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Från Presonus

Latencygåtan

By Wesley Elianna Smith
In the audio world, “latency” is another word for “delay.” It’s the time it takes for the sound from the front-of-house speakers at an outdoor festival to reach you on your picnic blanket. Or the time it takes for your finger to strike a piano key, for the key to move the hammer, for the hammer to strike the string, and for the sound to reach your ear.
Your brain is wired so that it doesn’t notice if sounds are delayed 3 to 10 milliseconds (ms). Studies have shown that sound reflections in an acoustic space must be delayed by 20 to 30 ms before your brain will perceive them as separate. However, by around 12 to 15 ms (depending on the listener), you will start to “feel” the effects of a delayed signal. It is this amount of delay that we must battle constantly when recording and monitoring digitally.

When Good Latency Goes Bad

Roundtrip latency in digital-audio applications is the amount of time it takes for a signal (such as a singing voice or a face-melting guitar solo) to get from an analog input on an audio interface, through the analog-to-digital converters, into a DAW, back to the interface, and through the digital-to-analog converters to the analog outputs. Any significant amount of latency can negatively impact the performer’s ability to play along to a click track or beat — making it sound like they’re performing in an echoing tunnel (unless they have a way to monitor themselves outside of the DAW application, such as a digital mixer or one of our AudioBox™ VSL-series interfaces).

What’s Producing the Delay: a Rogue’s Gallery

In practical terms, the amount of roundtrip latency you experience is determined by your audio interface’s A/D and D/A converters, its internal device buffer, its driver buffer, and the buffer setting you have selected in your digital audio workstation software (Mac^®) or Control Panel (Windows^®).
Converters. Analog-to-digital converters in your interface transform an analog signal from a microphone or instrument into digital bits and bytes. This is a ferociously complex process and takes a little more than half a millisecond on average. On the other end of a long chain we’re about to describe are the digital-to-analog converters that change the digital stream back into electrical impulses you can hear through a monitor speaker or headphones. Add another half-millisecond or so.