Audio: interface, rate and buffer

Setup > Audio is where you pick the interface, set the sample rate and buffer, and watch what the Mac is doing right now. Open it with ⌘, and AUDIO, or click the interface name or the LATENCY read-out on the top bar.

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1. 1The interface list: the one in use first, with its lamp and IN USE
2. 2Sample rate: only the rates this interface offers
3. 3Buffer, with its time in ms (one way) under each size
4. 4RECOMMENDED FOR LIVE: 48 KHZ, 64 in one click
5. 5CPU (AUDIO): how much of each buffer's time the processing takes
6. 6DROPOUTS: clicks since the last reset
7. 7MAC (EXACT) and, next to it, LOOP (MEASURED)
8. 8The advice line and its USE button

Picking the interface#

Every Core Audio interface the Mac sees is a row in the list, the one in use at the top. A green lamp and IN USE mark the open one, with a line like 48 in / 48 out - 48 kHz, buffer 64. Click another row to switch to it; Shadow Rack keeps the rate and buffer if the new interface can do them.

• REFRESH looks for interfaces again, for one you just plugged in.
• AUDIO MIDI SETUP opens macOS's own app, for the clock source and channel names.

All inputs and outputs of the interface are switched on when it opens – except a laptop's built-in microphone, which stays off: The built-in microphone stays off (it would feed back through the speakers).

When the interface disappears#

Shadow Rack holds on to the interface you picked. If its cable comes out, it never falls back to another device such as the Mac's speakers. The row turns red with NOT CONNECTED and Chosen - used again as soon as it's plugged in, and the top bar says Waiting for ... (unplugged?). Shadow Rack looks for it every few seconds and opens it again by itself as soon as it's back – nothing to click.

On the very first start Shadow Rack opens an interface by itself at 48 kHz / 64. If all it finds is the Mac's own microphone and speakers, the inputs start off, so nothing can feed back.

Sample rate and buffer#

The rates and buffer sizes are big buttons (so a finger on the tablet can use them too). You only see the rates the interface offers. Buffers go from 16 to 2048 samples, and under each one is its time at the current rate.

RECOMMENDED FOR LIVE: 48 KHZ, 64 sets both in one click. It's lit while that's what runs, and greyed out if the interface can't do 48 kHz or 64 samples. Start there and only go bigger if the advice line tells you to.

Running now#

The right-hand panel refreshes four times a second.

The plugins' own delays come on top of MAC and LOOP; the latency page explains how they add up.

The advice line#

Under the tiles Shadow Rack says what to do, often with a USE button that switches the buffer straight away:

• Clicks: a bigger buffer gives the plugins more time - 256 adds 5.33 ms. Then reset the counter. (red)
• The CPU is nearly full: clicks come next. 256 adds 5.33 ms. (red)
• The CPU has room: 64 would be 2.67 ms quicker. Watch the dropouts after. – only with no clicks, CPU under 35 % and a buffer bigger than 64. It never suggests going below 64.
• Running smoothly.

After you change the buffer, press RESET and watch the counter for a song or two before you trust the new setting.

Measuring the loop#

LOOP is the real latency of the whole chain, converters included. To measure it on any desk:

You get Measured: N samples at 48 kHz. It's remembered for this buffer size. If nothing comes back: The test sound didn't come back. Check the loop on the desk (USB out -> USB in) and the channels. If the interface has no inputs on, it says so.