In Podcast 3 I talked about the different formats for mixdown or foldown to get a stereo compatible mix from a 5.1 surround mix. I talked about Lo/Ro and Lt/Rt.
Lo/Ro (left only, right only) is a stereo down mix where the different channels are simply summed together.
Lt/Rt (left total, right total) is a stereo down mix where the different channels are processed before summing them together. This is also called matrix encoding.
Both LtRt and LoRo provide stereo compatible downmixes of surround content. The LoRo doesn’t have the phase shifted surround component to give the Pro Logic decoder something to do. Usually LoRo is set up so the Left & Right channels pass through unaffected the Centre channel gets mixed into both Lo & Ro at -3dB and Left Surround (Ls) is mixed into Lo at -3dB and Right Surround (Rs) at -3dB into Ro. Although these settings can be changed to suit the content to get the most compatible downmixes.
The new Avid MixDown plug-in in Pro tools 10 produces Lo/Ro
In Lt/Rt the surround signal is phase shifted so that it is shifted back by 90degs on the Lt output and 90deg forward on the Rt output so that the surround component is actually 180 deg (ie out of phase) so if you mix Lt/Rt down to mono the surround conponent cancels out. The full Dolby Pro Logic encoder is more complex than I have describes and also different for Pro Logic 1 to Pro Logic 2 but the basic concept is there. The most common Lt/Rt plug-in for Pro Tools comes from Paul Neyrinck’s stable as is the SoundCode Stereo Lt/Rt plug-in.
Both LtRt and LoRo are used in television so you must look at the delivery specs to see what the channel is asking for.
Then there is the whole issue of metadata. On a lot of HD channels that transmit 5.1 surround audio, they don’t transmit a stereo compatible mix as well. Stereo and mono viewers hear a downmix created in their TV or set top box. The settings of which are sent as metadata on the Dolby AC3 stream. The metadata is set when the file is encoded and so again we can determine the best settings and have the consumers equipment do the downmix to those settings. For live content the metadata is still transmitted but we have to make a best guess at it before transmission starts. Again the metadata setting for live content can be in the channel’s delivery spec, so check before you start.
Your LtRt mixes can appear to have more ‘airiness’, I suspect that is due to the phase shifting of the surround component. Out of phase audio will appear very wide across the stereo soundfield.