RME OCTAMIC XTC [52/62] Latency and monitoring

User’s Guide OctaMic XTC © RME

22.3 Latency and Monitoring

The term Zero Latency Monitoring has been introduced by RME in 1998 for the DIGI96 series

and describes the ability to pass-through the computer's input signal at the interface directly to

the output. Since then, the idea behind has become one of the most important features of mod-

ern hard disk recording. In the year 2000, RME published two ground-breaking Tech Infos on

the topics Low Latency Background, which are still up-to-date: Monitoring, ZLM and ASIO, and

Buffer and Latency Jitter, found on the RME website.

How much Zero is Zero?

From a technical view there is no zero. Even the analog pass-through is subject to phase er-

rors, equalling a delay between input and output. However, delays below certain values can

subjectively be claimed to be a zero-latency. This applies to analog routing and mixing, and in

our opinion also to RME's Zero Latency Monitoring. RME's digital receiver's buffer and the out-

put via the transmitter cause a typical delay of 3 samples. At 44.1 kHz this equals about 68 µs

(0.000068 s), at 192 kHz only 15 µs.

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 about 40 samples,

equalling one millisecond. A playback and re-record of the same signal via DA and AD (loop-

back) then causes an offset of the newly recorded track of about 2 ms.

Low Latency!

The OctaMic XTC uses latest AD-converters with special low latency filters, exceptional Signal

to Noise ratio, lowest distortion figures and lightning quick conversion. A delay of only 10 sam-

ples hasn’t been available just a few years back. But even the chip used for DA-conversion has

a lower delay than usual. The exact delays caused by the AD-conversion of the OctaMic XTC

are:

Sample frequency kHz 44.1 48 88.2 96 176.4 192

AD (12.6 x 1/fs) ms 0.28 0.26 0.14 0.13

AD (9.8 x 1/fs) ms 0.06 0.05

DA (28 x 1/fs) ms 0.63 0.58 0.32 0.29 0.16 0.15

These values are smaller than those available from even much more expensive devices. They

represent an important step in further reducing the latency in the computer-based recording

studio.