The Challenge of the Push-Pull Track
Today’s guest blogger is Heidi Holmstrom. Heidi works in the Motion Picture Preservation Lab, which is responsible for performing conservation and preservation work on motion picture records held across the National Archives.
There are many sound systems that have been used for motion picture films over the years. Some of the earliest relied on sound recorded to a disc or cylinder that had to be played back in sync with the film. Even after optical soundtracks became the industry standard, there were multiple optical systems available. Soundtracks recorded as variable area or variable density are easy to play back with standard equipment and fortunately most soundtracks we see are one of these two types. However, NARA also has a good number of films with push-pull soundtracks. They are most often associated with newsfilm from the 1930s. At NARA, we are most likely to see them in donated news footage, such as Motion Picture Films of March of Time Outtake Footage, compiled 1935 – 1951 (ARC Identifier 97891 / Local Identifier NWDNM(M)-MT-MTT).
Push-pull tracks cannot be read by standard equipment because the sound information contained on the two halves of the soundtrack must be processed and combined together. If the processing is not done, you can easily mistake the audio for a monologue by one of the teachers in a Charlie Brown cartoon.
NARA’s Motion Picture Preservation Lab recently installed a new system for reading optical sound that will finally allow us to decode push-pull soundtracks in house. A camera records a high-resolution image of the soundtrack that is then converted into audio information by software on a digital audio workstation. The software can isolate the two halves of the soundtrack image and process them together to output clear and undistorted audio. If preservation of the film original is required, the audio can be recorded optically onto stable polyester film stock.
Listen to our Push-Pull test to hear the difference!