"Downmixing" refers to the process of mixing a certain number of audio channels to create an output with fewer channels. The most common type of downmix in TV audio involves creating a stereo (2-channel) version of 5.1-channel content so that all program elements, regardless of their original channel routing, can be heard in the mix when listening on a typical stereo television.
The controls in the UPMAX ISC Downmix menu allow the downmix format to be specified, determine whether <<FUTURE - SOMETHING ABOUT LFE ON>>, and trim the gain of several channels and channel groups.
Downmix Target (1A)
This control allows UPMAX ISC to create a stereo downmix from any of its multichannel output formats when set to "Stereo," or create a 5.1-channel downmix from a 7.1, 7.1.2, or 5.1.4 upmix.
Center Trim (1B)
This control allows the gain of the audio originally mixed to the center channel to be reduced (by as much as 70dB) or boosted (by as much as 6db) when the downmix is created. Setting the Center Trim control higher than the Height and Surround Trim controls can help ensure that dialog - the most common component of the center channel and the most critical channel in terms of program intelligibility - doesn't get overwhelmed by music and effects.
Height Trim (1C)
This control allows the gain of the audio originally mixed to the height channels in 7.1.2 and 5.1.4 formats to be reduced or boosted when the downmix is created.
LFE Trim (1D)
This control allows the gain of the audio originally routed to the LFE channel in multichannel formats to be reduced or boosted when the downmix is created.
Surround Trim (1E)
This control allows the gain of the audio originally routed to the surround channels in multichannel formats to be reduced or boosted when the downmix is created.
LFE On (1F)
When enabled, this control includes audio originally routed to the LFE channel to be included in the downmix.
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Figure 1 - Downmix menu
Upmixing Controls That Affect the Downmix
The settings of the Center Gain and Surround Gain controls in the Basic UPMAX menu and the Front Gain LF, Front Gain HF, Front Height Gain LF, Front Height Gain HF, Rear Gain LF, Rear Gain HF, Rear Gain LF, and Rear Gain HF controls in the Advanced UPMAX menu will affect the gain of the downmixed signal differently compared to the upmixed signal.
In the case of the Center Gain control, which focuses on a single channel that is almost always reserved just for dialog, the translation from a multi-channel output to a downmixed output is fairly predictable and straightforward.
However, with a parameter such as the Surround Gain control - which encompasses four audio channels (Left Mid Surround, Right Mid Surround, Left Rear Surround, and Right Rear Surround) in 7.1 or 7.1.2 formats - it is important to remember that whatever content is present in all four channels will be rendered at the same level in the downmix. The same is true for the remaining controls relative to the channels they incorporate.