etc. |
                                                                     +----------------+

   protocol selection (first 0x2A37 frame, then 3-frame decision window):
       RR bit set                 ----> StandardRr
       no RR bit, no PMD service  ----> HrOnly
       no RR bit, PMD present     ----> PolarPpi   [unsubscribe 0x2A37,
                                                    write START_PPI to PMD CP,
                                                    subscribe to PMD data]

Citing

If you use HRBand-LSL in published work, a citation back to the repository is appreciated. There is no associated paper; the preferred form is a software citation:

@software{hrband_lsl_2026,
  author  = {Barradas Chac{\'o}n, Luis Alberto},
  title   = {HRBand-LSL: BLE heart rate to Lab Streaming Layer},
  year    = {2026},
  version = {0.2.0},
  license = {MIT},
  url     = {https://github.com/abcsds/HRBand-LSL},
  orcid   = {0000-0001-6756-5485}
}

For mobile / Android-side acquisition with the same outlet contract, see the sibling project RRStreamer.