Usb Audio - RME Audio MADIface USB User Manual

Core Audio's Safety Offset
Under OS X, every audio interface has to use a so called Safety Offset, otherwise Core Audio
won't operate click-free. The MADIface USB 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.

26.4 USB Audio

USB audio is in several ways different from PCI based audio interfaces. The MADIface USB
even more, as it tries to use the USB standard's full potential. Transmitting 64 channels simul-
taneously in both directions therefore requires modern hardware. While the MADIface USB will
work on older computers with for example a Core 2 Duo CPU and ICH 9 as USB hub, the CPU
load is noticeable and DPC is raised from the underlying USB driver of the operating system.
Newer computers, especially those already having USB 3 ports, usually show no such issues,
as their raw performance handles the USB load with ease.
Low CPU load and click-free operation even at 64 samples buffer size are indeed possible on
current computers. However, using older computers a simple stereo playback will already cause
a CPU load of more than 30%.
A computer blocked for a short time – no matter if ASIO or WDM – will loose one or more data
packets. Such problems can only be solved by increasing the buffer size (and with this the la-
tency).
The MADIface USB features a unique data checking, detecting er-
rors during transmission via USB and displaying them in the Settings
dialog. Additionally the MADIface provides a special mechanism to
continue recording and playback in spite of drop-outs, and to correct
the sample position in real-time.
Like any audio interface the MADIface USB should have a data transmission to the computer as
undisturbed as possible. The easiest way to guarantee this is to connect it 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 MADIface USB to a USB port
Start the Device Manager, choose View 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 MADIface USB. By re-
connecting to a different port this view now allows for a check at which of the two controllers the
MADIface is connected. With multiple devices it can also be checked if they are connected to
the same controller.
Furthermore this information can be used to operate an external USB drive without disturbing
the MADIface USB, by simply connecting the drive to the other controller.
Especially with notebooks it can happen that all internal devices and all the sockets/ports are
connected to the same controller, with the second controller not used at all. In that case all de-
vices have to use the same bus and interfere with each other.
User's Guide MADIface USB © RME
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