RME Audio Digiface AES, Руководство пользователя

Страница: 95 / 104

User's Guide Digiface AES
© RME
95
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 in real world operation analog record
and playback is unavoidable, the drivers include an offset value matching the Digiface's converter
delays.
Therefore, in a
digital loopback test a negative offset of about 3 ms occurs. This is no real prob-
lem, because this way of working is seldom, and usually the offset can be compensated manually
within the application. Additionally, keep in mind that even when using the digital I/Os usually at
some place an AD- and DA-conversion is involved (no sound without...).
Safety Buffer
An additional small safety buffer on the playback side has proven to be very efficient and useful.
It is therefore implemented in all RME interfaces. Under Windows the Digiface AES uses a fixed
additional buffer of 32 samples, under Mac 24 samples, which is added to the current buffer size.
The main advantage is the ability to use lowest latency at highest CPU loads. Furthermore, 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 Digiface uses a Safety Offset of 24 samples.
This offset is signalled to the system, and the software can calculate and display the total latency
of buffer size plus AD/DA offset plus 2 x Safety Offset plus Safety Buffer for the current sample
rate. The option
Short Safety Offset in the Settings dialog reduces this to 12 samples each.
38.3 USB Audio
USB audio is in several ways different from PCI based audio interfaces. A good performance and
click-free operation at a low buffer size are indeed possible on current computers. However, using
older computers a simple stereo playback will begin to cause a CPU load of more than 30%.
A computer blocked for a short time
– no matter if ASIO or WDM – will lose one or more data
packets. Such problems can only be solved by increasing the buffer size (latency).
The Digiface features a unique data
checking, detecting errors during trans-
mission via USB and displaying them in
the Settings dialog.
Additionally the Digiface provides a special mechanism to continue recording and playback in
case of drop-outs, and to correct the sample position in real-time.
Best USB 2 performance is achieved by connecting the Digiface AES to its own bus, which should
be no big problem as most USB 2.0 interfaces are a double bus design. A check in the Device
Manager can be done as follows:

Connect the Digiface AES to a USB 2 port

Start the Device Manager, View set to Devices by Connection

Select ACPI x86-based PC, Microsoft ACPI-Compliant System, expand PCI Bus
This branch normally includes two entries of a
USB2 Enhanced Host Controller . A USB Root Hub
can be seen, which then connects all USB devices, including the Digiface AES. By reconnecting
to a different port this view shows at which of the two controllers the Digiface AES is connected.
With multiple devices it can be checked if they are connected to the same controller.