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Forums > C64 Coding > Accurately Measuring Drive RPM
2020-08-03 16:07
chatGPZ

Registered: Dec 2001
Posts: 11378		 Accurately Measuring Drive RPM

To bring the discussion from 1541 Speed Test into the forum....

first lets recapitulate:

The general idea is: have a "marker" on a track, then measure the time for one revolution using timers. Generally there are different ways to achieve this:

- wait for the marker and toggle a IEC line. the C64 measures the time using CIA timer. this is what eg the well known "Kwik Load" copy does, the problem is that it is PAL/NTSC specific, and it can never be 100% exact due to the timing drift between drive and C64.

- wait for the marker and measure the time using VIA timers on the drive. the problem with this is that VIA timers are only 16bit and can not be cascaded, so you either have to measure smaller portions at a time, or rely on the wraparound and the value being in certain bounds at the time you read it.

now, to make either way slightly more accurate, a special kind of reference track can be used. typically this track will contain nothing except one marker - which makes the code a bit simpler and straightforward. this is what 1541 Speed Test does. the DOS also does something similar when formatting, to calculate the gaps. This obviosly has the problem that we are overwriting said track.

Now - the question isn't how to do all this, that's a solved problem. The question is, given a specific implementation, how *accurate* is it actually, and why?

The basic math to calculate the RPM is this:

expected ideal:
300 rounds per minute
= 5 rounds per second
= 200 milliseconds per round
at 1MHz (0,001 milliseconds per clock)
= 200000 cycles per round

to calculate RPM from cycles per round:
RPM = (200000 * 300) / cycles

two little test programs are here: https://sourceforge.net/p/vice-emu/code/HEAD/tree/testprogs/dri.. ... the first reads timer values between each sector header and then the total time for a revolution is accumulated from the delta times. the second leaves the timer running for one revolution and then indirectly gets the time for a revolution from that. to my own surprise, both appear to be accurate down to 3 cycles (in theory the second one should be more accurate, at least thats what i thought. i also expected some more jitter than just 3 cycles)

1541 Speed Test writes a track that contains one long sync, and then 5 regular bytes which serve as the marker. it then reads 6 bytes and measures the time that takes, which equals one revolution. somehow this produces a stable value without any jitter, which was a bit surprising to me too (i expected at least one cycle jitter, due to the sync waiting loops) (i am waiting for the source release and will put a derived test into the vice repo too)

So, again, the question is... how accurate are those and why? (a stable value alone does not tell its accurate). Some details are not quite clear to me, eg if we are writing a reference track, how much will that affect the accuracy of the following measurement? how will the result change when the reference track was written at a different speed than when doing the measuring? Will using a certain speedzone make it more or less accurate?

Bonus question: can we use https://en.wikipedia.org/wiki/Chinese_remainder_theorem with two VIA timers to make this more accurate? or is it a pointless exercise?
 
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2020-08-24 01:46
Zibri
Account closed

Registered: May 2020
Posts: 304		 Update:

Thanks to Groepaz who recently modified VICE (r38422) giving a bigger (way too big) speed range to the 1541 emulation,
now my program is not only the most accurate but the only one (or one of the very few) that supports drives at very high or very low speed settings. And still keeping maximum accuracy!

Go download the latest version as usual at: https://github.com/Zibri/C64-1541-Speed-Test

All major formats are available, PRG, D64, TAP and even WAV.

Note: in the D64 is also present a configuration program (CONFIGURE.PRG) that creates a customized version.

Regards,
Zibri

Note:
Big thanks to the downvoter who voted 3 this useful program out of misidrected anger from his life failures.
I know exactly who you are.
And no, it was not Groepaz, we bicker online but I think we respect each other but I might be wrong.
2020-08-30 18:00
chatGPZ

Registered: Dec 2001
Posts: 11378		 Quote:
As for the presentation, I still think some kind of plot would be useful for quick adjustment.

I have implemented this now, see the *plot.prg variants of the testprograms: https://sourceforge.net/p/vice-emu/code/HEAD/tree/testprogs/dri..

It would be cool if some people could run them on their setups and post the results (screenshots). Please tell exactly what kind of drive it is that you are using :)

Edit: when you run the program, it will take a second or two to calibrate (so it can align the graph to the middle of the screen, more or less). Then you can increase the resolution (decrease the divisor) for the timer value using F7,F5,F3,F1. Use the smallest divisor (widest plot) that still fits on Y. And dont forget to mention what the F-Key was that you pressed :)

Edit++: while you are at it - it might be a good time to calibrate the RPM of your drive to roughly 300RPM :)
2023-03-16 13:57
Mr. Mouse

Registered: Dec 2001
Posts: 235		 Mr Tester &D Neil Chriss already released a speed test in 1984 in his Mr Tester. A program of extraordinary magnitude, I must add.
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