# How to downsample my data readings from 0.5hz to 0.001667hz. Do I filter and downsample?

I have a device that, in the lab, takes voltage readings every 2 seconds. However in real-life application the device would only wake every 10 minutes and then take a 100 readings whilst awake. How do i change my collected data to mirror real-life application? Only then can I interpret if my device is working as it should. I see from reading various posts that downsampling alone is not appropriate.

• So, to get this straight: you get 100 values with 2s in between, then you go to sleep for 10 min - 200s = 400s, then you continue? Sounds like you really just need to throw away data. The question is whether that simulated or the areal-world collected data then still is useful, and that can't be answered without you telling us what kind of signal that voltage signal is. – Marcus Müller Nov 9 '19 at 14:33
• Yes Marcus you are correct in identifying the sleep/reading pattern. The values coming into Realterm are discrete values. So yes I need to keep 100 values, dump the next 300 values, keep 100 values, dump 300 .....I had thought this was downsampling but I wasnt sure how to keep 100 values rather than just 1, which is what I would get with a decimating factor. – Suzanne O'Callaghan Nov 9 '19 at 20:08
• Well it sounds like you have two ranges of time-frequency information-- You have high rate information with a sampling period of 2 seconds, and then you have low rate information with a sampling period of 10 minutes. As for the low rate information, you can simply average the 100 samples you get to get the one sample for each 10 minute period. As for the high rate, each capture will indicate what occured over that time interval (you could average the powers easily enough-- anything else in the very low freq range would be revealed in the avged one sample from each over all averaged samples). – Dan Boschen Nov 10 '19 at 5:39
• (The averaging and then keeping the one sample is the filter and downsample) – Dan Boschen Nov 10 '19 at 5:40
• Thanks Dan. So I filter first and downsample afterwards. What type of filter I should use in MATLAB? There are so many parameters linked to each filter description that I am lost wrt to knowing which to apply to my data. – Suzanne O'Callaghan Nov 10 '19 at 9:55