The SI prefixes allow us to turn big numbers with lots of zeroes into small numbers with less zeroes, making them easier to write, read and comprehend. For example, we’re all familiar with k for kilo, which means ‘multiply by 1000’. So 1kg means 1 x 1000 = 1000 grams, 2km means 2 x 1000 = 2000 metres, and 10kHz means 10 x 1000 = 10,000 Hertz. Thanks to the proliferation of computers and mobile devices, we’re also familiar with M for Mega which means ‘multiply by 1,000,000’, G for Giga which means ‘multiply by 1,000,000,000’, and T for Tera which means ‘multiply by 1,000,000,000,000’. If your mobile device has 64GB of memory, it has 64 Gigabytes of memory, or 64 x 1,000,000,000 = 64,000,000,000 bytes of memory.
Similarly, there are SI prefixes for small numbers with lots of zeroes. There is m for milli which means divide by 1000, u for micro which means divide by 1,000,000, n for nano which means divide by 1,000,000,000, and p for pico whch means divide by 1,000,000,000,000.
To understand comb filtering we need to deal with small increments of time, typically measured in thousandths of a second. To simplify those numbers we’ll make use of the SI prefixes m and u, giving us milliseconds (abbreviated to ms and representing thousandths of a second) and microseconds (abbreviated to us and representing millionths of a second). For example, 1ms = 1 millisecond = 1/1000s = 0.001s, and 1us = 1 microsecond = 1/1,000,000s = 0.000001s.
Earlier in this instalment we saw that the frequency of 440Hz has a period of 0.00227 seconds. We can simplify this figure by multiplying it by 1000 to turn it into milliseconds. So, 0.00227s x 1000 = 2.27ms. Conversely, to convert it back to seconds we divide it by 1000, so 2.27ms/1000 = 0.00227s.
ms, m/s and MS confusion
Please be careful not to confuse ms with m/s or MS. It’s important to remember the differences, and understand that ms represents milliseconds and is a measurement of time, m/s represents metres per second and is a measurement of velocity, and MS represents Mid/Side and is a stereo microphone technique and signal processing technique.
A quick hack to convert seconds (s) into milliseconds (ms) is to move the decimal point three places to the right. To convert milliseconds back into seconds move the decimal point three places to the left. Likewise, to convert seconds (s) into microseconds (us) move the decimal point six places to the right, and to convert microseconds back into seconds move the decimal point six places to the left. So 0.001s becomes 1ms, and 0.000001s becomes 1us. Conversely, 1ms becomes 0.001s, and 1us becomes 0.000001s. Interestingly, a figure such 0.00227 seconds (the period for 440Hz) can be expressed as 2.27ms or 2,270us. In both cases, moving the decimal point the appropriate number of places to the left brings it back to 0.00227 seconds. Check out the tables below for further examples…