I'm owning a Polar H10 device and I'm interested in the heart rate as well as RR-interval which I read out with the official bluetooth low energy API of Android. The Polar device sends every every second a package with the heart rate and the RR-interval. Now I have recognized that in every such package is a heart rate value but in some packages there are no RR-interval values (The value of the RR-interval is -1).
Why does this happen? Is my device broken or did I made a mistake in the implementation or does somebody else also face this issue?
Edit: Here is the code. In the method public void onCharacteristicChanged(BluetoothGatt gatt, BluetoothGattCharacteristic characteristic) I'm receiving changed values from the Polar Device. This method is triggered approximately every second. Then I parse the characteristic as follows:
public int[] parse(BluetoothGattCharacteristic characteristic) {
double heartRate = extractHeartRate(c);
Integer[] interval = extractBeatToBeatInterval(c);
int[] result = null;
if (interval != null) {
result = new int[interval.length + 1];
} else {
result = new int[2];
result[1] = -1;
}
result[0] = (int) heartRate;
if (interval != null) {
for (int i = 0; i < interval.length; i++) {
result[i+1] = interval[i];
}
}
return result;
}
private static double extractHeartRate(
BluetoothGattCharacteristic characteristic) {
int flag = characteristic.getProperties();
Log.d(TAG, "Heart rate flag: " + flag);
int format = -1;
// Heart rate bit number format
if ((flag & 0x01) != 0) {
format = BluetoothGattCharacteristic.FORMAT_UINT16;
Log.d(TAG, "Heart rate format UINT16.");
} else {
format = BluetoothGattCharacteristic.FORMAT_UINT8;
Log.d(TAG, "Heart rate format UINT8.");
}
final int heartRate = characteristic.getIntValue(format, 1);
Log.d(TAG, String.format("Received heart rate: %d", heartRate));
return heartRate;
}
private static Integer[] extractBeatToBeatInterval(
BluetoothGattCharacteristic characteristic) {
int flag = characteristic.getIntValue(BluetoothGattCharacteristic.FORMAT_UINT8, 0);
int format = -1;
int energy = -1;
int offset = 1; // This depends on hear rate value format and if there is energy data
int rr_count = 0;
if ((flag & 0x01) != 0) {
format = BluetoothGattCharacteristic.FORMAT_UINT16;
Log.d(TAG, "Heart rate format UINT16.");
offset = 3;
} else {
format = BluetoothGattCharacteristic.FORMAT_UINT8;
Log.d(TAG, "Heart rate format UINT8.");
offset = 2;
}
if ((flag & 0x08) != 0) {
// calories present
energy = characteristic.getIntValue(BluetoothGattCharacteristic.FORMAT_UINT16, offset);
offset += 2;
Log.d(TAG, "Received energy: {}"+ energy);
}
if ((flag & 0x16) != 0){
// RR stuff.
Log.d(TAG, "RR stuff found at offset: "+ offset);
Log.d(TAG, "RR length: "+ (characteristic.getValue()).length);
rr_count = ((characteristic.getValue()).length - offset) / 2;
Log.d(TAG, "RR length: "+ (characteristic.getValue()).length);
Log.d(TAG, "rr_count: "+ rr_count);
if (rr_count > 0) {
Integer[] mRr_values = new Integer[rr_count];
for (int i = 0; i < rr_count; i++) {
mRr_values[i] = characteristic.getIntValue(
BluetoothGattCharacteristic.FORMAT_UINT16, offset);
offset += 2;
Log.d(TAG, "Received RR: " + mRr_values[i]);
}
return mRr_values;
}
}
Log.d(TAG, "No RR data on this update: ");
return null;
}
The first element returned by the parse method is the heart rate and the second element is the RR-interval. It happens that sometimes the second element is -1 (i.e. no RR-interval detected).
There is nothing wrong with your Polar device or the software you posted.
The RR-interval measure may be missing from some transmitted packets and the if ((flag & 0x16) != 0) accounts for this case.
Suppose for example that your device send a heart measure every second and you have 50 beats/sec: there will be some intervals where the RR interval is not measured because there isnt a detected beat in that second (it is a simplified explanation, just to get the point).