RME Audio MADIface XT, Руководство пользователя

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User's Guide MADIface XT
© RME
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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 au-
dible 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. The exact delays of the MADIface XT are:
Sample frequency kHz 44.1 48 88.2 96 176.4 192
AD (43.2 x 1/fs) ms 0.98 0.9 0.49 0.45
AD (38.2 x 1/fs) ms 0.22 0.2
DA (28 x 1/fs) ms 0.63 0.58 0.32 0.29 0.16 0.15
Buffer Size (Latency)
Windows:
This option found in the Settings dialog defines the size of the buffers for the audio
data used in ASIO and WDM (see chapter 7 and 9).
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 MADIface XT is mainly a 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.
Safety Buffer
An additional small Safety Buffer on the playback side has proven to be very efficient and use-
ful. The MADIface XT uses a fixed additional buffer of 32 samples, which is added to the current
buffer size. The main advantage is the ability to use lowest latency at highest CPU loads. Fur-
thermore, the fixed buffer does not add to the latency jitter (see Tech Info), the subjective timing
is extraordinary.
Core Audio's Safety Offset
Under OS X, every audio interface has to use a so called
Safety Offset
on record and playback,
otherwise Core Audio won't operate click-free. The MADIface XT uses a Safety Offset of 16
samples. This offset is signalled to the system, and the software can calculate and display the
total latency of buffer size plus 2 x Safety Offset plus Safety Buffer for the current sample rate.