Topics covered in this article:
- Downmix and trim concepts
- Downmix and trim metadata, monitoring, and re-renders
- The trim and downmix controls window
- Downmix modes
- Trim controls
- Interaction of trim and downmix modes
- Layouts needed to audition trim values
Downmix and Trim Concepts
The Dolby Atmos Renderer provides metadata controls to determine how the 5.1 and 2.0 mixes are derived from a Dolby Atmos mix, impacting how they will sound during monitoring, on the re-renders, and when creating the 5.1 core created during encoding to the Dolby Digital Plus JOC codec used by streaming services.
Two sets of controls are used for this purpose: downmixes and trims.
Downmix and trim metadata in the master file used in the encoding process determines how the 5.1 core of a Dolby Digital Plus JOC bitstream will sound on legacy equipment, or when Dolby Atmos is decoded with a speaker layout of less than 7.1.
Downmix modes can be independently set for monitoring and re-renders if needed for both 5.1 and stereo. This can be useful in setting how stems will sound, as those will be remixed/summed later in localization workflows.
Downmix modes used for the master file and for re-renders can be optimized independently if needed. There are different downmix modes available based on mixers’ preferences and use cases. The 5.1 to 2.0 downmix happens after the 5.1 downmix, meaning that the 5.1 downmix mode will also impact the resulting 5.1 to 2.0 downmix selection.
Automatic trim mode provides optimized rendering to avoid any troublesome sonic buildup from overhead and rear content when monitoring with 2.0, 5.1, 5.1.2, and 7.1 speaker layouts. The automatic mode settings are sufficient for the vast majority of mixes. However, a manual mode is provided for more creative control, if needed. None of these modes affect how the rendered Dolby Atmos mix will sound when more than 5.1 floor speakers are used (i.e. 7.1.2, 7.1.4, 9.1.6 speaker layouts).
Downmix and Trim Metadata, Monitoring, and Re-renders
Downmix and trim metadata is written into the master file and can be edited/updated on a previously created .atmos master file that has been unlocked for editing. This metadata applies to the entire file and cannot be varied per “scene.”
The Trim and Downmix Controls Window
The Trim and Downmix Controls Window is used to set:
- Dolby Atmos-to-5.1 and 5.1-to-2.0 Downmix metadata for master files – when recording a master file or to edit metadata in an unlocked .atmos master file.
- Dolby Atmos-to-5.1 and 5.1-to-2.0 Downmix settings for monitoring and re-renders.
- Trim Controls for 5.1 and 2.0.
- Trim Controls for 5.1.2.
- Trim Controls for 7.1.
To open the Trim and Downmix Controls Window navigate to the menu bar Window > Trim and Downmix Controls, or press Command+T (Mac), or Ctrl+T (Windows).
- Select the desired 5.1 downmix type from the drop down menu under 5.1 downmix in master.
- If a different 5.1 downmix mode is needed for monitoring/re-renders, click on the link button (chain icon) to decouple the two.
-
- A separate drop-down menu will then be available to set the downmix mode for the 5.1 downmix in monitoring/re-renders.
- Select the desired 5.1-to-2.0 downmix type from the drop down menu under 5.1 to 2.0 downmix in master.
- If a different 5.1 to 2.0 downmix mode is needed for monitoring/re-renders click on the link button (chain icon) to decouple the two.
- A separate drop-down menu will then be available to set the downmix mode for the 5.1-to-2.0 downmix in monitoring/re-renders. Click on 5.1 and 2.0, 5.1.2, or 7.1 to highlight the selection
- Trim controls are available separately for 5.1 and 2.0, 5.1.2, and 7.1.
- Automatic Trim Controls (default) can be changed to Manual to unlock the Surround and Height Trims, Front/Back Balance controls for Overhead content, and on the Listener plane.
- These controls can be adjusted while monitoring input or playing back a master file. The proper monitoring layout should be selected from the Renderer main window (using the monitoring layout or numeric keyboard shortcuts) to match the layout of the Trim controls being modified.
- Automatic Trim Controls (default) can be changed to Manual to unlock the Surround and Height Trims, Front/Back Balance controls for Overhead content, and on the Listener plane.
Downmix Modes
To create a 5.1 downmix, two basic methods are used. The Dolby Atmos Renderer either renders to 7.1 (applying trims) and then downmixes to 5.1, or the Renderer downmixes directly to 5.1. There are two choices for either method.
7.1-to-5.1 Downmix Modes
For creating a 5.1 downmix from an initial 7.1 re-render, the mixer can choose either Standard (Lo/Ro) – default or Dolby Pro Logic IIx. Both determine how the rendered rear surrounds downmix to 5.1.
