User Manual

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User's Guide HDSPe AIO Pro © RME
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.
Low Latency
The HDSPe AIO uses the latest top AD- and DA-converters with special low latency filters, offer-
ing exceptional signal to noise and distortion figures in combination with a super-fast conversion.
The exact delays caused by the AD- and DA-conversion of the HDSPe AIO Pro converter chips
are:
Sample rate kHz
44.1
48
96
192
AD (5 x 1/fs) ms
0.11
0.10
0.06
0.026
DA Sharp (6.25 x 1/fs) ms
0.14
0.13
DA Sharp (5.63 x 1/fs) ms
0.06
0.029
These values represent an important step in further reducing the latency in the computer-based
recording studio. The added latency - at least from AD- and DA-conversion - can simply be ig-
nored.
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 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.
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 rec-
ord and playback is unavoidable, the drivers include an offset value matching the HDSPe AIO
Pro's converter delays.
Therefore, in a digital loopback test a minimal negative offset of a few samples occurs. Since
this small deviation is normally irrelevant, and since the offset can be corrected manually in the
DAW if in doubt, this is not a problem. Furthermore, even when using the digital I/Os, an AD and
DA conversion comes into play somewhere (no sound without DA conversion...).
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 AIO Pro uses a fixed addi-
tional buffer of 16 samples, under Mac 32 samples, which is added to the current buffer size.
Main advantage is the ability to use lowest latency at highest CPU loads.
Core Audios 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 HDSPe AIO Pro 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 safety offset for the current sample rate to inform the DAW in use.