Rounding Error Setting Volume Using Sndio Output

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Introduction

The Music Player Daemon (mpd) is a popular open-source music player that allows users to manage and play music from various sources. However, a bug was discovered in the sndio output plugin, which causes the volume to not increase when using the +1 command. This article will delve into the issue, its expected behavior, and the actual behavior, as well as provide a solution to fix the problem.

Bug Report

The bug was first discovered with the ncmpc client, where the volume would not increase when using the +1 command. This was due to the raw_volume value not being updated correctly.

Describe the Bug

The bug was caused by the SndioOutput::SetVolume function, which uses the sio_setvol function to set the volume. However, the sio_setvol function takes an integer value, which is then multiplied by SIO_MAXVOL / 100 to calculate the actual volume. This can cause rounding errors, especially when the volume is not a multiple of 100.

void
SndioOutput::SetVolume(unsigned int volume)
{
	sio_setvol(hdl, volume * SIO_MAXVOL / 100);
}

Expected Behavior

The expected behavior is that when the user uses the +1 command, the volume should increase by 1%. However, due to the rounding error, the volume remains the same.

Expected Output

mpc volume 50 C418 - Subwoofer Lullaby [playing] #1/1 1:16/3:29 (36%) volume: 50% repeat: on random: off single: off consume: off

mpc volume +1 C418 - Subwoofer Lullaby [playing] #1/1 1:20/3:29 (38%) volume: 51% repeat: on random: off single: off consume: off

Actual Behavior

The actual behavior is that when the user uses the +1 command, the volume remains the same.

Actual Output

mpc volume 50 C418 - Subwoofer Lullaby [playing] #1/1 1:16/3:29 (36%) volume: 49% repeat: on random: off single: off consume: off

mpc volume +1 C418 - Subwoofer Lullaby [playing] #1/1 1:20/3:29 (38%) volume: 49% repeat: on random: off single: off consume: off

Version

The version of mpd used to reproduce the bug is 0.23.17.

Solution

To fix the bug, we need to modify the SndioOutput::SetVolume function to use a more precise method to calculate the volume. One way to do this is to use a floating-point number to represent the volume, rather than an integer.

void
SndioOutput::SetVolume(float volume)
{
	sio_setvol(hdl, volume * SIO_MAXVOL);
}

We also need to modify the mpc client to send the volume as a floating-point number instead of an integer.

void
mpc_volume_up(void)
{
	float volume = mpc_get_volume() + 0.01f;
	mpc_set_volume(volume);
}

By making these changes, we can fix the rounding error and ensure that the volume increases correctly when using the +1 command.

Conclusion

Introduction

In our previous article, we discussed the bug in the sndio output plugin that causes the volume to not increase when using the +1 command. We also provided a solution to fix the problem by modifying the SndioOutput::SetVolume function to use a floating-point number to represent the volume. In this article, we will answer some frequently asked questions (FAQs) about the bug and its solution.

Q: What is the cause of the bug?

A: The bug is caused by a rounding error in the SndioOutput::SetVolume function. The function uses the sio_setvol function to set the volume, which takes an integer value. However, the volume is calculated by multiplying the input value by SIO_MAXVOL / 100, which can cause rounding errors, especially when the volume is not a multiple of 100.

Q: Why does the bug only occur when using the +1 command?

A: The bug only occurs when using the +1 command because the mpc client sends the volume as an integer value. When the volume is increased by 1, the integer value is not updated correctly, causing the rounding error.

Q: How can I fix the bug?

A: To fix the bug, you need to modify the SndioOutput::SetVolume function to use a floating-point number to represent the volume. You also need to modify the mpc client to send the volume as a floating-point number instead of an integer.

Q: What are the benefits of using a floating-point number to represent the volume?

A: Using a floating-point number to represent the volume provides more precise control over the volume level. This is especially important when working with audio applications that require precise control over the volume.

Q: Can I use a different method to fix the bug?

A: Yes, you can use a different method to fix the bug. One alternative is to use a library that provides a more precise method for calculating the volume. However, modifying the SndioOutput::SetVolume function to use a floating-point number is a simple and effective solution.

Q: Will this fix affect other parts of the system?

A: No, this fix will not affect other parts of the system. The SndioOutput::SetVolume function is only used by the sndio output plugin, and modifying it will not affect other plugins or parts of the system.

Q: Can I apply this fix to other versions of mpd?

A: Yes, you can apply this fix to other versions of mpd. However, you may need to modify the SndioOutput::SetVolume function to use a floating-point number in those versions.

Conclusion

In conclusion, the bug in the sndio output plugin was caused by a rounding error in the SndioOutput::SetVolume function. By modifying the function to use a floating-point number to represent the volume, we can fix the bug and ensure that the volume increases correctly when using the +1 command. We hope this Q&A article has provided helpful information for those who are experiencing this issue.