Downmixing LtRt vs LoRo. What’s the Difference?
There are different techniques for mixing and sound processing, such as the use of Lt/Rt and Lo/Ro formats. Let’s talk about the importance and differences between these two, as well as the advantages and disadvantages of each, and their use in different contexts and media.
And let’s start by saying that in a multitrack recording, multiple microphones are used to record multiple sound sources separately, thus creating individual audio channels. The process of downmixing, also known as ‘fold-down,’ combines some of these audio channels into a single one (usually, an audio engineer does this job…). Some consumer electronic devices, such as DVD players or audio cards, can downmix automatically. For instance, they can convert a surround sound signal (four channels) into a stereo signal (two channels) for playback on two monitors.
Now, when we talk about downmixing, we are not only talking about audio signals: in radio and communication, downmixing is associated with the creation of an intermediate frequency signal by modulating a complex frequency on a baseband carrier — and this involves many things.
So, Lt/Rt is a type of downmix. It is designed to be interpreted by a Dolby Pro Logic decoder, which allows it to obtain a Dolby Surround L, C, R, and S signal. In addition, the Lt/Rt downmix can also be reproduced in stereo sound systems, such as headphones or hi-fi systems. Basically, Lt/Rt is a matrix encoding used to convert 5.1 mixes to stereo.
When a Lt/Rt encoded mix is passed through a decoder, the channels are reopened in 5.1. However, if the mix is listened to in a stereo sound system, phase cancellations can occur due to the information present in the rear channels, which can lead to listening problems, hence to listen to a Lt/Rt encoded mix, it is necessary to use a Dolby Pro Logic II decoder.
Lt/Rt matrix encoding works by summing the sounds of the surrounding channels to create an “S” signal that is then added to the mix. The phase shift in Lt/Rt is -90 degrees for the left channel and +90 degrees for the right one if a real Dolby encoder is used. It is possible to fake this phase shift with a 180-degree and 0-degree one, but it may be necessary to add a slight delay of a few milliseconds on the “S” channel to avoid audible phase problems. Although this mix can be decoded by Dolby Pro Logic II to recreate a pseudo 5.1 mix, it is not fully compatible with mono playback.
What about Lo/Ro? This one is used to ensure program compatibility in mono systems. Unlike Lt/Rt, which is set to a Dolby Pro Logic decoding system to bring the signal back to 5.1, Lo/Ro destroys the separation of information in the front/rear channel signal so that a Dolby Pro Logic system has no problem reproducing the signal in 5.1. Then, Lo/Ro encoding is used to convert a 5.1 program into a pure stereo signal by correcting phase problems for correct stereo program listening.
In the Lo/Ro mix, the left channel is added to the center (with a 3dB reduction) and the left rear channel (also with a 3dB reduction). The right channel is similarly summed, with a 3dB reduction in the center and right rear channels. The resulting mix is fully compatible with mono and stereo systems. For a 5.1 mix, the LFE channel is muted.
For example, if in a 5.1 mix, a voice is placed in the rear channels to give a sense of spatiality or to place a character off in a situation in front of the protagonist, if it is encoded in Lt/Rt when decoded you will hear that voice again in the rear. Nevertheless, if listened to undecoded in a stereo player, that voice may disappear since it is in counter-phase on the L and R channels, causing program information to be lost. With Lo/Ro encoding, the phase problem is solved and a new mix is created in the L and R channels at a lower volume, creating a sense of distance without compromising the intelligibility of the program.
Both formats offer different approaches to creating a stereo listening experience. We already mentioned that Lt/Rt is useful when working with a mono signal, as in the mixing of movies and television programs. With this format, a stereo image can be created from a mono signal by using a decoding matrix. This approach can be particularly effective in mixing dialog and sound effects, as it helps to place sounds in a stereo space.
As for the Lo/Ro format, because it uses two separate signals to create a more detailed stereo image, it is useful when working with original stereo recordings, such as in mixing music and in some cases movies and TV shows. Using separate stereo signals allows greater flexibility in the placement of sounds in the stereo field, which can be important in creating an immersive and detailed listening experience.
We hope the difference is clear to you, and that you know when to use one or the other. If you need additional advice for your audio or video projects, don’t hesitate to contact Enhanced Media Sound Studio. We want your work to achieve excellence and we want to be part of it.
