How to check bit-perfect playback on a Mac

Why a Mac resamples

Core Audio mixes every app into one stream per output device, at the device’s current sample rate. Whatever rate that is, 44.1, 48 or 96 kHz, every file that does not match is converted on the way. Apple’s converter is excellent, so this is not a disaster, but it means the DAC is no longer receiving the samples in your file.

Check the device rate

1. Open Audio MIDI Setup (Applications → Utilities, or Spotlight).
2. Select your output device in the left column: the built-in speakers, headphones or your USB DAC.
3. Read Format on the right, for example “2 ch 24-bit Integer 48.0 kHz”.
4. Now play a file and compare. A CD rip is 44.1 kHz; hi-res downloads are usually 96 or 192. If the Format still says 48 kHz, the Mac is resampling.

Many USB DACs also show the incoming rate on a display or a coloured light. That is the most direct check there is.

The manual fix

You can set the Format by hand to the file’s rate before you play it. It works, but only for one rate at a time: an album at 44.1 followed by one at 96 means changing it again, and forgetting means resampling.

The automatic fix

A player can set the device rate itself. MP95 for Mac does it for every track: it reads the file’s rate, picks it from the list the device supports and switches the device through Core Audio. Watch Audio MIDI Setup while it plays and you will see the Format change between albums. When you quit, MP95 puts the original rate back.

Bit depth and volume

Sample-rate matching is one check, not a complete bit-perfect test. EQ, effects, ReplayGain, headphone correction, volume scaling and system processing can alter the samples. Check the actual output information and the active processing controls.

A player’s status label describes what it can observe. This article does not include an independent digital capture proving every supported Mac and DAC combination.

One catch: DACs without a volume

When an output has no hardware volume control, attenuation may happen digitally. That can change samples even if the sample rates match. Do not turn an unfamiliar headphone or amplifier setup to full volume merely to pursue a bit-perfect label. Set a safe listening level and identify where volume is controlled.