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5. Sampling issues

5.5 Absolute latency

The latency of a signal chain in a typical networked audio system at a sampling frequency of 48 kHz is around 4 milliseconds. The main factors are distribution latency, DSP latency and AD/DA latency. In recording studio’s with isolated control rooms - where the listener never hears the sound source directly - latency is not a problem at all. If the listener can see the sound source (but not hear it), a latency of up to 20 milliseconds (corresponding with the PAL video frame rate of 50 Hz) is allowed before video and audio synchronisation mismatch can be detected - mainly due to the slow reaction time of the eyes and the visual cortex in the human brain.

In live audio systems however, the audio signals radiated by the sound sources are often mixed with the processed output of the audio system - causing problems if the two signals have similar amplitude. With high latencies, a ‘delay’ is perceived by listeners and performers, which is specially inconvenient for musicians because it disturbs musical timing. Low latencies cause comb filtering, disturbing timbre and pitch. This problem is most prominent for vocal performances where a singer hears his or her own voice acoustically and through bone conduction, as well as through a monitor system.

For in-ear monitoring, latencies of more than 5 milliseconds might cause single sounds to be heard as two separate audio signals, disturbing musical timing. Below 5 milliseconds, latency causes comb filtering - which is also a problem, but can be adapted to by experienced performers. As a rule of thumb, high quality vocalist in-ear monitoring ideally requires a system latency (from microphone input to headphone output) of less than 5 milliseconds, preferably lower.

For monitoring systems using loudspeakers, the issue is less significant because the acoustic path from the loudspeaker to the ear is included in the signal chain, adding about 4 milliseconds or more acoustic delay. Although the latency is noticeable, because of the acoustic reflections the comb filter effect is less audible.

For Front Of House (FOH) Public Address (PA) speaker systems, the absolute latency of a networked system is much less significant because the distance of the listener to the sound source normally is several meters, adding tens of milliseconds acoustic delay - making the system’s latency less significant. Also, FOH PA speaker systems normally are situated in front of the performers to prevent feedback, in some cases compensating the latency difference between system PA output and the source’s direct sound for the audience. Figure 510 and table 502 show a typical networked live audio system and the latencies and acoustic delays that occur. Table 503 shows an average typical latency perception of musicians (pianist, vocalist, guitarist) analysed in numerous test sessions by Yamaha.

table 502: latency and acoustic delay in a typical networked live audio system

signal path latency
& acoustic delay (ms)
to performer
monitor speaker
to performer
PA speaker
to audience
sound source
to audience
A/D conversion 0.5 0.5 0.5 n/a
network i/o rack -> FOH 1 1 1 n/a
DSP 1 1 1 n/a
network FOH -> i/o rack 1 1 1 n/a
D/A conversion 0.5 0.5 0.5 n/a
monitor speaker @ 2m* n/a 6 n/a n/a
PA speaker @ 20m* n/a n/a 58 n/a
sound source @ 23m* n/a n/a n/a 67
- - - -
total latency (ms) 4 10 62 67

* speed of sound = 343 m/s

table 503: average monitor system’s absolute latency perception by musicians (Pianist, Vocalist, Guitarist)

signal path latency for in-ear monitor systems for floor monitor systems
1.15 - 2 ms Playable without any big problem. Playable.
2 - 5 ms Playable, however tone colour is changed. Playable.
5 - 10 ms Playing starts to become difficult. Latency is noticeable. Playable. Although latency is noticeable, it is perceived as ambience.
>10 ms Impossible to play, the delay is too obvious. Impossible to play, the delay is too obvious.

>>5.6 Relative latency

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