is turned on to increase the dynamic range of the main program by 8 dB. Track-3 (Center Front) is applied to the Sensurround system for program sounds. No other signal is recorded on Tracks 2 and 4. 70mm Sensurround prints were issued only outside of the US. In addition, Earthquake was the only Sensurround film recorded in stereo or issued with 70mm prints. All other Sensurround films were 35mm mono optical in the Sensurround Mod-II or Mod-III format. Sensurround Mod-II Even before Earthquake ended its theatrical run, MCA began a program to re-engineer the Sensurround system to enable the recording of the deep bass on a standard 35mm optical soundtrack, with no external rumble generator being required, as well as improve fidelity and the overall effect and to simplify the system. In addition, dbx Type-II noise reduction was incorporated to increase the dynamic range and reduce audible noise created by the optical process. MCA also changed the way the control tones were used, allowing the Sensurround horns in the front and back of the theater to be controlled independently. This allowed for more creative effects, such as a sound being panned from front to back of the theater. Re-engineering the system was important to MCA because there were times during dialogue sequences when the sound mixers wanted the Sensurround running but did not want the dialogue to shift to the back of the theater. Having separate control of the front and back groups of Sensurround horns allowed this. The drivers in the Sensurround horns were improved to extend their frequency response higher so they could reproduce the low notes in music, thus allowing sound mixers to incorporate music into the Sensurround system (this was used to good effect in Rollercoaster.) Advancements by RCA in optical soundtrack recording led MCA engineers to the realization that an optical track could reliably record and reproduce bass down to 10 Hz. As well, removal of the Academy equalization would allow the high frequency response to be extended upward by an additional octave. The addition of dbx Type-II noise reduction and its attendant increase in useful dynamic range created a high-fidelity sound from a conventional optical print. MCA called this re-engineering of the system "Sensurround Mod-II" and the market name of the system was changed from just "Sensurround" to the "Sensurround Special Effects System". Unlike the original version of Sensurround, Mod-II (and later, Mod-III) were