Part of the ATSC audio system, also known as Dolby Digital (AC-3), is the ability to send programming with any number of discrete audio channels up to 5.1 to any viewer- regardless of how many speakers they might have. Reproducing 5.1 channels of audio through a lesser number of speakers requires a process called downmixing. Simply put, downmixing combines the Left, Right, Center, Left surround, and Right surround channels in a somewhat logical manner to drive stereo or mono speakers.

The first rule of downmixing is to discard the LFE channel. This channel carries (or should only carry) low frequency information that is of little use to stereo or mono speakers, unless the goal was to overdrive them. Further, the LFE channel bandwidth is limited to 120Hz, a bit too low for most small speakers to reproduce accurately. It is thus critically important to remember that stereo and mono listeners will not hear any audio placed in the LFE channel.

Creating a stereo downmix, also known as LoRo (Left only/Right only), is very straightforward. The Lo output is created by adding Left, plus Center (at –3dB), plus Left surround (at –3dB), and the Ro output is created similarly by adding Right, plus Center (at –3dB), plus Right surround (at –3dB). This downmix equation is also sometimes referred to as an ITU Downmix, and is fully stereo and mono compatible with very few side effects. However, the downmix must be slightly different for a surround compatible output.

Coming up in Part II: Stereo downmixing is easy, but creating a two-channel downmix of a 5.1 channel program that is matrix surround (LtRt) compatible while sort of preserving stereo and mono is much more difficult.