LFE 10dB attenuation and boost

[Beber]

Hey, guys.

Is this common knowledge? While discussing multichannel audio editing in DaVinci Resolve with Gemini, this came up:

When editing audio tracks, there's an invisible digital trap regarding the LFE channel. If you do not correct it during your editing phase, your final export will lack punch, and your subwoofer will sound completely flat. Here is why it happens and how to fix it.

1. Why is there a -10dB difference?
- In movie theater / mixing stages: to prevent extreme sub-bass frequencies from clipping and distorting the digital master track, the industry standard (Dolby and DTS) dictates that the LFE channel is recorded on the disc 10dB lower than the other 5 channels.
- On playback: your home theater AV Receiver automatically detects the movie track and boosts the subwoofer output by +10dB to restore the correct theatrical balance. Everything sound perfectly heavy.

2. The trap in video editors (DaVinci Resolve, Premiere, etc.)
When you extract a DTS-HDMA track to uncompressed multichannel PCM (WAV) using tools like Shutter Encoder or eac3to, you get raw digital audio streams. When you import these 6 separated mono tracks into your timeline, your video editing software does not know track 4 is an LFE track. It treats it like a regular audio track. If you edit, crossfade and export your project as-is, that 10dB attenuation becomes permanent in your new file. Your final AV Receiver will not boost it, and your mix will lose its cinematic rumble.

So basically, when editing a track you ripped from a DVD or Blu-ray that you converted to PCM to work on, you must take into account that you'll have to boost the LFE channel by +10dB.

[LucasGodzilla]

2. The trap in video editors (DaVinci Resolve, Premiere, etc.)
When you extract a DTS-HDMA track to uncompressed multichannel PCM (WAV) using tools like Shutter Encoder or eac3to, you get raw digital audio streams. When you import these 6 separated mono tracks into your timeline, your video editing software does not know track 4 is an LFE track. It treats it like a regular audio track. If you edit, crossfade and export your project as-is, that 10dB attenuation becomes permanent in your new file. Your final AV Receiver will not boost it, and your mix will lose its cinematic rumble.

So basically, when editing a track you ripped from a DVD or Blu-ray that you converted to PCM to work on, you must take into account that you'll have to boost the LFE channel by +10dB.

Technically from a metadata level, if you re-export the mix in a format that interprets an LFE channel correctly, then it should be fine when playing the audio back through a proper decoder? This would apply to PCM, DTS-HD, TrueHD, etc (just about any audio format frankly).

However, that said, I personally have been operating on the assumption that if you are working on surround downmixes, then technically that's where the attenuation comes into play.

For example: Since Dolby Atmos drops the LFE entirely in a binaural bounce, I just mix it in as a front-center channel / object and attenuate all the other channels by -10 dB to avoid clipping.

[Beber]

The examples I gave Gemini that made it came up with this were my intentions to edit the DTS HDMA tracks of Sin City to create the 5.1 mix of my extended cut project, and my intent to make a hybrid cut of the CDS mix and the DTS HDMA of T2 Special Edition. So we're dealing with 5.1, and I'll try to work on those in DaVinci Resolve 15 free version which doesn't go beyond 5.1, I believe. Apparently, according to Gemini, my workflow that consists in transcoding these original tracks to PCM so that I can work on them in Resolve requires to boost the LFE channel by +10dB for the reasons explained above. The idea being to export my work in PCM 5.1 and potentially transcode that to FLAC 5.1 to gain space.

[Beber]

Okay, so I just had Gemini elaborate on the subject: this whole thing only applies if your target is PCM, FLAC, AAC and such. However if your target is Dolby or DTS, proprietary theatrical codecs, the encoders know all this and are dealing with it on their own, so you definitely don't need and you definitely should not do it if you go for, say, a DTS-HDMA final rendering, or you'll get a double-boosted LFE output on your AV Receiver. That was an important precision. Lucas was right when mentionning metadata. It's all about flags that dictates the AV Receiver to boost a Dolby or DTS LFE channel, flags that are not part of PCM, FLAC, AAC and such, hence the AV Receiver not triggering an automatic boost on the LFE and the necessity to apply it on your own in the editing process. So to decide to apply it or not, you need to know what final codec your audio track will be rendered in.

[commandrbond]

Hmm. I just imported a DTS-HD MA 5.1 track into Audacity. The LFE channel is sitting only 3db below 0. Boosting by 10db before exporting to say, FLAC, would cause bad clipping.

