Appsys ProAudio ADX-32B, Руководство пользователя

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User's Manual
10. DIGITAL SIGNAL QUALITY
10.1. Latency
Latency has – in contrast to traditional analog systems – always been a topic in digital audio technology. High
latency can lead to unwanted effects, such as phasing, hall, echo etc. and can seriously affect audio performance.
By design, other digital snake solutions (e.g. Ethernet based systems) have a relatively high propagation delay,
because audio data has to be sampled, buffered, converted, transmitted, buffered and then finally converted back
to the original format. These delays can easily add up to several tens or hundreds of milliseconds which makes
them unusable for many applications, especially for monitoring purposes.
One design goal of the ADX was to create a system with one of the lowest latencies on the market. This is
achieved by performing no conversion at all, and only minimal buffering with clock alignment.
These measures allow an excellent overall latency of less then 1µs, which is not noticeable (sound waves
travel only 0.3mm during this delay)!
10.2. Jitter
In any digital audio transmission, the clock signal picks up a certain amount of jitter (clock phase noise). Jitter can
affect audio performance (SNR) if a jittered clock is directly used for D/A or A/D conversion, and can even lead to
drop-outs or data errors if the jitter values is so high that it is impossible to determine the correct value of a bit.
For proper operation of ADAT connections, jitter is also an important issue because the duration of the “start of
frame” mark is used to calculate the sample point of the subsequent bits in the data stream. In contrast to bi-
phase-mark encoding used by SPDIF and AES/EBU, the ADAT signal uses NRZ encoding which allows the double
data rate, but makes it more difficult to extract the proper bit timing.
To cope with jitter, each ADAT signal is fed to a PLL circuitry which cleans the jitter by recovering proper timing
characteristics before the signal is output.
The overall jitter of an ADAT signal transmitted over 100m (330ft)
Cat5 cable is only ±1.5ns typ. and is in the same range of the jitter usually introduced by a standard short fiber
optic connection. This feature allows seamless integration even with “picky” equipment (like the Presonus
Digimax FS microphone preamp, or Yamaha consoles at 48kHz) which refuses proper clock tracking when there
is a higher amount of jitter in the signal.
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