This article describes the process of creating an 'Aggregate Device' with the Audio MIDI Setup utility in Mac OS X computers.
Creating an aggregate device is a way to group multiple audio interfaces into one virtual device so that an audio application can speak to a single, solitary virtual audio interface.
Certain audio software cannot interface to multiple audio device drivers. In these cases, it is necessary to create an aggregate device if you wish to use multiple interfaces. For example, interfacing with Logic simultaneously via a device using optical and another device using USB requires an aggregate device to be created encompassing these two devices. This new group of devices appears as a single aggregate device to the audio application.
Instructions for creating an 'aggregate device’
Open Audio MIDI Setup
Choose "Open Aggregate Device Editor" from the 'Audio' menu located on the top menu bar (or press 'shift-command-A')
Press the '+' button to create a new aggregate device
Choose which devices you would like to interface with your audio application
Click on the 'Done' button
This new aggregate device should now be seen by the audio application.
A digital audio device achieves bit-transparency if it passes digital audio data without affecting the data in any way. This device may be hardware, software, or a combination of both.
Testing for Bit Transparency
To determine if a device is bit-transparent, it can be tested by sending a pseudo-random bit sequence through it and monitoring the digital output. We typically generate the pseudo-random sequence using an Audio Precision digital signal generator. The Audio Precision includes a digital analyzer that is programmed to detect the pseudo-random sequence produced by the generator. The analyzer detects any differences between the generated sequence and the received sequence. The number of differences is tallied by a counter. A digital channel that is bit transparent will show no differences between the transmitted and received pseudo-random sequences.
Dither is a type of intentional variation (noise) which is added to a digital audio signal to avoid distortion caused by quantization errors.
Dithering in audio
Dither is often used when an analog signal is being quantized into a finite number of digital levels. It is also often used when a digital signal is being quantized into a fewer number of bits per sample corresponding to a fewer number of digital levels.