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5. Sampling issues

5.6 Relative latency

Similar to the design challenge of coping with acoustic delays in speaker systems, latency in networked systems cause timing and comb filtering problems if multiple signal paths exist from sound source to listener. An example is a stereo overhead of a drum kit, where a certain portion of the outputs of the left and right microphones contain identical - correlated - signals. When these signals arrive at a listener with different latencies, the timing difference as well as the comb filtering can be detected by listeners - even for very small latencies down to 6 microseconds.

Figure 511 shows the comb filter effect caused by mixing two identical signals with a latency difference of 20 microseconds (roughly one sample at 48 kHz). Although the cancellation frequency of the comb filter effect lies at 24 kHz - above the hearing limit of 20 kHz, the slope already starts to attenuate frequencies within the hearing range, so the effect can be detected by listeners independent from the detection of the time difference itself. At 20 kHz, the attenuation is -10 dB. For latency differences of two samples or more, the cancellation frequency moves inside the hearing range causing clearly audible interference patterns - similar to the interference patterns created by the acoustic delay when using multiple loudspeakers.

Many digital mixing consoles and digital signal processors have delays built in to manipulate latency - for example to correct relative latency problems.

Networks (timing protocol) and DSP processes (latency compensation) have built-in latency correction mechanisms that align latencies of all signal paths in the system. However, it is important to remember that this is only valid for the default path - eg. a default signal chain in a mixer, or a single pass through a network. If an external process is inserted in the signal path, eg. a graphic equalizer, the latency of that signal path increases with the latency of the graphic equalizer relative to all other signal paths in the mixer. If the equalizer is connected through a network, the network latency has to be added as well.

Another possible cause for latency differences is the use of different digital components for correlated signals. Different components - including A/D and D/A converters - may add different latencies to the signal paths. In general, it is advisable to route correlated signals through the same type of components - preferably only one.

Figure 512A shows a system where 0,2 milliseconds (10 samples) latency difference is caused by using different A/D and D/A converters, and another 2 milliseconds (100 samples) is added by the inserted graphic equalizer and the extra pass through the network. Figure 512B shows the same application using the same A/D and D/A converter devices for both signal paths, and applying a manual compensation delay in the mixing console output for signal path 2, resulting in the same latency for both signal paths.

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