RME Audio HDSPe RayDAT, User Manual

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User's Guide HDSPe RayDAT
© RME
27.3 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 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 RayDAT operates-
buffered, and in combination with the optical transmitter causes a typical delay of 3 samples. At
44.1 kHz this equals about 68 µs (0.000068 s), and about 15µs at 192 kHz.
Oversampling
While the delays of digital interfaces can be disregarded altogether, the analog inputs and out-
puts do cause a significant delay. Modern converter chips operate with 64 or 128 times over-
sampling plus digital filtering, in order to move the error-prone analog filters away from the audi-
ble frequency range as far as possible. This typically generates a delay of one millisecond. A
playback and re-record of the same signal via DA and AD (loopback) then causes an offset of
the newly recorded track of about 2 ms.
Buffer Size (Latency)
Windows: This option found in the Settings dialog defines the size of the buffers for the audio
data used in WDM and ASIO (see chapter 8).
Mac OS X: The buffer size is defined within the application. Only some do not offer any setting.
For example iTunes is fixed to 512 samples.
General: A setting of 64 samples at 44.1 kHz causes a latency of 1.5 ms, for record and play-
back each. But when performing a digital loopback test no latency/offset can be detected. The
reason is that the software naturally knows the size of the buffers, therefore is able to position
the newly recorded data at a place equalling a latency-free system.
AD/DA Offset under ASIO and OS X: ASIO (Windows) and Core Audio (Mac OS X) allow for the
signalling of an offset value to correct buffer independent delays, like AD- and DA-conversion or
the Safety Buffer described below. An analog loopback test will then show no offset, because
the application shifts the recorded data accordingly.
Because the HDSPe RayDAT is a completely digital interface, and the delays introduced by
external AD/DA-converters or other digital interfaces are unknown to unit and driver, the drivers
include the digital offset values (3 / 6 / 12 samples). Therefore the delays caused by external
converters have to be taken care off in the record software, which usually means that the user
has to enter specific offset values manually.
Note: Cubase and Nuendo display the latency values signalled from the driver separately for
record and playback. The current driver includes a safety offset of 32 samples for the playback
side only, which will be included in the shown value.