Mono or Stereo for SFX?

[The Almighty Red]

So, something I'd been thinking on for a few days, and was unable to find anything specific regarding this (unless I overlooked it) and thought I would ask.
Is there any significant reason to use mono over stereo when implementing sound effects? I have noticed some mods with custom sounds, despite being very high quality sounds, are still in mono when I have found the same ones available in stereo.
I personally have noticed a difference when the sound is played in GZ where the stereo sounds a little more like it's coming from the actual source as opposed to the flat mono. So basically my question comes down to, are there any drawbacks to using stereo over mono that possibly effect how the sounds are played in the engine, or is it just a case of I should "just go with what sounds better"?

[Nash]

For stereo sounds, GZDoom will forcefully downmix them to mono if the sounds are originating from a physical sound source in the world (for example - sounds coming from monsters (with the exception of boss monsters), sounds coming from the ambient sound things, etc). You'd be wasting file space making such sounds in stereo.

If a stereo sound originates from a currently-viewed camera - AKA a "head" sound (for example - player's gun firing sounds, UI sounds) - they will be played stereo, for as long as you are viewing said camera. But the same stereo gun sound, when originating from another player (in multiplayer) - they will, again, be downmixed to mono.

tl;dr: if a sound is intended to be played from an in-world sound source, use mono. For player gun sounds and UI sounds, they should definitely be in stereo.

[Nash]

I personally have noticed a difference when the sound is played in GZ where the stereo sounds a little more like it's coming from the actual source as opposed to the flat mono

Sounds originating from objects in the map will always be downmixed to mono. Do you, by any chance, have HRTF enabled? HRTF does some fancy trickery to make sounds sound more "rich" - positional sound sources, despite being mono (as far as the sound file is concerned), do sound a bit more "spacious" when HRTF is enabled.

[The Almighty Red]

> [quote]
> Sounds originating from objects in the map will always be downmixed to
> mono. Do you, by any chance, have HRTF enabled? HRTF does some fancy
> trickery to make sounds sound more "rich" - positional sound
> sources, despite being mono (as far as the sound file is concerned), do
> sound a bit more "spacious" when HRTF is enabled.
> [/quote]

I have HRTF on "auto" which I think is the default. To my recollection I have not changed that setting, but you're saying the sound being more "rich" is more to do with the HRTF setting effecting the sound and being stereo kind of amplifies it?

[Nash]

Are you listening through speakers, or a headphone? What I've found is that, when HRTF is set to "Auto" (and yes, you are right, it is indeed the default) - it does exactly what it says on the tin - on normal stereo speakers, the HRTF doesn't kick in, but when I plug in a headphone, the HRTF kicks in.

[The Almighty Red]

> Are you listening through speakers, or a headphone? What I've found is
> that, when HRTF is set to "Auto" (and yes, you are right, it is
> indeed the default) - it does exactly what it says on the tin - on normal
> stereo speakers, the HRTF doesn't kick in, but when I plug in a headphone,
> the HRTF kicks in.

Ah, okay, this makes a whole lot more sense to me now. I generally use headphones and rarely use speakers while editing so I had not noticed the sound playing without HRTF. It seems to me like the HRTF through headphones in combination with stereo SFX is making them sound more vibrant but it's not really true stereo; it's just the HRTF making the sound (despite still being downscaled to mono) sound more 3D. The stereo is just making it sound a bit cleaner and crisper.

[Chris]

If the sound is the same for the left and right stereo channels, there's not much reason for it to be stereo. It's wasting memory and CPU resources by needlessly mixing two channels to play the same sound and simulate a single channel. Having a stereo sound auto-downmixed to mono for "world" sounds may also be slightly louder than intended since it's actually two channels playing the same sound together from the same location, instead of one sound from one location. And given surround sound or HRTF playback, for a "head" sound (that's played from the viewer's position, area/boss sounds, etc), there is an audible difference between a mono sound in the center position, and a stereo sound creating a phantom mono sound in the center position. The former is clearer and sharper, while the latter creates more crosstalk (with the former, a single sound reaches both ears at the same time, while with the latter, the left channel reaches the left ear first followed by the right ear, while the right channel reaches the right ear first followed by the left ear, creating a subtle echo and related phase interference).

