Develop Reference

BLE Heart Rate Senser Value Interpretation

I have an Android App where I get Heart Rate Measurements from a Polar H10 Device.
I'm totally lost on how to interpret the heart rate. Various links to the bluetooth.com site are resulting in 404 errors unfortunately.
The characteristics value is i.e.
[16, 59, 83, 4]
From what I understood the second byte (59) is the heart rate in BPM. But this does not seem to be decimal as the value goes up to 127 and then goes on -127, -126, -125, ... It is not hex either.
I tried (in kotlin)
characteristic.value[1].toUInt()
characteristic.value[1].toInt()
characteristic.value[1].toShort()
characteristic.value[1].toULong()
characteristic.value[1].toDouble()
All values freak out as soon as the -127 appears.
Do I have to convert the 59 to binary (59=111011) and see it in there? Please give me some insight.
### Edit (12th April 2021) ###
What I do to get those values is a BluetoothDevice.connectGatt().
Then hold the GATT.
In order to get heart rate values I look for
Service 0x180d and its
characteristic 0x2a37 and its only
descriptor 0x2902.
Then I enable notifications by setting 0x01 on the descriptor. I then get ongoing events in the GattClientCallback.onCharacteristicChanged() callback. I will add a screenshot below with all data.
From what I understood the response should be 6 bytes long instead of 4, right? What am I doing wrong?
On the picture you see the characteristic on the very top. It is linked to the service 180d and the characteristic holds the value with 4 bytes on the bottom.

See Heart Rate Value in BLE for the links to the documents. As in that answer, here's the decode:
Byte 0 - Flags: 16 (0001 0000)
Bits are numbered from LSB (0) to MSB (7).
Bit 0 - Heart Rate Value Format: 0 => UINT8 beats per minute
Bit 1-2 - Sensor Contact Status: 00 => Not supported or detected
Bit 3 - Energy Expended Status: 0 => No Present
Bit 4 - RR-Interval: 1 => One or more values are present
So the first byte is a heart rate in UInt8 format, and the next two bytes are an RR interval.
To read this in Kotlin:
characteristic.getIntValue(FORMAT_UINT8, 1)
This return a heart rate of 56 bpm.
And ignore the other two bytes unless you want the RR.

It seems I found a way by retrieving the value as follows
val hearRateDecimal = characteristic.getIntValue(BluetoothGattCharacteristic.FORMAT_UINT8, 1)
2 things are important
first - the format of UINT8 (although I don't know when to use UINT8 and when UINT16. Actually I thought I need to use UINT16 as the first byte is actually 16 (see the question above)
second - the offset parameter 1
What I now get is an Integer even beyond 127 -> 127, 128, 129, 130, ...

Cross Platform Random Reproducibility

I need to use a random function, but also have it repeating on different devices (PC / iOS / Android).
I'm running this sample code, to shuffle a vector:
#include <algorithm>
#include <iostream>
#include <iterator>
#include <random>
#include <vector>
int main() {
std::mt19937 generator(1337);
std::cout << "Your seed produced: " << generator() << std::endl;
std::vector<int> v = { 1, 2, 3, 4, 5, 6, 7, 8, 9, 10 };
std::shuffle(v.begin(), v.end(), generator);
std::copy(v.begin(), v.end(), std::ostream_iterator<int>(std::cout, " "));
std::cout << "\n";
return 0;
}
Output from two different PCs (windows):
Your seed produced: 1125387415
10 6 8 1 7 2 4 3 5 9
Output from iOS:
Your seed produced: 1125387415
9 1 4 6 7 8 5 3 10 2
Why am I getting different results?
Is there another dependency relating to the OS itself?
How is it possible to get this to work cross-platform?

std::mt19937 is rigorously defined by the standard and has no room for platform specific/implementation defined behaviour, your problem doesn't lie here.
The problem is with std::shuffle, which in no way says how it is supposed to use the random number generator, just that is has to use it.
Which unfortunately means, if you want a reproducible shuffling behaviour, you might need to implement your own.

The std::shuffle function’s third argument changed. It was a function object that should return a value in the range of [0,n). That means you could pass a std::uniform_int_distribution. Now it takes a random bit source like std::mt19937. Which does your library expect?

Android studio double to date format

I'm making a project on android studio.
I must convert a number into an hour, minutes and seconds date.
The number is: 0,18865386
The result must be: 04:31:40
I've obtained this date whit excel (used for test calc) just changing the cell format to HOURS but i don't understand how to calculate it.
Any solutions?

