On 2006-09-20 20:27:41 -0700, "Radium" said:

The ERPI system, Fox-Case's Movietone, and De Forest's Phonofilm use
variable-density recording film audio

But, no one makes variable density sound recoding film, nor do the labs
know how to process VD properly.

And, I'd bet nearly every WECo RA-1231 ever made has been converted
from its original VD mode to VA mode, and to Stereo Variable-Area, at
that.

RA-1231s are still in new production, and are pretty much as WECo
designed them in 1947, and with the valve as WECo designed in 1937, but
changed for SV-A.