OS X Audio Playback - Setup Guide

From Benchmark

Benchmark's Guide for Audio Playback using Mac OS X

A simple guide to configure your computer for optimal audio quality

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*this article was researched using OS X 10.4.6. Some information in this article may not apply to different versions of this software, though most information can be applied to most versions of OS X. Email us to report any errors or discrepancies, or if you would like more information on this topic.

Overview: OS X Audio

Bit-transparency is possible when using OS X for audio playback, but the user must configure the settings correctly. Specifically, the sample-rate of CoreAudio must be set manually within Audio MIDI Setup. It will not automatically change according to the sample-rate of the file being played. If the sample-rate is not set to match that of the audio file being played, the sample-rate will be converted, usually causing severe distortion. Other considerations about specifics media player are important as well. Please read the article pertaining to the specific media player being used.

Audio MIDI Setup

Mac OS X has an audio control interface called Audio MIDI Setup. This can usually be found in the 'Utilies' folder (Applications -> Utilities -> Audio MIDI Setup). We recommend adding an icon for 'AudioMIDI Setup' to the 'Dock' for instant access.

The settings in 'AudioMIDI Setup' determine what device will handle your audio input and output. It also sets the sample-rate and word-length of the input and output audio. This can be troublesome, especially if you have mixed playlists with different sample-rates.

Set Word-Length and Sample-Rate

It is recommended that the word-length always be set to the highest possible value (preferably 24 bit). The sample-rate should be set to the sample-rate of the audio you are listening to. If you're not sure about the sample-rate of an audio file, check the source disk, if possible. Audio CD's almost always have a sample-rate of 44.1kHz and word-length of 16-bit.

Set Volume Controls

All digital volume controls should be set to 1.00 (under 'Value' in Audio MIDI Setup). Depending on the nomenclature, it is also known as 0.0dB, or 100%, or 'Unity Gain'. Read more about how digital volume controls affect audio.

Known Problems for Audio with Mac OSX

Fixed output sample-rate

• The output sample rate does not automatically change to follow the sample rate of the file being played.
• The user must set output sample rate to match the sample rate of the file being played. Failure to do so will introduce severe distortion due to sample rate conversion.
• Playlists with mixed sample rates cannot be played without sample rate conversion.

THD+N due to sample-rate conversion and word-length truncation

We measured the distortion of a MacBook playing audio with iTunes through the optical (S/PDIF) output (which comes factory-built into the computer). The measurements indicate not only OS X performance, but iTunes as well. These distortion measurements illustrate how important it is to properly configure your software.

A 16-bit 10 kHz sine wave played through iTunes on OSX 10.4.6, without any sample-rate conversion or word-length reduction, and with the iTunes volume control set at 100%. Notice there is no audio energy, aside from the fundamental 10 kHz, above the noise floor.

A 24-bit 10 kHz sine wave played through iTunes on OSX 10.4.6, with sample-rate conversion from 48kHz to 44.1kHz, and with the word-length truncation (rather then dithered) from 24-bit to 16-bit. iTunes volume control was set at 100%. All audio energy outside of 10 kHz is due to distortion.

This image is a graph of the optical (S/PDIF) audio output of the Macbook while a test tone is played through iTunes. The audio test tone is a dithered, pure 10kHz, -1dBFS signal, with a sample-rate of 44.1kHz and word-length of 16-bit. The optical output is set to 44.1kHz and 16-bit; therefore, the signal is NOT sample-rate converted NOR word-length reduced. When the 44.1kHz, 16-bit test tone is played with proper audio settings through the optical output, the digital audio is bit-transparent (perfect).

This image is a graph of the optical (S/PDIF) audio output of the Macbook while a test tone is played through iTunes. The audio test tone is a dithered, pure 10kHz, -1dBFS signal, with a sample-rate of 48kHz and word-length of 24-bit. The optical output is set to 44.1kHz and 16-bit; therefore, the signal is sample-rate converted and word-length reduced (iTunes truncates to 16-bit, but ideally it should be dithered). The distortion and noise in this illustration defers from that in the illustration to the left because of sample-rate conversion and word-length truncation. With proper handling of the digital audio signal, the digital output should measure almost as good as the illustration to the left.