Standard Lo/Ro sums the rear and side surrounds in equal measure, using the following coefficients:
Ls = 0 dB × Lss + 0 dB × Lrs
Rs = 0 dB × Rss + 0 dB × Rrs
Pro Logic IIx is a matrix-encoding technology originally developed for encoding rear surrounds of 7.1 mixes into 5.1 to provide 7.1 playback in home theaters from 5.1 sources. When used to downmix rendered 7.1-to-5.1, the intended use case may indeed be for the 5.1 Pro Logic IIx to be decoded to 7.1; however, some mixers simply prefer the sound of a 5.1 mix encoded using Pro Logic IIx.
ProLogic IIx uses the following downmix coefficients:
Ls = Lss + (–1.2 dB × Lrs) + (–6.2 dB × Rrs)
Rs = Rss + (–6.2 dB × Lrs) + (–1.2 dB × Rrs)
Direct 5.1 Downmix Modes
When creating a direct 5.1 downmix from the Renderer, the mixer can choose Direct Render or Direct Render with Room Balance.
- Direct Render renders to 5.1 (without first downmixing via 7.1) to recreate the 7.1 sound field at the central listening position using phantom imaging between the surround speakers and front speakers in order to maintain rear surround and side surround panning intent which is lost when using a summing downmix.
- Direct Render with Room Balance renders to 5.1 as above but avoids some of the possibly unfavorable characteristics of Direct Render. Because Direct Render uses phantom imaging between the surround speakers and front speakers, it sometimes sounds ‘front heavy’ and can also result in comb filtering artifacts between the fronts and surrounds. Direct Render with Room Balance applies an updated Dolby rendering algorithm that reduces the comb filtering effects associated with phantom imaging of objects positioned halfway between the front and rear of the room. Room balance refers to how the Renderer deals with content that is panned between the midpoint and rear of the room. Using room balance, the content is presented at a constant level in the surround speakers between the rear and midpoint of the room, avoiding any need for phantom imaging until it is in the front half of the room.
5.1-to-2.0 Downmix Modes
All of the 2.0 downmix options are created by first creating a 5.1 downmix from the 7.1 render of the Dolby Atmos mix, and then downmixing from 5.1-to-2.0 in the same manner as 2.0 consumer products. The choice of the 5.1 downmix method above affects the subsequent 5.1 to 2.0 downmix.
Lo/Ro - default: Downmixes from 5.1-to-2.0 using the following coefficients:
Lo = L + (–3 dB × C) + (–3 dB × Ls)
Ro = R + (–3 dB × C) + (–3 dB × Rs)
Lt/Rt (Pro Logic II): Downmixes from 5.1-to-2.0 using the following coefficients:
Lt = L + (–3 dB × C) – (–1.2 dB × Ls) – (–6.2 dB × Rs)
Rt = R + (–3 dB × C) + (–6.2 dB × Ls) + (–1.2 dB × Rs)
Lt/Rt (Pro Logic II) w/Phase 90: Downmixes from 5.1-to-2.0 using the following coefficients:
Lt = L + (–3 dB × C) – (–1.2 dB × Ls) – (–6.2 dB × Rs)
Rt = R + (–3 dB × C) + (–6.2 dB × Ls) + (–1.2 dB × Rs)
A 90-degree phase shift to Ls/Rs is applied prior to feeding the downmix, which reduces undesirable phase cancellation, improves imaging, and enables proper matrix decoding. It is recommended to use the 90-degree phase shift for any Lt/Rt downmixes.
Trim Controls
When set to manual, the mixer can audition the trim controls in real time:
- Surround and Height sliders apply trims (in dB) to the default fold-down behavior for the surround and overhead content.
- Overhead and Listener Plane sliders set how height content (Overhead) and surround content (Listener Plane) fold down, pushing more towards the front or rear of the soundscape.
Note that trim controls apply to the entire file. Manual control should not be used to optimize one scene at the expense of the rest of the content.
Interaction of Trim and Downmix Modes
The trim controls for 5.1 and 2.0 are applied concurrently with the selected downmix modes. It is important to be aware of the characteristics of manual trims when selecting downmix mode. For example, adjustments to the Front/Back balance of the Listener Plane can impact the effectiveness of Direct Render with Room Balance.
Monitoring Layouts Needed to Audition Trim Controls
The default monitor layout of the Dolby Atmos Renderer is 7.1.4. If the mixer wants to make manual adjustments to trims for 5.1.2, a custom monitoring layout will need to be created in the Room Setup window to add the Ltm/Rtm in Speaker Setup and to add a monitoring layout for 5.1.2 in the Monitoring tab.
Navigation
Next: Appendix F - Recording Live Re-Renders
Previous: Appendix D – Dolby Atmos Content Delivery Signal Flow