Push-pull

Push-pull refers to an optical recording format that was used to reduce noise and increase audio fidelity before the widespread adoption of magnetic recording. It utilized two audio tracks of the same recording printed side by side with one track 180 degrees out of phase from the other.

When playing back a push-pull optical, the two tracks are combined together in a mixing console, which causes phase cancellation, essentially rendering the audio silent. Then, using the "phase reverse" switch in the console, one of the tracks has its phase reversed, in turn bringing back the sound, but with less noise and better fidelity. The phase cancellation reduces the noise and the phase reversal increases fidelity, thus improving the signal-to-noise ratio by 10 to 20 percent. Push-pull tracks can be variable area or variable density optical sound tracks.

Once very common for recording large orchestral scores, push-pull tracks were also used to record M&E tracks. This was done to achieve better overall fidelity in the music and effects before mixing them with dialog for the final composite track. This was a high-budget form of recording and mixing prior to the days of magnetic recording; soundtracks for large films such as Gone With the Wind were recorded using this method.