[Richard Crowley]

"dE|_" wrote ...
A more cut/fade tamper resistant method of watermarking might be to record
through a slow flanger. Any interruptions would stick out like a sore
thumb then surely? I still like John's idea of parallel analogue tapes,
these can be compared to the flanging & checksummed digital media and be
used in court.

Mechanical (not magnetic) recordings like those old dictation machine loops
were once considered pretty tamper-proof. But not practical these days. I
would consider running a flash recorder and immediately handing the SD card
(or whatever) to the attorney to seal as evidence before leaving the room.

I'm confident that spending some quality time with Google would reveal
what are considered legally acceptable forms of recording these days.
Seems quite likely that any DIY, home-made scheme would never be
deemed admissible.