RaceCapture/Pro MK2: RPM noise filtering

If you’re running RaceCapture/Pro MK2 and you’re experiencing noise in the form of high RPM spikes, try the beta firmware we just made available that just might solve your problem.

Where it should help

  • If you’re using CoilX tapping directly into your ignition coil, and you’re still seeing periodic spikes due to extra noisy ignition systems
  • If you’re tapping into low voltage ECU tachometer signal that should be a clean square wave, but noise on that line triggers high RPM spikes

How it works

Ideally, the RPM signal should be a clean square wave between 5 and 12v. RaceCapture/Pro MK2 has built in Schmitt Trigger buffers that will clean up a signal – to a point.  In severe environments, noise can exist in the signal which shows up as false triggers and subsequent RPM spikes / glitches.  Typically, this noise occurs shows up as a ringing signal following the actual pulse – often seen as a component of the ignition pulse flyback.

The RPM firmware in this beta release works by triggering on the falling edge of the signal, and ignores noisy signals for a period of 4.5 ms after the primary pulse triggers.  Why 4.5 ms? When measuring RPM from an engine, this is roughly the right duration to handle an RPM signal ranging from 0 to about 15K RPM.

noise_filtering2
Note, this filtering is only enabled for RPM mode: If Frequency or Period sampling mode is selected the filter will not be active.

Limitations

If you have an inverted signal that should trigger on the rising edge then this RPM filter patch won’t help.  Also, if you have severe noise that extends past the 4.5 ms range then your results may also be sub-optimal.
RPM_noise_filtering_rising_edge_falling
For firmware 2.9.0 we will add additional configuration options to select rising or falling edge triggering in addition to configuring the duration of the noise filter outside of the default 4.5 ms range. For this beta version (and upcoming 2.8.8), we selected the configuration we felt would benefit most scenarios.

Test Jig

Gotta test, right? Using our Ford EDIS crank-fired ignition test platform we tapped into one of the primary windings from the coil pack, feeding it right into the RaceCapture/Pro timer input. It’s not the connection we recommend for measuring RPM, but it does provide an ultra noisy signal which can be viewed as the acid test for our new filtering algorithm.
RPM_filtering_test_jig

Like many test setups, it looks like a hot mess – but it does the job

Using the Beta firmware

Assuming you have RPM already connected, all you have to do is apply the new firmware and monitor your engine’s RPM – no need to update any additional settings, providing you were getting correct RPM readings aside from the noise. In case you need to double check, use the following screen shot to aid your configuration. Ensure your channel mode is RPM, Timer Speed is Medium and Pulse Per Revolution is set correctly (usually it’s 1,2 or 3 depending on the number of cylinders and coils)

RPM_settings
It’s always good to back up your configuration, even though your config will be preserved if you’re running the latest 2.8.x release.

Testing!

We’ve tested this beta firmware with good results, even when using the extremely noisy signal from our ignition test jig. Now it’s your turn to try out this firmware! We’d really like to hear if it makes an improvement – or more importantly, if you’re still seeing glitches. Let us know!