🚀 go-pugleaf

I have a Zenith Space Commander 200, one of the first wireless TV remote
control units ever made (~1960).  It has 4 ultrasonic gongs inside, and
when you push one of the buttons, it emits a tone in the range 35-50 KHz
for a fraction of a second.

I want to build a receiver to decode these tones.  Any suggestions on what
to use as a receiving transducer?  I have an ultrasonic microphone of
unknown ancestry, but it seems to be too narrow band judging by the scope
trace I get out of it.  Also, what kind of circuit to decode the tones?
I could build four hi-Q tuned circuits out of LC or active filters, but
maybe some of you digital hacks could suggest some kind of one-chip
discriminator that would do the trick.  Maybe a modem chip?  I think I
could blunder my way through the design if I had some part numbers to
look up.

I want to retain the remote controller in its original form, so suggestions
involving replacing its innards with IR are not useful.  (I'm a TV collector,
and plan to use this with a 25" GE manufacturing prototype from 1966,
in case you're curious).