When did The Movie Industry Switch To Magnetically recorded soundtracks?

When you see an old movie on TV (from the 1930’s), you frequently hear a great deal of noise on the sound track, and often a 60 Hz hum as well. I understand that befor the 1950’s, movie sound tracks were optically encoded on the side of the film…this was later replaced by a magnetic tape. Can these old soundtracks be cleaned up? And, when the transfer to DVD is done, what process isused to transfer the old optical sound track?

The audio (if it hasn’t changed since film school) is recorded on magnetic tape but encoded on the optical track for the final printing.

Remember that stock, mics, and recording devices…as well as now commom practice of grounding (60cycle hum)…have all made significant advances.