If the left and right stereo channels aren't copies of each other, then whether or not to use stereo depends on whether the sound is intended to be a "head" sound. If not, then it's always going to be downmixed anyway, so it's better off as mono to save space and CPU resources. The downmix you do in your audio editor can produce the same result as downmixing it in real-time (or perhaps better, depending on your editor's capabilities). If it is intended to be a "head" sound at least some of the time, and provides a useful stereo effect, then it may be fine to keep as stereo.

[The Almighty Red]

If the sound is the same for the left and right stereo channels, there's not much reason for it to be stereo. It's wasting memory and CPU resources by needlessly mixing two channels to play the same sound and simulate a single channel. Having a stereo sound auto-downmixed to mono for "world" sounds may also be slightly louder than intended since it's actually two channels playing the same sound together from the same location, instead of one sound from one location.

The sounds in question I had been using are true stereo and not a doubled up mono sound. I don't bother using "stereo" if they're just duplicate mono tracks in a stereo format because as you pointed out, it would be entirely moot.

And given surround sound or HRTF playback, for a "head" sound (that's played from the viewer's position, area/boss sounds, etc), there is an audible difference between a mono sound in the center position, and a stereo sound creating a phantom mono sound in the center position. The former is clearer and sharper, while the latter creates more crosstalk (with the former, a single sound reaches both ears at the same time, while with the latter, the left channel reaches the left ear first followed by the right ear, while the right channel reaches the right ear first followed by the left ear, creating a subtle echo and related phase interference).

If the left and right stereo channels aren't copies of each other, then whether or not to use stereo depends on whether the sound is intended to be a "head" sound. If not, then it's always going to be downmixed anyway, so it's better off as mono to save space and CPU resources. The downmix you do in your audio editor can produce the same result as downmixing it in real-time (or perhaps better, depending on your editor's capabilities). If it is intended to be a "head" sound at least some of the time, and provides a useful stereo effect, then it may be fine to keep as stereo.

So if I am to understand this correctly, what it comes down to is, if it is a genuine stereo sound and it is being played as a head sound it has a significant effect and can be utilized for a more crisp, rich stereo sound; however, if it is not a head sound it will make absolutely no difference as it will always be played as a center mono regardless of the sound channels?

[Chris]

So if I am to understand this correctly, what it comes down to is, if it is a genuine stereo sound and it is being played as a head sound it has a significant effect and can be utilized for a more crisp, rich stereo sound; however, if it is not a head sound it will make absolutely no difference as it will always be played as a center mono regardless of the sound channels?

Basically, yeah. For non-head sounds, it will be played as a panned mono sound, since both channels will pan together toward the sound source.

If the sound is sometimes a head sound and sometimes a world sound (e.g. weapon sounds that the player can fire as a head sound, and enemies or other players can fire as a world sound), it's fine to leave as stereo and let it be dynamically downmixed as needed. But if it's always a world sound, there's no point since it'll always downmix.

[The Almighty Red]

So if I am to understand this correctly, what it comes down to is, if it is a genuine stereo sound and it is being played as a head sound it has a significant effect and can be utilized for a more crisp, rich stereo sound; however, if it is not a head sound it will make absolutely no difference as it will always be played as a center mono regardless of the sound channels?

Basically, yeah. For non-head sounds, it will be played as a panned mono sound, since both channels will pan together toward the sound source.

If the sound is sometimes a head sound and sometimes a world sound (e.g. weapon sounds that the player can fire as a head sound, and enemies or other players can fire as a world sound), it's fine to leave as stereo and let it be dynamically downmixed as needed. But if it's always a world sound, there's no point since it'll always downmix.

Great! This is exactly the information I needed. Thank you everyone!