Technical Background; Lock And Synccheck; Latency And Monitoring - RME Audio HDSPe AIO Pro User Manual

27. Technical Background

27.1 Lock and SyncCheck

Digital signals consist of a carrier and the data. If a digital signal is applied to an input, the re-
ceiver has to synchronize to the carrier clock in order to read the data correctly. To achieve this,
the receiver uses a PLL (Phase Locked Loop). As soon as the receiver meets the exact fre-
quency of the incoming signal, it is locked. This Lock state remains even with small changes of
the frequency, because the PLL tracks the receiver's frequency.
If an ADAT or SPDIF signal is applied to the HDSPe AIO Pro, the unit indicates LOCK, i. e. a
valid input signal. This information is presented in the HDSPe AIO Pro's Settings dialog. In the
status display SyncCheck, the state of all clocks is decoded and shown as simple text (No Lock,
Lock, Sync).
Unfortunately, LOCK does not necessarily mean that the received signal is correct with respect
to the clock which processes the read out of the embedded data. Example [1]: The HDSPe AIO
Pro is set to 44.1 kHz internally (clock mode Master), and a mixing desk with ADAT output is
connected to input ADAT. The status display will show LOCK immediately, but usually the mixing
desk's sample rate is generated internally (it is Master too), and thus slightly higher or lower than
the HDSPe AIO Pro's internal sample rate. Result: When reading out the data, there will fre-
quently be read errors that cause clicks and drop outs.
Also when using multiple inputs, a simple LOCK is not sufficient. The above described problem
can be solved elegantly by setting the HDSPe AIO Pro from Master to AutoSync (its internal
clock will then be the clock delivered by the mixing desk). But in case another un-synchronous
device is connected, there will again be a slight difference in the sample rate, and therefore
clicks and drop outs.
In order to display those problems, the HDSPe AIO Pro includes SyncCheck. It checks all
clocks used for synchronicity. If they are not synchronous to each other, the status display will
show LOCK. If they are synchronous to each other (i. e. absolutely identical), the status display
will change to SYNC. In example 1 it would have been obvious that the entry LOCK is shown in
SyncCheck after connecting the mixing desk.
In practice, SyncCheck allows for a quick overview of the correct configuration of all digital de-
vices. So one of the most difficult and error-prone topics of the digital studio world finally be-
comes easy to handle.

30.2 Latency and Monitoring

The term Zero Latency Monitoring has been introduced by RME in 1998 for the DIGI96 series
of audio cards. It stands for the ability to pass-through the computer's input signal at the inter-
face directly to the output. Since then, the idea behind has become one of the most important
features of modern hard disk recording. In the year 2000, RME published two ground-breaking
Tech Infos on the topics Low Latency Background, which are still up-to-date: Monitoring, ZLM
and ASIO, and Buffer and Latency Jitter, both found on the RME website.
How much Zero is Zero?
From a technical view there is no zero. Even the analog pass-through is subject to phase errors,
equalling a delay between input and output. However, delays below certain values can subjec-
tively be claimed to be a zero-latency. This applies to analog routing and mixing, and in our opin-
ion also to RME's Zero Latency Monitoring. The term describes the digital path of the audio data
from the input of the interface to its output. The digital receiver of the HDSPe AIO Pro can't op-
erate un-buffered, and together with TotalMix and the output via the transmitter, it causes a typi-
cal delay of 4 samples. At 44.1 kHz this equals about 90 µs (0.00009 s), at 192 kHz only 21 µs,
and is identical for both ADAT and SPDIF/AES.
User's Guide HDSPe AIO Pro © RME
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