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google / walt

A device for measuring latency of physical sensors and outputs on phones and computers
Apache License 2.0
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tap latency need accelerometer #120

Open AlphaZhang1990 opened 2 years ago

AlphaZhang1990 commented 2 years ago

Now tap latency need accelerometer. If the tap speed is different, the result will different. Is there any way to test tap latency not use accelerometer?

adlr commented 2 years ago

Sorry, touch up and touch down latency need the accelerometer.

kamrik commented 2 years ago

Yes, as Andrew mentioned, tap latency measurement needs the accelerometer. It is used to detect when the probe physically touches the screen. We experimented with other methods, but those were way less accurate.

The result should not depend much on speed, I would expect the variability from speed to be on the order of 1 ms. One way to reduce it is to always do a fairly quick and firm tap.

AlphaZhang1990 commented 2 years ago

Can I know what is the sample rate of the accelerometer?

adlr commented 2 years ago

Mark may know for sure. Looking at the code, the accelerometer is probed in a tight loop without fixed frequency. Thus I would expect it's thousands of times per second. Note that this is an educated guess, not confirmed.

kamrik commented 2 years ago

It should be at least several kHz. The accelerometer output is analog and as Andrew mentioned, the microcontroller samples it in a tight loop with no sleep. The ADC on Teensy is known to handle 44.1 kHz audio sampling well, so I would expect it to be in the tens of kHz. But I don't remember ever measuring the actual rate.

AlphaZhang1990 commented 2 years ago

OK，I will try quick and firm tap to reduce the result change.