What detector or generation of detectors am I describing?

[Detectorist#1]

Hi Altra,
Well, the idea of SD2000 is very good for Australia's places and for these years - absolutely true! Now, we try to make MDs for different places with different solutions. No universal solution as we all know!

[Altra]

The other patent US5537041 describes BBS - ML Sovereign, Etrac and CTX3030. These operate on continuous Tx current flow, while the SD is discontinuous Tx

[Detectorist#1]

Hi Altra,
Yes, this patents are still valid and what is the problem? - what new we find now days?

[Altra]

Those patents are expired, so no problem!. When I said "valid" I meant the concept or idea is still good after all these years.

[detectormods]

My description is the initial Minelab design but the way it works is much the same as all P.i designs over the years with technology based improvements. Yes 30 years ago.