Query on crackpot scheme for recording audio with extended bandwidth

[Electronic M]

You may be able to do this with SDR (software defined radio) many SDRs can record the continuous RF signal and replay and low power retransmit the RF signal. You have to pick your ADC/DAC carefully if you want post detector recording, or buy a mixer/oscillator to heterodyne the signal between it's Actual frequency and a frequency the DAC/ADC can process.

It's also possible on some high end reel tape recorders to record up to 100khz IIRC.