Latency and CPU

Everything that goes through a computer arrives a little later. With Shadow Rack the delay has two parts: what the Mac adds (the interface's buffers, the same for every channel) and what each channel's plugins add. Shadow Rack shows both exactly, keeps the channels that must stay together in time, and tells you when the buffer is too small or too big.

The Mac's part#

The audio goes into the Mac one buffer at a time and comes out one buffer later. At 48 kHz with a buffer of 64 samples, that's 1.33 ms in and 1.33 ms out. The MAC figure on the top bar's LATENCY display is exactly this: one input buffer plus one output buffer, from USB in to USB out. It doesn't depend on the plugins.

The plugins' part#

Many plugins delay the signal: lookahead limiters and de-essers, linear-phase EQs, some compressors. Shadow Rack shows it everywhere:

• In the chain, each row has its DELAY in ms - yellow above 1 ms, red above 5 ms.
• In the mixer, under each strip's meters: the channel's whole delay, for example 5.00 ms.
• At the bottom of the channel view, the LATENCY line adds it up: LATENCY 4.20 ms (plugins 2.10 + 2.10 PA). It turns red when the plugins add more than 5 ms.

When a plugin changes its delay while running (a lookahead switched on, a different mode), Shadow Rack follows it straight away.

How channels stay in time#

Each channel is delayed only by its own plugins - channels don't wait for each other unless they must. Two cases where they must:

New York buses - automatic. A New York copy has to come out with its channel to the sample, or the two comb-filter. So every channel that feeds a New York bus is delayed by exactly as much as that bus's plugins take; the bus's read-out shows the range (0.00-15.00 ms). Nothing to set. See New York buses.

Keep in time with the PA - when you ask. Channels that feed the speakers together (left and right mains, the subs, delay lines) must not drift apart because one of them has a limiter with lookahead. Right-click those channels in the mixer (select them all first) and tick Keep in time with the PA. Every channel in this group is then delayed to match the slowest of them. Their delay read-outs turn yellow - PA 4.20 ms - and the message says ... kept in time with the PA. The setting is saved in the session.

Bypassing a plugin never changes the timing: the dry signal is delayed exactly like the plugin. When a delay has to change, it changes smoothly, without a gap or a click.

The read-outs#

12345678

1. 1Buffer sizes, with the time each one takes
2. 2RECOMMENDED FOR LIVE: 48 KHZ, 64
3. 3CPU (AUDIO): how much of each buffer's time the processing uses
4. 4DROPOUTS: clicks since the last reset
5. 5MAC (EXACT): in + out buffer
6. 6LOOP (MEASURED): the whole way out of the desk and back
7. 7SLOWEST CHANNEL: the channel with the most plugin delay, and its total
8. 8Advice, with a button that applies it

On the top bar, always visible:

• CPU - how much of the available time the audio processing uses, with a bar: green, yellow, red.
• DROPS - dropouts (clicks) since the start; red above 0. Click it to reset the counter.
• LATENCY - MAC (exact) and LOOP (measured, or not measured). Click it to open Setup > Audio.

In Setup > Audio, the Running now panel shows the same in big, plus the slowest channel - its name and +15.00 ms = 21.90 ms in all, the loop plus its plugins - and one line of advice. More on that page: Setup > Audio.

Each plugin's own CPU share is in its chain row: yellow above 10 %, red above 25 %.

Measuring the loop#

LOOP is the whole trip: out of your desk, through the Mac, back into the desk. You measure it once per buffer size:

The result - Measured: 331 samples at 48 kHz - is remembered for this buffer size. If the chirp doesn't come back, check the loop on the desk; if the interface has no inputs switched on, enable them first.

Choosing a buffer size#

Start at 48 kHz, 64 samples - the RECOMMENDED FOR LIVE: 48 KHZ, 64 button sets it. Then watch DROPS during the soundcheck with the full show loaded:

• Clicks – the advice offers the next bigger buffer (128 adds 2.67 ms). Set it, then reset the counter.
• CPU nearly full - a bigger buffer before the clicks come.
• No clicks, CPU low, buffer above 64 - the advice offers a smaller one: quicker, but watch the dropouts after.

Changing the interface, rate or buffer while channels play asks first: Restart the audio? - everything through the rack goes quiet for a moment while the interface starts again.

Shadow Rack uses all the Mac's cores by itself: channels are processed side by side, in parallel. There's nothing to set.