Downmixing (Lt/Rt and Lo/Ro) Stereo Downmix: Left Total/Right Total and Left only/Right only

What downmixing is

Downmixing converts a multichannel soundtrack into fewer channels, almost always stereo, so one 5.1 stream can serve every speaker setup. ATSC A/52, the standard behind Dolby Digital (AC-3), says it is important that downmixing "be standardized so that program providers can be confident of how their program will be reproduced." Dolby's Hybrik documentation names three outputs: Lo/Ro (left only/right only) for headphones and stereo TVs, a mono mix derived from Lo/Ro, and Lt/Rt (left total/right total) for Pro Logic or Pro Logic II decoding. The standard Lo/Ro and Lt/Rt equations leave the LFE channel out, and TV Technology calls discarding it the first rule of downmixing. A/52 treats LFE downmixing as optional, noting an ideal downmix would play it 10 dB above left and right, and TV Technology points out that LFE is limited to 120 Hz and is of little use to stereo speakers.

How Lo/Ro works

A/52 gives the Lo/Ro equations as Lo = 1.0 L + clev C + slev Ls and Ro = 1.0 R + clev C + slev Rs, with no phase tricks. The coefficients come from metadata. The center mix level clev can be 0.707 (-3.0 dB), 0.595 (-4.5 dB) or 0.500 (-6.0 dB), and the surround mix level slev can be 0.707 (-3 dB), 0.500 (-6 dB) or 0, which switches the surrounds off. Because everything is summed in phase, Lo/Ro also survives mono: the mono sum is M = 1.0 L + 2.0 clev C + 1.0 R + slev Ls + slev Rs. A/52 therefore says Lo/Ro is preferred when a mono signal is wanted. TV Technology calls it an ITU downmix and says it is fully stereo and mono compatible.

How Lt/Rt works

Lt/Rt uses opposite polarity for the surrounds. The A/52 equations are Lt = 1.0 L + 0.707 C - 0.707 Ls - 0.707 Rs and Rt = 1.0 R + 0.707 C + 0.707 Ls + 0.707 Rs. Sound On Sound describes Lt/Rt as the Pro Logic compatible version, with matrixed center and mono surround channels carried inside the left and right information, which a Pro Logic decoder can pull back out. Per TV Technology, the encoder can pre-shift the surrounds by 90 degrees so the downmix lands at 270 degrees in Lt and 90 degrees in Rt, which reduces but does not eliminate cancellation. Sound On Sound adds that this phase shift is usually inaudible but can show on music-only recordings, so it can be switched off, and the metadata then flags that no Lt/Rt downmix is possible. The cost is mono: A/52 states that if the Lt/Rt downmix is combined to mono, the surround information is lost, because Lt + Rt cancels the surround terms.

Worked example

Take a scene with L = 0.5, C = 0.8, Ls = 0.4 and default coefficients of 0.707. Before scaling, Lo = 0.5 + 0.707 x 0.8 + 0.707 x 0.4 = 1.348. A/52 warns that unscaled downmixes can overload, and gives the worst-case scale factors: for Lo/Ro the coefficient sum is 1 + 0.707 + 0.707 = 2.414, so everything is multiplied by 0.4143 (a 7.65 dB cut); for Lt/Rt the sum is 3.121, so the multiplier is 0.3204 (a 9.89 dB cut). Lo scales to 1.348 x 0.4143 = 0.559. Dialogue only in the center at 1.0 gives Lo = Ro = 0.707 (-3 dB), and the unscaled mono sum is 2 x 0.707 = 1.414, which A/52 halves for mono to avoid overload. That is why dialogue can sound different between a stereo and a mono feed.

Metadata and where you meet it

Dolby Digital carries the choices. A/52 Annex D defines the dmixmod field, where 01 means Lt/Rt downmix preferred and 10 means Lo/Ro preferred, plus separate center and surround levels for each mode (ltrtcmixlev, ltrtsurmixlev, lorocmixlev, lorosurmixlev). The center options run from 1.414 (+3.0 dB) down to 0.500 (-6.0 dB) in 1.5 dB steps, plus off. Sound On Sound adds that the universal fields default to -3 dB for center and surrounds, and only recent decoders use the extended set. That is why one program can fold down differently on two devices. TV Technology says Lt/Rt is the default for most cable and satellite set-top boxes on their two-channel RCA output, and Sound On Sound lists the DP569 Dolby Digital encoder and DP570 Multichannel Audio Tool as the era's authoring hardware. A/52 also sets downmix coefficient accuracy at ±0.25 dB.

Common confusions

Lo/Ro is not "better surround"; it is the safer stereo. If you want a receiver to rebuild surround from stereo, Lt/Rt is the signal that helps, and the receiver then runs a Pro Logic style decoder, covered in Dolby Surround upmixer. Quiet dialogue after a downmix can come from the center-level setting or from dynamic range, so read night mode, dialnorm and dialogue enhancement. Downmix coefficients also change absolute dialogue level, a point A/52 makes for SPL-calibrated systems. Dolby's Hybrik documentation says dialnorm, dynamic range control and downmixing work together, and that an incorrect dialnorm value or a program that misses its loudness target can degrade the result, since the combination of DRC and dialnorm is what prevents overload when downmixing. Hybrik adds that automated downmixing is built into every Dolby decoder, constructing a stereo downmix from a 5.1 source and a mono downmix from the stereo, so one multichannel program can feed everyone. If a familiar movie sounds wrong in stereo, test it in each downmix mode before blaming the speakers. For the encoding side see Dolby Digital (AC-3), and for room practice see the apartment soundbar guide.