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AMS (Audio MIDI Setup), Audio Devices section
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Apple’s AMS (Audio MIDI Setup) utility is where you make OS-related settings for your audio and MIDI devices. You can find it in Macintosh HD>Applications>Utilities.

On top of the AMS screen you will find buttons for “Audio Devices” and “MIDI Devices”. Clicking on either one allows you view and edit either audio or MIDI settings.

Audio Devices section:

Any settings you make in this screen will apply to any audio software you are running on your computer. However, if an audio application allows you to select the audio device to use from within the software and you change the application’s default audio settings, this usually overrides settings made in AMS. When leaving these settings unaltered within the application it should use the AMS settings. Many basic audio applications in MacOS X do not have their own audio settings window, which is where AMS comes in as an important configuration tool.

There are four pull-down menus here, all of which allow you to select audio devices. Here is why and what every one of them does:


Default Input (upper left):
Selects the input device software applications will be using.


Default Output (upper right):
Selects the output device software applications will be using.


System Output (second from top, right):
Selects which audio device the Operating System sends alert sounds to.


Properties For (second from top, left):
Selects which device’s setting options will be displayed in the lower half of the AMS window. These settings can be made for any device, regardless of what’s selected for Default Input- or Output, so always make sure to select the correct device here first.


Lower half of AMS – optional device settings:
Not all fields are available for all devices. If displayed functions are “grayed out”, the device either doesn’t have this functionality, or has its own set of controls for it – for example through the device’s own control panel (e.g. volume settings, mute, etc.).


Clock Source:
If your audio device has a digital input, you can switch between internal- and external word clock sync here.


Configure Speakers button
If you selected an audio device with more than a stereo output, this button will become available. Clicking it allows you to configure the speaker configuration to make sure the correct signal is being sent to the correct speakers. When clicking this button for the first time, a Stereo Speaker set will be active. To configure surround speaker setups, click on the “Multichannel” button and select the desired speaker set type, e.g. “5.1 Surround”. In MacOS 10.3.6 or higher, you need to do this at least once before any signal can be sent to any other channels than front L/R. Click “Apply” to make the changes active, and “done” to close this window. Please note that some M-Audio products (e.g. Revolution Series, Sonica Theater) require you to select a surround speaker set in the device’s control panel in addition to the settings made in AMS.


Format (Input side, left):
Allows you to set the input Sample Rate, Bit Depth and number of input channels for the selected device.


Format (Output side, left):
Allows you to set the output Sample Rate, Bit Depth and the number of output channels for the selected device. Additionally, if your audio device has a digital output and supports AC-3 or DTS streaming, a setting named “Encoded Digital Audio” will be available here. This is used for example when playing surround encoded DVDs. In most cases, the DVD player application will set this automatically and you don’t have to change any settings. In fact, attempting to change these settings with an audio application already open may cause the application to crash.


Volume / Mute control field:
If your audio device supports this AMS feature and/or doesn’t have its own controls for volume/mute in its control panel, you will be able to set the individual channel levels and/or mute them here.


Additional notes:
Some of the settings in AMS mirror settings made in another (simpler) configuration tool within MacOS X, the “Sound” preferences window, which can be found in System Preferences. Changing the audio Input- or Output device in either AMS or “Sound” will be reflected in both places.