Digital Sound & Music: Concepts, Applications, & Science, Chapter 5, last updated 6/25/2013
transmission instead of an analog transmission, you can cut your latency down by at least two
milliseconds because you’ll have eliminated two signal conversions.
Buffer size contributes to latency as well. Consider a scenario in which a singer's voice is
being digitally recorded (Figure 5.20). When an audio stream is captured in a digital device like
a computer, it passes through an input buffer. This input buffer must be large enough to hold the
audio samples that are coming in while the CPU is off somewhere else doing other work. When
a singer is singing into a microphone, audio samples are being collected at a fixed rate say
44,100 samples per second. The singer isn't going to pause her singing and the sound card isn't
going to slow down the number of samples it takes per second just because the CPU is busy. If
the input buffer is too small, samples have to be dropped or overwritten because the CPU isn't
there in time to process them. If the input buffer is sufficiently large, it can hold the samples that
accumulate while the CPU is busy, but the amount of time it takes to fill up the buffer is added to
the latency.
The singer herself will be affected by this latency is she's listening to her voice through
headphones as her voice is being digitally recorded (called live sound monitoring). If the
system is set up to use software monitoring, the sound of the singer’s voice enters the
microphone, undergoes ADC and then some processing, is converted back to analog, and reaches
the singer’s ears through the headphones. Software monitoring requires one analog-to-digital and
one digital-to-analog conversion. Depending on the buffer size and amount of processing done,
the singer may not hear herself back in the headphones until 50 to 100 milliseconds after she
sings. Even an untrained ear will perceive this latency as an audible echo, making it extremely
difficult to sing on beat. If the singer is also listening to a backing track played directly from the
computer, the computer will deliver that backing track to the headphones sooner than it can
deliver the audio coming in live to the computer. (A backing track is a track that has already
been recorded and is being played while the singer sings.)
Latency in live sound monitoring can be avoided by hardware
monitoring (also called direct monitoring). Hardware monitoring splits the
newly digitized signal before sending it into the computer, mixing it directly
into the output and eliminating the longer latencies caused by analog-to-digital
conversion and input buffers (Figure 5.25). The disadvantage of hardware
monitoring is that the singer cannot hear her voice with processing such as
reverb applied. (Audio interfaces that offer direct hardware monitoring with
zero-latency generally let you control the mix of what’s coming directly from
the microphone and what’s coming from the computer. That’s the purpose of
the monitor mix knob, circled in red in Figure 5.24.) When the mix knob is turned fully
counterclockwise, only the direct input signals (e.g., from the microphone) are heard. When the
mix knob is turned fully counterclockwise, only the signal from the DAW software is heard.
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