The SDK offline recording makes it possible to record data into Polar device internal memory. The recording continues even though the BLE connection is lost while recording.
To enable the offline recording the feature FEATURE_POLAR_OFFLINE_RECORDING must be opted in at the time of Polar BLE SDK instantiation. Once the BLE connection is created to the Polar device the bleSdkFeatureReady callback is called by the SDK with feature FEATURE_POLAR_OFFLINE_RECORDING indicating the offline recording functionality is available in Polar device and it is ready to be used. From this point onwards the offline recording API functions, exposed in the PolarOfflineRecordingApi interface, are available and usable.
To know which data types are supported in the particular Polar device, the getAvailableOfflineRecordingDataTypes function can used to check the availability.
The API PolarOfflineRecordingApi has the basic functions to start startOfflineRecording and stop stopOfflineRecording offline recording. Status query of API function getOfflineRecordingStatus indicates if device is already having active recordings on going.
The listing of recordings saved in device memory can be done with listOfflineRecordings. The listing returns all the entries (PolarOfflineRecordingEntry) in the device or empty list if no recordings found. Using the PolarOfflineRecordingEntry it is possible to read the record with the getOfflineRecord and delete it with removeOfflineRecord.
To know what are the capabilities of each PolarDeviceDataType in offline recording, the settings shall be queried by the requestOfflineRecordingSettings. The wanted settings are then provided as parameter for startOfflineRecording.
SDK Mode
The offline recording can be used in SDK mode. The SDK mode provides wider range of settings to be used, to know the available settings in SDK mode for offline recording the settings can be queried by the requestFullOfflineRecordingSettings.
Note
Usually devices don't provide as high sampling rates when doing offline recording as when doing streaming over BLE. The reason is that device internal memory is limited in size and could get filled very quickly, and it's also more power hungry to store data in non-volatile memory instead of streaming it over the BLE radio, which results in much shorter device lifetime.
Offline recording triggers
Triggers are the way to automatically start the offline recording. The options are TRIGGER_SYSTEM_START and TRIGGER_EXERCISE_START. In case the trigger is set to TRIGGER_SYSTEM_START the offline recording is started every time the Polar device is switched on, e.g. when VeritySense device power button is pressed by the user to turn device on. In case the trigger is set to TRIGER_EXERCISE_START the offline recording is started every time exercise is started in Polar device.
With the VeritySense TRIGGER_EXERCISE_START will trigger offline recording if either the exercise or swimming mode is started by the user of VeritySense.
The API setOfflineRecordingTrigger is used to setup the trigger. When the offline recording is automatically started by the trigger the recording will end in two conditions, either recording is stopped by stopOfflineRecording or user switch off the device. To disable the trigger the setOfflineRecordingTrigger function is called with the option TRIGGER_DISABLED
Security
The offline record saved in device memory can be encrypted by providing the 128bit AES key as parameter for startOfflineRecording request. If providing the key with the startOfflineRecording request, the exact same key must be provided in getOfflineRecord to be able to read out the offline record. Each of the started recoding may be started with different key. The key management is not provided by the SDK, but the application developer shall implement the key management. It is recommended the security option is used, otherwise it might be possible by others to read out recordings in VeritySense.
The device does not automatically erase offline recordings to make space for new ones. There are 2 different memory limits :
- Memory limit 1 : Once free space is below that limit, attemping to start a recording or enable any trigger will return
ERROR_DISK_FULLerror. - Memory limit 2 : Once free space is below that limit, all active offline recordings are automatically stopped and triggered recordings are disabled.
To anticipate this scenario, the disk space can be queried to the device by calling getDiskSpace function at any time.
| Device | From ver. onwards | Offline recording triggers | Security | Memory limit 1 | Memory limit 2 |
|---|---|---|---|---|---|
| Polar Verity Sense | 2.1.0 | Yes | Yes | 2 MB | 300 KB |
| Polar 360 | 1.0 | No | No | 2 MB | 2 MB |
The online streaming and offline recording do not work same time for the same data type. For example if either accelerometer offline recording or the accelerometer online streaming is started, the attempt to start the online streaming or offline recording at this state will return the error ERROR_ALREADY_IN_STATE.
The offline recording read getOfflineRecord and delete removeOfflineRecord can be called while the offline recording is recording, but that is not recommended. It is recommended to stop the offline recording by stopOfflineRecording before trying to access the recording with getOfflineRecord or removeOfflineRecord functions.