Everything You Always Wanted To Know About Trim and Downmix (but were afraid to ask) – Part Two
Trim and Downmix Part Two – with Ceri Thomas and Nathaniel Reichman
Immersive Master Pro is the only application that supports Batch Trim and Downmix. This allows users to change the metadata for multiple Dolby Atmos masters in one step. But with power comes responsibility. In this two-part interview, Immersive Machines’ co-founder Nathaniel Reichman talks with Ceri Thomas about this feature. Ceri is the founder of the consulting company IMMERSV, and is widely regarded as an expert in immersive sound.
DOWNMIX CONTROLS
Nathaniel Reichman: Now over to Downmix. When we make a 5.1 and 5.1.x mix the first choice is called “LoRo - default.” I’ve always thought LoRo meant a discrete stereo mix—‘Left-only, Right-only’—so I’m confused to see it associated with downmixing a surround mix. Is this a misuse of the term, or is there a deeper Dolby definition?
Ceri Thomas: “LoRo – default” is really describing that stage to stereo from 5.1. The 5.1 portion is pretty direct beyond the trims that have been specified.
LtRt is where the definition becomes deeper with Dolby. It stands for “Left Total/Right Total” and refers to the matrixed two channel form of encapsulating a multi channel mix into fewer channels. This goes back to the original Dolby Stereo created with, in its most modern iteration, the DMU (Dolby Mastering Unit) in theatrical mixing stages. This matrixing allowed surround mixes to be stored in the audio track space along the edge of a film reel and then decoded in theatres for surround playback. It also acted as a fallback in case something went wrong.
So LoRo is an accurate description of what is happening in the renderer, as it is not matrixed. To further complicate the situation, there is LtRt and LtRt Pro Logic which gave discrete surround (LCRLsRs) versus a mono surround (LCRS), but that’s more than anyone needs to know.
Nathaniel Reichman: Is anyone still using Dolby Pro Logic II, II w/ Phase 90, IIx? The last time I was asked to make a Pro Logic LtRt was for a TV show in 2016. Are these included here just for backwards compatibility?
Ceri Thomas: Well, I struggle to say it’s not used but definitely used less and less frequently. In the US I’d say it’s probably more prevalent than in other territories, largely due to systems like DVB being more widely utilized outside of the US. Any time you see Dolby Surround on a TV show, it’s likely Pro Logic II. The phase 90 refers to a 90-degree phase shift that is applied to LFE and helps with a number of things not least bass management, by phase shifting the LFE there is less likelihood of cancellation.
“[In] a soundbar plus satellites situation...the listener plane balance will control how much of the sound field is reproduced by the soundbar and how much goes to the satellites. This can be really important when the full surround field is utilized as it might mean your mix being cohesive and balanced or leaning front/back too much.”
Nathaniel Reichman: Whenever I listen to Direct Render vs. Direct Render with room balance, I always choose the former, because it seems like a more natural conversion from immersive. But maybe I’m missing something here. What does room balance do that I should know about?
Ceri Thomas: Room Balance is more associated with Post workflows as a generality. It’s a method to automatically prevent the side-surround information from leaning towards the screen channels and the same for the wide information. Direct Render on the other hand makes a stronger attempt to keep the overall balance of the mix in the room regardless if that meant side/wide information becoming part of screen channels.
For music only, Direct Render is definitely the way to go in 95% of circumstances. However, if that album project you’ve been working on suddenly requires stems and more for placement in a movie soundtrack, Direct Render with room balance can quickly make a beneficial change for the theatrical ‘gaffer’ (Dialog and Music) mixer, which will be appreciated.
Similarly, if a broadcast TV show wanted Mix Minus for a performance, then switching them to LtRt Pro Logic might be preferable. So, they are worth knowing about as tools and having a global, batch conversion utility in Immersive Master Pro is definitely of benefit to the marketplace, so thank you for that!
DELIVERY
Nathaniel Reichman: Zooming out for a moment to look at the big picture- Trim and Downmix parameters affect all the channel-based re-renders that we’ve been talking about, so they’re baked into the sound of my 2.0 or 5.1 WAVs, for example. But those parameters are also included in the metadata of an ADM that I might send out. In what context would a consumer hear something differently solely because of this metadata?