1 corresponds to 24 hours.
In hours: 0,18865386 means 24 x 0,18865386 ~= 4,5 hours which corresponds to your 04:31:40.
If you need more precision, e.g. in seconds:
The number of seconds in a day = 24 x 60 x 60 = 86400.
Then 0,18865386 means 86400 x 0,18865386 = 16300 seconds = 04:31:40.

Synchronize Dataset - Multiple users, multiple timestamps

I'm trying to understand what is the best way to go about doing this:
Basically I will have a bunch of Android/iOS users who will be logging sensor data on their phone, except each user will have its own timestamp depending on when people start the app etc. So say I have two users:
User 1: (5,45), (6,34), (8,32)
User 2: (5,35), (7,32), (9,32)
The format is (time t, and some arbitrary value).
What would be the best way to synchronize the two datasets? Do I have to write an algorithm which is then going to go back and say on User 2, insert the following entry (6,35). So eventually the new data looks like:
User 1: (5,45), (6,34), (7,34), (8,32), (9,32)
User 2: (5,35), (6,35), (7,32), (8,32), (9,32)
This could be very data intensive though, because I am expecting to have about 300 users, and each will have about 36,000 data entries. Any advice would be appreciated - also something I could do on the app side that could help that would be appreciated.
I believe I am going to have to do something like this when I have all my data - but since this is an actively developing project I thought I'd get some advice first.

You could create a DataFrame from each user's data. Then store them all in a Panel. Pandas will line them all up based on the timestamp and you can use the forward fill method, ffill, to propagate the values:
In [62]: df1 = DataFrame([45,34,32], index=[5,6,8], columns=['value'])
In [63]: df2 = DataFrame([35,32,32], index=[5,7,9], columns=['value'])
In [64]: p = Panel({'user1': df1, 'user2': df2})
In [75]: p.ffill().to_frame().unstack()
Out[75]:
user1 user2
minor value value
major
5 45 35
6 34 35
7 34 32
8 32 32
9 32 32
Or, you could do the same thing using just Series and DataFrames. I guess it depends on what you want to do with it:
In [78]: s1 = Series([45,34,32], index=[5,6,8])
In [79]: s2 = Series([35,32,32], index=[5,7,9])
In [80]: df = DataFrame([s1,s2])
In [81]: df
Out[81]:
5 6 7 8 9
0 45 34 NaN 32 NaN
1 35 NaN 32 NaN 32
In [82]: df = DataFrame([s1,s2]).T
In [83]: df
Out[83]:
0 1
5 45 35
6 34 NaN
7 NaN 32
8 32 NaN
9 NaN 32
In [84]: df.ffill()
Out[84]:
0 1
5 45 35
6 34 35
7 34 32
8 32 32
9 32 32

Polar Wearlink Bluetooth packet

i am looking at the code of a project called MyTracks:
http://code.google.com/r/jrgert-polar-bluetooth/source/browse/MyTracks/src/com/google/android/apps/mytracks/services/sensors/PolarMessageParser.java?r=ebc01faf49550bc9801633ff38bb3b8ddd6f5698
Now I am having problems with the method isValid(byte[] buffer). I don´t understand what exactly is he checking here. We want to know if the first byte in the array is the header containing 0xFE. I don´t quite understand the following lines :
boolean goodHdr = ((buffer[0] & 0xFF) == 0xFE);
boolean goodChk = ((buffer[2] & 0xFF) == (0xFF - (buffer[1] & 0xFF)));
return goodHdr && goodChk;
any ideas?

Ewoks is correct, refer to this blog post:
http://ww.telent.net/2012/5/3/listening_to_a_polar_bluetooth_hrm_in_linux
"Digging into src/com/google/android/apps/mytracks/services/sensors/PolarMessageParser.java we find a helpful comment revealing that, notwithstanding Polar's ridiculous stance on giving out development info (they don't, is the summary) the Wearlink packet format is actually quite simple.
Polar Bluetooth Wearlink packet example
Hdr - Len - Chk - Seq - Status - HeartRate - RRInterval_16-bits
FE - 08 - F7 - 06 - F1 - 48 - 03 64
where
Hdr always = 254 (0xFE),
Chk = 255 - Len
Seq range 0 to 15
Status = Upper nibble may be battery voltage
bit 0 is Beat Detection flag."

&0xff simply converts signed byte to unsigned int for doing the comparison

First line is checking is received buffer are starting with 0xFE as it should be with this Polar Wearable.
Second line is checking if length byte is correct as well because it's value by specification is 255-value writen is size byte..
This together is super simple verification that messages are correct (more complicated implementation would include CRC or other verification methods). cheers