HRTF Results in Lost Lows and Highs in All Sounds, even 2D

[metallicafan212]

Hi, I am a developer, part of the OldUnreal group that develops and maintains free legal licensed releases for Unreal 1998, Unreal Tournament 1999, and (most recently) Unreal Tournament 2004 (https://github.com/OldUnreal).

As part of these patches, we swapped from using the old OpenAL to soft, as we want to support as many platforms as possible, however a constant complaint we see is that people think that the audio in our patches sounds "bad" and digging into it, it's usually HRTF causing it.

Now, I understand that this issue is coming from the model used, and that headphones automatically get detected (I've looked through some of the past issues), and that we should be giving the user a way to disable HRTF directly in our settings (in the works), but the last resort should be turning off HRTF, not the first. 3D audio is awesome after all, and gives a much more immersive experience.

Now, to alleviate some of the issues, I did add code to try and disable HRTF at least on "2d" audio, for UI elements or music. It works for music, but mono sources still are missing lows and highs, as (from looking into it), they're still routed through the HRTF functions

openal-soft/alc/alu.cpp

Line 1537 in 9335813

if(!props.mPanningEnabled)

I did try disabling that (as a test, returning the direct channel setting instead of off) and it does work, but it results in much much quieter audio for those sources.

This is how I disable HRTF on "2d" audio like UI and music:

alSourceiDirect(SoundContext, sid, AL_SOURCE_RELATIVE, AL_TRUE);
alSourceiDirect(SoundContext, Source.Source, AL_DIRECT_CHANNELS_SOFT, AL_REMIX_UNMATCHED_SOFT);
alSourceiDirect(SoundContext, sid, AL_SOURCE_SPATIALIZE_SOFT, AL_FALSE);

What option(s) do we have here? Do we just need to add a equalizer to 2d mono sources, or all sources when HRTF is enabled?

We're trying to not fork and ship modified versions as that just means another library in the long list we have to maintain (and there's issues with linux if we do that), so we would either like to work with you to fix the issue here, or work in our codebase to alleviate it for our games.

If needed, I can upload examples. I usually use my personal project patch (HP2) to prototype stuff like audio before going back to our patches, so I've been playing around in there and have an example of what it should sound like vs what it sounds like, including the source audio files.

[kcat]

Unfortunately mono sources ignore the AL_DIRECT_CHANNELS_SOFT setting, and the AL_SOURCE_SPATIALIZE_SOFT setting only alters whether it's processed for 3D spatialization (world position panning and attenuation, velocity/doppler pitch adjustment), but it still goes through HRTF for virtual panning (which is what allows surround formats to give a virtual surround effect with headphones).

However, the latest versions do contain an in-progress extension to do 2D panning, AL_SOFTX_source_panning, which can be enabled on a mono source to have them be treated as stereo. And that interacts with AL_DIRECT_CHANNELS_SOFT to have it skip HRTF like other stereo sources. Being in-progress does technically mean it's subject to change, but at this point I don't think there needs to be any alterations beyond changing the extension name from AL_SOFTX_source_panning to AL_SOFT_source_panning (removing the X to indicate it's no longer experimental).

In alc/inprogext.h, there is:

#ifndef AL_SOFT_source_panning
#define AL_SOFT_source_panning
#define AL_PANNING_ENABLED_SOFT                  0x19EC
#define AL_PAN_SOFT                              0x19ED
#endif

which you can copy out to your own sources to use. And after ensuring either AL_SOFTX_source_panning or AL_SOFT_source_panning is supported, you can set it:

alSourceiDirect(SoundContext, Source.Source, AL_PANNING_ENABLED_SOFT, AL_TRUE);
alSourceiDirect(SoundContext, Source.Source, AL_DIRECT_CHANNELS_SOFT, AL_REMIX_UNMATCHED_SOFT);

You can leave AL_PAN_SOFT at 0 to keep the sound center-panned. Though if you do want to pan it left or right, you can specify a float value between -1 to +1, which applies an additional left- or right-side gain on top of the source gain, with the calculation effectively being:

LeftGain = AL_PAN_SOFT > 0.0f ? 1.0f - AL_PAN_SOFT : 1.0f;
RightGain = AL_PAN_SOFT < 0.0f ? 1.0f + AL_PAN_SOFT : 1.0f;

[metallicafan212]

Thank you for the quick response! Going to try this now, seeing in the source code that this only applies if the format is mono, so I don't need to add any additional code on my side to only do this on mono sources. Need to recompile my "universe" with a unmodified soft (gotta love vcpkg...).

But still: for 3d audio, is there no way to compensate for the HRTF losses? Is the reduction in range just unavoidable? I saw that you mentioned that you can use different models that may not have that issue, we're in the same issue as you (licensing, and then we would have to ship that model with our patch), but is there a known compensation we can apply?

I thought about just recording two samples: sweeping waveform with HRTF off, sweeping waveform with it on, compare the output and then just EQ adjust all sources, but I don't know if that would be applicable to all sounds in all instances (given with 3d...). I'm not an audio expert unfortunately, just a hobby volunteer that knows more about Unreal Engine than is probably healthy.

[metallicafan212]

That works, thanks! Only issue is that it feels like mono sources are quieter in that path than with HRTF on....

I'll record and measure the output on a single sound to double check.

[metallicafan212]

Ok I think my ears were just playing tricks on me, the output looks fine! Will send to one of my testers as a blind test and see how he takes it.

Still leaves the 3D audio issue, but this at least solves the most "glaring" issue that people have (they notice it in the menus immediately when the game starts, at least with 3D audio it's in the world space and covered up by a lot of stuff going on at the same time).

[kcat]

But still: for 3d audio, is there no way to compensate for the HRTF losses?

The only real way is to use a different HRTF that better balances the bass and treble. I have thought about switching to one of the sadie datasets SteamAudio uses, though I'm not sure. sadie_h12 is smaller in size but less dense than the current (fewer directional impulse responses, so there will be more apparent blending for sounds in various directions), while sadie_d1 is larger in size than current, and it has more horizontal resolution and less vertical resolution. That they also sound different adds complication to deciding which, if either, would be better to use.