The filters invented by Espenschied were of too low a bandwidth
for voice use at a carrier frerquency of 60kHz. Warren Mason found a
solution in 1929 using a full lattice filter. To quote Kinsman:
"Mason expanded on this work in a landmark paper published in 1934."
and "... which was the basis for all crystal filter designs generated
in the next 20 years." The paper, 'Electric Wave Filters Employing
Quartz Crystals as Elements', was published in the Bell System
Technical Journal, Vol 13, July 1934.

73

John KC0G

Thank you for the expansion. I have Kinsman, his work on monolithics alone
are worth the price of the book.

Regards
W4ZCB