An idea could be a retriggerable timer, eg, MC14538/CD4538.
Set it to 1.0 s (not S, that's siemens).
If you have the fast pulse train (0.55 s), it won't time out, and the output will stay stable.
With 1.6 s pulse train, it will time out, and you'll have 0.6 s long pulses with a 1.6 s period on the output.
How you want to process this further is a separate question.