RME Audio MADIface XT, User Manual

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98
User's Guide MADIface XT
© RME
30.2 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 a MADI signal is applied to the MADIface XT, the unit indicates LOCK, i. e. a valid input sig-
nal. This information is presented in the MADIface XT 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: The MADIface XT is
set to 44.1 kHz internally (clock mode Master), and a mixing desk with MADI output is con-
nected to the card's MADI input. 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 MADIface XT internal sample rate. Result: When reading out the data, there will
frequently be read errors that cause clicks and drop outs.
Also when using multiple clock signals, a simple LOCK is not sufficient. The above described
problem can be solved elegantly by setting the MADIface XT from Master to AutoSync (its inter-
nal clock will then be the clock delivered by the mixing desk). But in case the card is clocked to
word clock, this signal can also be un-synchronous, and there will again be a slight difference in
the sample rate, and therefore clicks and drop outs.
In order to display those problems, the MADIface XT 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 the example above it would have been obvious immediately that the entry
LOCK is shown in
SyncCheck
instead of SYNC, right after connecting the mixing desk. With
external synchronisation via word clock, both entries Word Clock and MADI must display
SYNC.
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.
30.3 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 er-
rors, equalling a delay between input and output. However, delays below certain values can
subjectively be claimed to be a zero-latency. This applies to analog routing and mixing, and in
our opinion 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. RME's digital receivers operate buffered,
and together with TotalMix and the output via the transmitter cause a typical delay of 3 samples.
At 44.1 kHz this equals about 68 µs (0.000068 s), and about 15µs at 192 kHz.