Stereo Audio

The human ear hears sounds in stereo, and the brain uses the subtle differences in sounds entering the left and right ears to locate sounds in the environment. To re-create this sonic experience, stereo recordings require two audio channels throughout the recording and playback process. The microphones must be properly positioned to accurately capture a stereo image, and speakers must also be spaced properly to re-create a stereo image accurately.

If any part of the audio reproduction pathway eliminates one of the audio channels, the stereo image will most likely be compromised. For example, if your playback system has a CD player (two audio channels) connected to only one speaker, you will not hear the intended stereo image.

Important: All stereo recordings require two channels, but two-channel recordings are not necessarily stereo. For example, if you use a single-capsule microphone to record the same signal on two tracks, you are not making a stereo recording.

Identifying Two-Channel Mono Recordings

When you are working with two-channel audio, it is important to be able to distinguish between true stereo recordings and two tracks used to record two independent mono channels. These are called dual mono recordings.

Examples of dual mono recordings include:

  • Two independent microphones used to record two independent sounds, such as two different actors speaking. These microphones independently follow each actor’s voice and are never positioned in a stereo left-right configuration. In this case, the intent is not a stereo recording but two discrete mono channels of synchronized sound.

  • Two channels with exactly the same signal. This is no different than a mono recording, because both channels contain exactly the same information. Production audio is sometimes recorded this way, with slightly different gain settings on each channel. This way, if one channel distorts, you have a safety channel recorded at a lower level.

  • Two completely unrelated sounds, such as dialogue on track 1 and a timecode audio signal on track 2, or music on channel 1 and sound effects on channel 2. Conceptually, this is not much different than recording two discrete dialogue tracks in the example above.

The important point to remember is that if you have a two-track recording system, each track can be used to record anything you want. If you use the two tracks to record properly positioned left and right microphones, you can make a stereo recording. Otherwise, you are simply making a two-channel mono recording.

Identifying Stereo Recordings

When you are trying to decide how to work with an audio clip, you need to know whether a two-channel recording was intended to be stereo or not. Usually, the person recording production sound will have labeled the tapes or audio files to indicate whether they were recorded as stereo recordings or dual-channel mono recordings. However, things don’t always go as planned, and tapes aren’t always labeled as thoroughly as they should be. As an editor, it’s important to learn how to differentiate between the two.

Here are some tips for distinguishing stereo from dual mono recordings:

  • Stereo recordings must have two independent tracks. If you have a tape with only one track of audio, or a one-channel audio file, your audio is mono, not stereo.

    Note: It is possible that a one-channel audio file is one half of a stereo pair. These are known as split stereo files, because the left and right channels are contained in independent files. Usually, these files are labeled accordingly: AudioFile.L and AudioFile.R are two audio files that make up the left and right channels of a stereo sound.

  • Almost all music, especially commercially available music, is mixed in stereo.

  • Listen to a clip using two (stereo) speakers. If each side sounds subtly different, it is probably stereo. If each side sounds absolutely the same, it may be a mono recording. If each side is completely unrelated, it is a dual mono recording.

Interleaved Versus Split Stereo Audio Files

Digital audio can send a stereo signal within a single stream by interleaving the digital samples during transmission and deinterleaving them on playback. The way the signal is stored is unimportant as long as the samples are properly split to left and right channels during playback. With analog technology, the signal is not nearly as flexible.

Split stereo files are two independent audio files that work together, one for the left channel (AudioFile.L) and one for the right channel (AudioFile.R). This mirrors the traditional analog method of one track per channel (or in this case, one file per channel).