How Audio Hijack sets sample rates

A sample rate represents how frequently audio data is recorded for playback through your speakers: common sample rates are 44.1 kHz (44,100 samples per second) and 48 kHz (48,000 samples per second). When using Audio Hijack, audio is captured at a fixed sample rate which is set each time a session runs.

The sample rate for an active session is displayed as part of the panel displaying information at the bottom of the session window:

Each time a session is activated, the sample rate which will be used is automatically determined, and locked in. While the session is running, this rate will not change, even if the sample rates of any sources or outputs change. When necessary, Audio Hijack will automatically resample source audio to match the session's rate.

How Audio Hijack sets the session's sample rate

To determine what sample rate to use, Audio Hijack checks all sources and outputs at the time the session is started, then applies the following rules:

  • Audio Hijack will always run at a minimum sample rate of 44.1 kHz.

  • If a System Audio block is connected to an Output Device block, Audio Hijack will check the sample rate of the selected output device. If that device is using a sample rate higher than 44.1 kHz, the session will run at that higher sample rate.

  • If an Application block is present in the session, Audio Hijack will check the sample rate of all of your Mac's output devices. The session will then run at the highest rate found among the connected devices.

  • If an Input Device source is present in the session, Audio Hijack will also check the sample rate of the selected input device, in addition to the sample rates of all output devices attached to the Mac. The sample rate of the session will then be set to the highest rate found.

Adjusting device sample rates

If you start a session and find that its sample rate is not what you expect, you'll want to check the sample rates of your connected devices. You can adjust the format of your devices with MacOS's Audio MIDI Setup utility, found in the Utilities folder of the main Applications folder. When you have the device configured as desired, restart your Audio Hijack session.

Sample rates for recordings

By default, the Recorder block follows the sample rate setting of your session automatically. However, the MP3, AIFF, and WAV recording formats offer custom sample rate settings, if desired. You can set a custom sample rate by clicking the Advanced Recording Options section, which will reveal additional sample rate options. When you select a specific sample rate for your recording, the Recorder block will resample audio as needed.

The MP3 and AAC formats will record at a maximum of 48 kHz, automatically adjusting the sample rate based on the sample rate of your session as well as the bit rate of your recording file. You can customize the sample rate of MP3 recordings, but do be aware that the bit rate of the recording dictates the available sample rates. For example, an 8 Kbps MP3 recording tops out at 24 kHz, while a 320 Kbps MP3 recording will offer custom sample rate options between 32 kHz and 48 kHz.

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