[Beber]

Hmm. I just imported a DTS-HD MA 5.1 track into Audacity. The LFE channel is sitting only 3db below 0. Boosting by 10db before exporting to say, FLAC, would cause bad clipping.

I'm asking Gemini. I'll get back to you.

[Beber]

## 💡 Clarifying the DTS-HD MA Import in Audacity & The FLAC Export Conundrum
To answer your concern regarding clipping and the nature of the DTS-HD MA source file: You are 100% correct about the clipping risk, but the fact that the source is a DTS-HD MA track does not make the -10dB LFE rule obsolete.
Here is the breakdown of what is happening under the hood in Audacity, and why your final FLAC will still suffer from the -10dB drop if left uncorrected.
## 1. The "Invisible" PCM Transcoding inside Audacity
DTS-HD Master Audio is a proprietary, encoded "wrapper" (essentially like a .zip file). No audio editing software—including Audacity or DaVinci Resolve—can display waveforms or process audio directly inside a raw encoded DTS stream.
When you drag and drop your DTS-HD MA file into Audacity, the software utilizes its FFmpeg libraries to instantly decode and expand the track into raw, uncompressed linear PCM in the background. The 6 visual tracks you see on your screen are no longer DTS; they are pure PCM. At this very moment, the proprietary DTS metadata envelope and its automatic cinema instructions are stripped away.
## 2. The Final Destination: The FLAC Export
Since your goal is to export your project into a FLAC 5.1 container (which is an open, generic IT audio format), the final file will carry no Dolby or DTS licensing "flags".
When you play this FLAC 5.1 file through a media player (like a Zappiti, Zidoo, or PC) hooked up to your AV Receiver, the receiver will detect a generic multi-channel LPCM/FLAC stream. It will completely bypass its internal automatic "+10dB LFE Movie Boost" and apply a flat 0dB gain across all channels. Because the LFE track was stored with that built-in -10dB studio attenuation, your subwoofer will sound 10dB too quiet on playback.
## 3. How to solve the clipping issue?
You are absolutely right about the math: if your LFE track peaks at -3dB and you apply a brute-force +10dB gain, you will hard-clip the digital ceiling (exceeding 0dBFS) and destroy the track with nasty digital distortion.
To circumvent this, you have the two standard preservation workflows used by restoration archivists:

* The Software Fix (with safe peak management): If you want the +10dB boost baked into the FLAC file for plug-and-play convenience on media players, do not just raise the volume slider. Apply a Limiter effect in Audacity. Set the Hard Limit ceiling to -0.1 dB and apply a +10dB input gain. The limiter will safely raise the overall volume of the low-end environment by 10dB while cleanly taming the few extreme heavy peaks (the explosions) without a single bit of digital clipping.
* The Pure Hardware Fix (No digital alterations): If you want to keep the bitstream data absolutely uncompressed and untouched, do not apply the +10dB boost in Audacity. Export your FLAC with the LFE channel peaking at -3dB. However, you must add a disclaimer for the users of your fan-edit: they will need to manually engage a +10dB LFE offset directly in their AV Receiver settings (via a Quick Select preset or HDMI input level) when watching your release.

[Adis]

+10 dB boost in AV Receiver makes sense if Bass Management is activated - you need LFE program 10 dB above the redirected bass program from "small channels" on LFE output (cinch/jack/xlr) - and is optional.
My 2c.

[Beber]

+10 dB boost in AV Receiver makes sense if Bass Management is activated - you need LFE program 10 dB above the redirected bass program from "small channels" on LFE output (cinch/jack/xlr) - and is optional.
My 2c.

Gemini 100% agrees with you and also said that modern AV Receivers have the capability to differentiate both kinds of bass (the actual LFE and the small channels bass) so that only the LFE ones are boosted even when the subwoofer is assigned to deliver both kinds of bass. But that only happens automatically on a standardised profesionnal codec such as Dolby and DTS and not with PCM or FLAC since they don't include "flags" to trigger that boost, which makes all the more important to boost the LFE channel by 10dB on your own directly at the source when you edit your audio track in a PCM/FLAC workflow.

[stwd4nder2]

This topic has come up before, but has anyone ever recording the output of their AVR to see how different codecs are decoded? Seems like an easy way to get a definitive answer.