Mix_FadeOutGroup() : "stair effect" fadeout

[SDLBugzilla]

This bug report was migrated from our old Bugzilla tracker.

Reported in version: unspecified Reported for operating system, platform: All, All

Comments on the original bug report:

On 2015-03-11 14:41:51 +0000, JosBas wrote:

I've noticed this with headphones, and now it is confirmed, as I recorded the stereo output.

Mix_FadeOutGroup() is not a normal linear fadefout, but a step function ("staircase" function): see http://gget.it/e46s3stu/5.jpg

Input: http://gget.it/uk2oi63z/sine1.wav

Output with 200ms fadeout: http://gget.it/zjh61iz2/sine1_fadeout_with_pygame.wav (You'll notice stair effect.)

Notes: *Tested on both Win7/PC and RaspberryPi.

• The "width" of the constant pieces of the staircase function is exactly the audio buffer size (1024 samples in my example).

On 2015-03-11 18:46:35 +0000, Ryan C. Gordon wrote:

This happens because we change the channel volume during fading once per callback to feed the audio device, so you get stairs like that.

At the moment, it makes no attempt to interpolate the fade between samples in a single callback, so you get a more bigger staircase the larger your sample size.

This could be fixed without changing the API, but there aren't plans to do so at the moment (patches are welcome, though!). I'll leave this bug open for now, though.

--ryan.

On 2015-03-11 18:56:47 +0000, JosBas wrote:

Yes, the problem is that the volume change is just a "multiplication by a constant number" for each call of the callback. If B[i] is the current buffer (0<=i<=n), currently this is what happens: Out[i] = B[i] * k , and k will decrease only for next callback call.

Instead we could multiply the buffer by a linear function :

Out[i] = B[i] * L[i] where L[i] is a linear decreasing function.

@RyanC.Gordon, can you point me in which file exactly this code happens (I don't know SDL source at all), I can maybe try to write a patch.

Thanks.

On 2015-03-14 19:18:26 +0000, Philipp Wiesemann wrote:

In SDL_mixer source it is "mixer.c" which uses SDL_MixAudio() from SDL.

In SDL source it is "src/audio/SDL_audio.c" for SDL_MixAudio() and "src/audio/SDL_mixer.c" for SDL_MixAudioFormat().

SDL_MixAudioFormat() applies the volume and would need to be copied or changed.

On 2015-08-10 08:31:53 +0000, Yash wrote:

Hi JosBas

You found a solution for this ?

On 2017-10-21 19:24:11 +0000, Sam Lantinga wrote:

Yeah, the fading should really be an interpolated volume application.