Ceri Thomas: DD+ is a 5.1 channel core, and that is what makes it backwards compatible to non-Atmos receivers etc. It is also relevant for Soundbar plus Satellite playback mechanisms, in-car playback depending on the car system; let’s say the decoder in the device is specified for 5.1.4 rendering, the renderer tells the decoder to output 9 main elements streams and an LFE, those 9 elements are to all intents and purposes channels for the renderer. At 448kbps, the decoder actually outputs 15 elements plus the LFE, giving 6 elements that are object audio only; it’s more complex than that in truth, but it’s a decent rule of thumb. Thinking primarily about a soundbar plus satellites situation, the 5.1.4 soundbar itself is going to predominantly reproduce LCR plus a portion of the overhead field, assuming the rear satellites have some form of upward firing speakers, too, then that will be a split of the overhead information between the soundbar and the rears. The listener plane balance will control how much of the sound field is reproduced by the soundbar and how much goes to the satellites. This can be really important when the full surround field is utilized as it might mean your mix being cohesive and balanced or leaning front/back too much.
In a car that decodes 5.1.2, it could mean the difference between rear passengers getting purely reverb/delay versus main content too.
If a BluRay is the destination for your mix, which uses Dolby TrueHD instead of DD+JOC then the channel based core of the format is 7.1 instead of 5.1. The Trim and Downmix controls adjust accordingly to that extended capability and listener plane resolution.
Ceri Thomas has spent more than 20 years working with and around some of the best creative talent in the world across experiences in film, television, live broadcast, virtual reality and music. He has been a trusted partner of Oscar, Emmy and GRAMMY winners, as well as many of the world’s biggest brands, outlets and studios. He defined standards at Dolby Laboratories for creation in Dolby Atmos for VR and later for Dolby Music, where he also developed training programs and studio specifications for many of the world’s top music studios and mixers. At Apple Music, he championed higher quality in every aspect of Spatial Audio, including helping to establish the foundation of Spatial Audio QC.
Ceri Thomas, CEO of IMMERSV
Nathaniel Reichman: What about ADMs that are going to Apple Music or Apple TV? Does the Trim and Downmix metadata change the sound of a mix in Apple’s Spatial Audio?
Ceri Thomas: It’s important to clarify the division of labor here before answering. An ADM getting delivered to a streaming platform, Apple Music/TV+ or any other OTT streamer such as Netflix, Amazon etc; will get encoded into a bitstream of some variety upon ingest. That is done with Dolby encoding capabilities and is the Co portion of codec. That bitstream then gets requested with the title that a consumer, Listener/Viewer, wants and hits the Dolby decoder portion of the pipeline which decodes the bitstream based on what the renderer communicates is available, this is the dec portion of the codec. The Trim and Downmix Metadata is used during these portions of the media pipeline.
The Render portion that follows is where each playback becomes its own beast, whether it’s Sonos TruePlay, Apple Spatial as branded virtualizers or in the home theatre market Dirac, Storm, Trinnov, Synthesis, etc. control how those decoded signals are treated for the optimum playback for that device or virtualizer.
All that said, if you are using Apple Music on an Apple TV device playing into a Sonos Soundbar, the decoder and render likely resides within the Sonos and therefore TruePlay is in operation rather than Spatial. If listening on an Apple Speaker or Apple Headphone based output then you’re listening to Spatial Audio.
So a very long answer, to be very specific in saying that metadata contained within the BWAV ADM will control the playback on every system to some degree.
Nathaniel Reichman: And Trim and Downmix has no impact on the sound of the AC-4 binaural mix that we hear coming from the renderer, and also on TIDAL and Amazon Music, right?
Ceri Thomas: It would to some degree, based on the fact that Off is essentially LoRo, the trim metadata would affect this. It would affect the Near, Mid and Far only in as much as the interaction between the LoRo and those render modes.
So while it’s unlikely to make an enormous difference, it would be wrong to say it has no impact. Whether it is even worth considering though will depend on how a mix has used those Render modes.
Nathaniel Reichman: Dolby has a little asterisk on Stereo Direct that says “Stereo Direct is a monitoring re-render layout, it is not a downmix setting that can be written to the master.” What do they mean by that?
Ceri Thomas: Basically, it’s not something that an encoder would be able to output so don’t rely on it for a deliverable if derived stereo is part of the plan. Example would be, if the ADM was the sole deliverable and all downstream encodes were made from it including the 2.0, then Stereo Direct would not be available to the encoder as the 2.0 to create.
The mixer can monitor the Stereo Direct as a live render, can record it as a live re-render, and can select it as an offline re-render output, so can be used as a deliverable alongside the ADM. This circumstance would be the Atmos-first workflow where a stereo asset is also needed.
Nathaniel Reichman: Thank you, Ceri, for taking the time to talk with us. So many of us in the industry are grateful for your leadership and generosity over the years. At Immersive Machines, we hear from users of Immersive Master Pro every day—and they’re all committed to pushing the boundaries and doing their best work in these evolving immersive formats.
…thanks for reading! in case you missed Trim and Downmix Part One click on News & Articles above to read the first part of this deep dive with Ceri Thomas.