Mac OS X’s Volume keys: What exactly do they control?

Posted by Pierre Igot in: Macintosh
November 28th, 2006 • 3:24 pm

This is a problem that has bugged me for a long time in Mac OS X, and it shows no sign of going away.

Because I don’t like hearing alert sounds and various other system sounds while I am listening to music, I use the following audio set-up: I have a pair of Apple speakers connected to my PowerWave USB audio device, and a pair of headphones connected to my Mac’s headphones jack on the front panel.

In Mac OS X’s System Preferences, in the “Sound” preference pane, I choose “Headphones” as the output channel in the main “Output” tab (so that iTunes and other applications using audio play through the headphones), and then, in the “Sound Effects” tab, I choose “PowerWave Composite” as the channel to play alerts and sound effects through.

This way, when I am listening to music in iTunes with the headphones, if there is a system alert (like Mail receiving messages, etc.), it doesn’t interfere with the music and plays through the external speakers connected to the PowerWave instead.

In such a set-up, however, the question is: What do the Volume keys on the keyboard control? Do they control the overall volume level of audio playing through the headphones, or do they control the volume level of the alert sounds playing through the speakers?

Given that the “Sound Effects” tab in the “Sound” preference pane has its own volume slider labelled “Alert volume,” which is separate from the main volume slider labelled “Output volume,” you would think that it would be straightforward: the Volume keys on the keyboard should control the Output volume” slider, i.e. the overall volume level of the audio played through the main output channel, i.e. the headphones in my case.

But now consider this screen shot:

Volume levels

Do you see what I see? In this screen shot, the volume level as indicated by the slider at the bottom of the preference pane is pretty high, but the volume level when I press the Volume keys, which is indicated by the translucent volume indicator that appears temporarily on top of all windows while I press the keys, is completely different!

And indeed, when I change the volume level using the Volume keys, the little “tick” sound that Mac OS X makes to indicate the volume level is actually playing through… the external speakers, i.e. the output channel that I have selected for my alert sounds. Yet, when I look at the volume slider for the alert sounds, it doesn’t move while I press the Volume keys either!

This is utterly confusing. Right now I am faced with a situation where there appears to be three different volume levels:

  1. the volume level indicated by the “Alert volume” slider
  2. the volume level indicated by the “Output volume” slider
  3. the volume level indicated by the translucent grey overlay when I press the Volume keys on my keyboard

I must admit I have no idea how I ended up with such a situation. Maybe it’s since the last power outage we had last week. I don’t know. I guess I am going to have to reboot my machine to see what happens.

Out of curiosity, I tried unplugging the PowerWave altogether. Mac OS X switched to playing everything (including alerts) through the headphones, and then the Volume keys started working again to control the overall output volume.

But then as soon as I plugged the PowerWave back in, the situation came back to what I have described above. I can understand the system being confused about the two volume levels (Alert volume and Output volume) and which volume level is controlled by the Volume keys on the keyboard. But based on the visual feedback I am getting, right now the Volume keys are not controlling either, but a third volume level that seems to come on top of the Alert volume (and has nothing to do with the overall Output volume level)!

Now, there have been periods where things have worked in a more predictable way, i.e. the Volume keys would actually control the overall Output volume level and I would hear the volume ticks through my headphones. But I have also repeatedly experienced this other situation, where there appears to be a third “phantom” volume level that has no corresponding slider in the graphic user interface for sound in Mac OS X.

Presumably the problem is related to the fact that I use two different devices (the PowerWave and the headphones) at the same time, but surely Mac OS X should be able to deal with this, especially since it has built-in support for the user of two different sound channels, one for system sounds and another one for actual audio output from applications. It also cannot be a problem with the PowerWave software, because… there is no PowerWave software. The PowerWave is a USB audio device and it uses Mac OS X’s built-in drivers to work with Mac OS X. You don’t have to install any software when you plug the PowerWave into your machine. You just plug it in, and Mac OS X recognizes it automatically.

(Incidentally, third-party applications can also be designed to play their own alert and sound effects through the appropriate device. I know this, because Evan Gross, the developer of Spell Catcher X, has designed his application to use that channel to play its application’s alerts. This way, the alerts played by Spell Catcher X don’t interfere with my listening to music with the headphones either. But of course there are other third-party products that are not that smart and play all their sounds through the main output channel, including alert sounds.)

So for now, the mystery remains. What exactly is this third volume level that my Volume keys appear to be controlling at present?

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