XO Wave: Resolution in XO Wave (Technical Note)

This technical note briefly describes XO Wave's approach to digital audio sample resolution. XO Wave has two audio engines, a 32-bit audio engine, which uses IEEE 754 32-bit floating point arithmetic, and (in XO Wave Pro) an optional 64-bit audio engine, which uses IEEE 754 64-bit floating point arithmetic, also known as "double precision" floating point. The term "internal format" will be used below to refer to these data types.

Dither is used by default when resolution is reduced. These cases are described below. See our technical note Dither in XO Wave for more details on the dither used in XO Wave.

QuickTime is used for a number of conversions on the Macintosh platform. For example, importing an MP3 file into XO Wave requires conversion to an uncompressed format (AIFF or WAV) using QuickTime. All QuickTime conversions are done entirely by the QuickTime libraries, not by XO Wave, and therefore may produce files that have been reduced in resolution, or even truncated. To import high resolution audio into an XO Wave session, it is best to use AIFF, AIFC or WAV file formats because they do not need to be converted. For QuickTime output, an effort is made to deliver the highest possible resolution to QuickTime (which may then perform its own conversion).

Input: XO Wave converts all incoming data into its own internal format before doing any processing. This includes source files as well as input from sound devices. No arithmetic operations are performed on incoming data streams before this conversion takes place. Although this generally results in bit-transparency and minimal truncation and noise, there are some (very unusual) circumstances when bit-transparency cannot be guaranteed. For example, a 32-bit input file cannot be processed transparently using 32-bit floating point internal format.

Intermediates: In general, all intermediates are maintained using at least internal format. In some cases, such as in recursive filters, some values are stored at even higher resolution, because of the sensitivity to cumulative noise on these values. During normal operation, XO Wave does not produce low resolution intermediates for any purpose. All cross fades, effects, automation and other processing is performed in real-time using at least internal format. The only exception to this is when a file's format needs to be converted as it is being imported into XO Wave by Apple's QuickTime library.

Output (Playback): XO Wave normally passes 32-bit (single-precision) floating point numbers to the operating system or sound card driver. Because the final resolution and bit-reduction method used by the operating system and sound-card is often unknown, and because there may be further processing of the audio after it is output by XO Wave (for example, XO Wave's output may undergo sample rate conversion or be combined with other sounds), output dither can be controlled explicitly in the Audio Hardware window. The default dither is appropriate for 16-bit output, and is also generally appropriate for simulating 16-bit output on higher resolution hardware. Possible settings are 16-bit, 8-bit and off.

Output (Export): For output to QuickTime formats, XO Wave delivers internal format audio to QuickTime, without dither, since pre-dithering is not appropriate for most commonly used QuickTime codecs. 8- and 16-bit AIFF and WAV files are, by default, appropriately dithered before bit-rate reduction. 24- and 32-bit files do not require dither.

Output (CD Mastering): CD Masters, like 16-bit AIFF exports, default to dither appropriate for 16-bit data.