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How Sound Editing Works

Mechanical Sound Editing
Before computer editing, sound editors used reel-to-reel audio tape players to record sound.
Before computer editing, sound editors used reel-to-reel audio tape players to record sound.
Siede Preis/Getty Images

Before computers came into wide use for sound editing in the 1990s, everything was done with magnetic tape. To make edits using magnetic tape, you literally had to cut the tape, remove the piece of audio that you didn’t want and splice the tape back together again.

The machine of choice for mechanical audio editing was the reel-to-reel tape recorder. With this piece of equipment, you could record and playback audio from circular reels of magnetic audiotape. You also needed several pieces of specialized editing equipment: a razor blade, an editing block and editing tape.

Here’s the basic cut-and-splice editing process using magnetic tape:

  1. Find the initial edit point (or in point), which is the starting point on the tape for the section of audio you want to remove. This is done through a process called scrubbing, where the sound editor slowly rocks the reels back and forth to find the precise point to make the cut.
  2. Using a grease pencil, make a mark on the tape directly over the tape recorder’s playhead.
  3. Play the tape until you reach the first sound you want to keep, called the out point. Also mark that edit point with a grease pencil.
  4. Remove the tape from the reel-to-reel and place it in an editing block. The editing block contains special grooves at 45º and 90º angles.
  5. Line the first edit point up with the 45º groove, cut the tape along the groove with a razor blade. Do the same with the second edit point.
  6. Using special editing tape, tape the two loose ends of magnetic tape back together, leaving no space in between.
  7. Put the tape back on the reel-to-reel and test the edit. You may need to cut more off one of the ends, or maybe you already cut too much!

When magnetic tape was invented in the late 1940s, one of its greatest advantages was that it could hold multiple audio channels without creating a lot of excess noise. This allowed for a process called overdubbing or multi-track recording.

For the first time, the sound editor could isolate and individually edit each piece of audio (dialogue, sound effects, music) and record them as their own track. That’s called predubbing. Then the individual tracks could be recorded on top of each other -- overdubbed -- on a single piece of magnetic tape. The first prototypes of magnetic tape could only handle two audio tracks at a time, but later versions could hold hundreds.

In the next section, we’ll see how the same basic sound editing techniques entered the digital age.