Print This Page

5. Sampling issues

5.4 Timing issues

Signal propagation through cables happens at close to light speed - 299.792.458 meters per second(*5Q). The speed is limited by the cable’s insulation - an unshielded cable will transfer audio signals at approximately 96% of the light speed, a coaxial cable at appr. 66%. Because of this limitation, a 100 meter coaxial cable will induce approximately 0,5 microseconds delay. Semiconductors also induce delays - a 74HC4053 analogue switch (an integrated circuit used in many remote-controlled head amps) typically adds 4 nanoseconds delay to the signal path(*5R).

Analogue signal distribution and processing circuits operate with this limitation - which normally does not affect audio quality unless for example the cabling spans more than 1,8 kilometers, lifting the delay above the 6 microseconds hearing limit.

Digital systems however operate at much lower speeds because signals are chopped in samples and then distributed and processed sample-by-sample - which requires a multiple of the sample time involved. For digital audio systems with a 48kHz sampling rate, the sample time is 20.8 microseconds - well above the 6 microseconds hearing limit. Where dynamic range and frequency range of digital audio systems have developed to span almost outside the reach of the human auditory system, time performance is a bottleneck that can not be solved. Instead, system designers and operators will have to study the time performance of their digital systems in detail, and take countermeasures to limit the consequences for audio quality as much as they can for every individual system design.

With analogue systems, engineers only had to concern themselves with the acoustic timing issues of microphone and speaker placement. With the introduction of digital (networked) audio systems, this task is expanded with the concern for digital timing or latency. The acoustic and digital timing issues basically cause the same effects, requiring a combined approach to achieve a high audio quality.

The following issues concerning time performance of digital audio systems will be covered in the following chapters:

table 501: timing issues in digital systems

issue typical delay (48kHz) cause
network latency 1 ms one pass through a networked distribution system
processing latency 1 ms a digital mixer’s signal processing
conversion latency 0.5 ms AD or D/A conversion
clock phase 0.01 ms synchronisation and receiver circuits
jitter 0.000005 ms PLL, electronic circuits, cabling, network

>>5.5 Absolute latency

Return to Top