I am writing an app that logs the accelerations of the mobile phone every 40ms (at 25Hz). This frame rate can be held on average, but sometimes I am experiencing delays of 5'000ms - 50'000ms between timeframes. I am wondering why this happens.
Here you have a graph of the delays where you can see that they occur quite often:
Here's what I am doing (which might be bad):
- The activity points to a accelerometer logger class (singleton, pure java, no android class extensions).
- The accelerometer logger singleton continues to log in the background.
- The accelerometer logger saves every log directly to the sqlite db.
- I am also logging GPS data in the background.
- The DAO (Data Access Object) assigns every Log to a LinkedBlockingQueue and saves them in a separate thread.
Here's what I think might be the problem:
- Maybe i have to implement further lifecycle methods, or extend a specific android class, so that the accererometer logger gains priority (or just set the priority somewhere).
- I might use the
event.timestamp
instead ofSystem.currentTimeMills()
. (I would prefer not to do this, as some sensors have different timezones, thats why i useSystem.currentTimeMillis()
, but if necessary I'd switch.)
Do you have any experience with this or suggestions where the problem could probably lie?
Here's my code:
@SuppressLint("NewApi")
public class AccelerometerLogger implements SensorEventListener {
private static AccelerometerLogger singleton = new AccelerometerLogger();
private LoggerDao loggerDao;
private SensorManager sensorManager;
private Sensor accelerometer;
private double acceleorometerRate = 25; // Hz
int accelerometerDelayMicroseconds = (int) (Math.round(((1/this.acceleorometerRate)*1000000.0)));
private AccelerometerLogger()
{
this.loggerDao = LoggerDao.getInstance();
}
public static AccelerometerLogger getInstance()
{
return singleton;
}
public void start(Context context)
{
this.sensorManager = (SensorManager) context.getSystemService(Context.SENSOR_SERVICE);
this.accelerometer = this.sensorManager.getDefaultSensor(Sensor.TYPE_ACCELEROMETER);
int accelerometerMinDelay = this.accelerometer.getMinDelay();
//Log.d("lggr-r", "desired delay: "+this.accelerometerDelayMicroseconds+" microseconds");
//Log.d("lggr-r", "provided min delay: "+accelerometerMinDelay+" microseconds");
if(accelerometerMinDelay < this.accelerometerDelayMicroseconds)
{
this.sensorManager.registerListener(this, this.accelerometer, this.accelerometerDelayMicroseconds);
//Log.d("lggr-r", "listener registered for desired rate: "+this.acceleorometerRate+"Hz (delay of "+this.accelerometerDelayMicroseconds+" microseconds).");
}
else if(accelerometerMinDelay==0)
{
this.sensorManager.registerListener(this, this.accelerometer, SensorManager.SENSOR_DELAY_FASTEST);
// Log.d("lggr-r", "listener registered for streaming api. only changes will be notified (interrupt).");
}
else
{
int providedRate = (int) Math.round(1 / (accelerometerMinDelay / 1000000.0));
this.sensorManager.registerListener(this, this.accelerometer, SensorManager.SENSOR_DELAY_FASTEST);
// Log.d("lggr-r", "can't read at the desired rate ("+this.acceleorometerRate+"Hz), app will read at "+providedRate+"Hz instead (delay of "+accelerometerMinDelay+" microseconds).");
}
}
public void stop()
{
this.sensorManager.unregisterListener(this);
}
@Override
public void onAccuracyChanged(Sensor sensor, int accuracy)
{
// String name = sensor.getName();
// Log.d("lggr", "the accurracy of "+name+" changed to "+accuracy+".");
}
@Override
public void onSensorChanged(SensorEvent event)
{
// lazy load loggerDao (TODO: fix all of those)
if(this.loggerDao == null)
{
this.loggerDao = LoggerDao.getInstance();
}
String values = "";
for(float value : event.values) values += value+",";
values = values.substring(0,values.length()-2);
// long timestamp = System.currentTimeMillis();
// Log.d("lggr", "acc = {time:"+timestamp+", data: ["+values+"]}");
AccelerometerSample accelerometerSample = new AccelerometerSample();
accelerometerSample.setTimestamp(System.currentTimeMillis());
accelerometerSample.setValues(event.values);
this.loggerDao.save(accelerometerSample);
}
}
Apparently the problem only happens on the Samsung Galaxy SIII mini. I've tested it with a Samsung Galaxy SII (custom ROM) and the delays were always about 0.04s (ranging between 0.005 and 0.12s - much better).
Do you have any suggestions why this happens on the Samsung Galaxy SIII mini?
UPDATE:
Ben Voigts answer which purposed to use the event.timestamp
has improved the delays significantly. Still, i am experiencing sometimes some longer delays. Do you know how I can further improve them?