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.
The Nyquist Frequency is the highest frequency that can be represented by a digital system. It is exactly 1/2 of the sample rate. Frequencies above the Nyquist Frequency will be incorrectly represented in the digital system. Frequencies above the Nyquist Frequency alias (or fold back) into the region below the Nyquist frequency. A frequency 1 kHz above the Nyquist Frequency will fold back to 1 kHz below the Nyquist Frequency. Likewise a frequency 2 kHz above the Nyquist Frequency will fold back to 2 kHz below the Nyquist Frequency.
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.
Windows 7, like Windows Vista, requires the user to set the sample-rate for streaming audio. In contrast, Windows 2000 and XP simply streamed at the sample-rate inherent to the audio file and/or media player being used.
In our testing, we found evidence that Windows 7 converted the audio to the sample-rate that is set in the 'Properties' menu for the playback device, unless the rate is set to the sample-rate of the audio being played. However, the sample-rate conversion within Windows 7 performs extremely well, causing no detectable amounts of distortion (below -140 dBFS).
Unfortunately, certain computers seem to have trouble maintaining a steady USB stream when running Windows 7 (even computers that worked perfectly with XP and/or Vista). We have found that turning off the 'energy saving' options will reduce the amount of interruptions in the USB stream.
This article will guide you in properly configuring your operating system and media player.