How to add sound to amateur films (1954)

Part Erasing When you record on a previously used tape, the erase head is normally energised to remove the earlier recording. If you disconnect the erase head, or somehow arrange that the tape does not come in contact with it, the original recording is partly, but not completely removed. The partial erasure is produced by the supersonic bias (p. 65) in the record/playback head. It agitates the magnetic particles just as it would in the erase head, but it does not act long enough or strongly enough to effect complete erasure. One method of mixing sounds on tape makes use of this principle. First of all record one sound, music for example. Then rewind and play back the tape until you reach the point at which you are going to add the second recording, perhaps a passage of commentary. At this stage, place a slip of paper over the gap in the erase head and fix it there with self-adhesive tape. This will not interfere with the passage of the magnetic tape, but will hold the magnetic tape clear of the erase head far enough to restrict the amount of erasure. You can now switch to "record", record your commentary and at the end switch to "playback" once more. When you reproduce the whole recording, you find that where you have superimposed the commentary, the music falls in volume because it has been partly erased. This system is easy to use, but it suffers from two drawbacks. Partial erasure affects the higher frequencies more than the lower ones and the music, therefore, sounds muffled and "woolly" where it accompanies the commentary. In many cases this may be useful since it ensures that the commentary is not rendered unintelligible. On the other hand, it can be a nuisance when you want to mix two sounds without modifying either. The big difficulty is that you cannot make corrections to passages of commentary superimposed on music. If you make a slip in your commentary you have to erase the tape completely in order to remove the offending passage, 82