What Is Downmixing? Part 2: Surround (LtRt)
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. This is the default downmix mode of most cable and satellite set-top boxes for two-channel audio output, usually via red and white RCA or pin-jack connectors. The LtRt signal is more complex to generate and relies on upstream Dolby Digital (AC-3) encoding equipment to perform some pre-processing to get it right.
Creating an LtRt signal is very similar to creating LoRo--at first. The LFE channel is discarded, then left channel and center channel (at -3 dB) are added, and then the right channel and the center channel (at -3 dB) are added. Handling the surround channels marks the point of departure from similarity. Left surround and right surround are added together to create a single "S" signal. This S signal is added out of phase (180 degrees) to the left and center signal to create Lt, and added in phase (zero degrees) to right and center signal to create Rt.
It should be apparent that without some help, signals added 180 degrees out of phase could cause serious problems. This is where the upstream processing comes into play. As part of the Dolby Digital (AC-3) encoding process, the surround channels can be selectively phase shifted by 90 degrees prior to encoding. After decode, this pre-applied 90-degree phase shift adds to what is done during downmix, resulting in the "S" signal being at 270 degrees in the Lt signal and at 90 degrees in the Rt signal. This minimizes, but does not eliminate, cancellation effects. Note also that the "S" information is still 180 degrees out of phase between Lt and Rt, resulting in complete cancellation if these signals are downmixed to mono. These complex phasing issues can cause problems with certain content that may have similar signals in surround and front channels and this must be carefully explored and monitored.