Allen & Heath 15 GS_R24 FireWire/ADAT Module User Guide
ALLEN & HEATH FIREWIRE CONTROL PANEL
DPC Latency Checker.
This tool in the control panel allows the deferred procedure call delays to be measured and recorded. It is important to check
that this to know that your computer is capable of streaming audio data to and from the FireWire bus and to your audio soft-
ware application.
Please note that this is not the latency measurement of the audio data transfer between the Allen & Heath FireWire device
and the computer. The DPC latency is the timing delays imposed on certain instructions used by your computer to process
audio data.
Click the DPC tab.
Start the checker by clicking the check box. The checker can be reset at any time.
The shot above shows a healthy reading, with a maximum measured timing of 41 microseconds. The Recommended Operation
Mode displayed is Normal which allows low buffer sizes to be used, hence giving fast data transfer speeds and low FireWire
bus latency.
There are Safe modes which may be recommended if high DPC timings are measured.
The shot below shows what happens when a video application is loaded and an MPEG movie is played separately on the com-
puter. A large DPC timing is detected caused by the computer loading a new application and buffering the video data, a figure
of 4.298 milliseconds is displayed and the Recommended Operation Mode is changed to Safe Mode Level 1.
If the FireWire bus was currently streaming in Normal mode and low buffer sizes then audio dropout would almost definitely
happen resulting in clicks at the very least, and interruptions in the audio software application.
A variety of things can cause high DPC latencies – network connections being particularly notorious, also other peripherals
connected to USB ports or on the FireWire bus, video graphics cards and settings, and of course other applications as in this
example that use up processor power.
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