RME Audio Hammerfall Digiface, User Manual, Page: 76 / 78

76
User's Guide HDSP System Digiface
© RME
Note: Cubase and Nuendo display the latency values signalled from the driver separately for
record and playback. These values equal nearly exactly the buffer size (for example 3 ms at
128 samples) on RME's digital interfaces.
Core Audios Safety Offset
Under OS X, every audio interface has to use a so called satety offset , otherwise Core Audio
won't operate click-free. The Digiface uses a safety offset of 32 samples. This offset is sig-
nalled to the system, and the software can calculate and display the total latency of buffer size
plus plus safety offset for the current sample rate.
34.3 DS - Double Speed
When activating the
Double Speed mode the Digiface operates at double sample rate. The
internal clock 44.1 kHz turns to 88.2 kHz, 48 kHz to 96 kHz. The internal resolution is still 24
bit.
Sample rates above 48 kHz were not always taken for granted, and are still not widely used
because of the CD format (44.1 kHz) dominating everything. Before 1998 there were no re-
ceiver/transmitter circuits available that could receive or transmit more than 48 kHz. Therefore
a work-around was used: instead of two channels, one AES line only carries one channel,
whose odd and even samples are being distributed to the former left and right channels. By
this, you get the double amount of data, i. e. also double sample rate. Of course in order to
transmit a stereo signal two AES/EBU ports are necessary then.
This transmission mode is called
Double Wire in the professional studio world, and is also
known as S/MUX (abbreviation for Sample Multiplexing ) in connection with the ADAT format.
The AES3 specification uses the uncommon term Single channel double sampling frequency
mode .
Not before February 1998, Crystal shipped the first 'single wire' receiver/transmitters that could
also work with double sample rate. It was then possible to transmit two channels of 96 kHz data
via one AES/EBU port.
But
Double Wire is still far from being dead. On one hand, there are still many devices which
can't handle more than 48 kHz, e. g. digital tape recorders. But also other common interfaces
like ADAT or TDIF are still using this technique.
Because the ADAT interface does not allow for sampling frequencies above 48 kHz (a limita-
tion of the interface hardware), the Digiface automatically uses the
Sample Multiplexing method
in DS mode. One channel's data is distributed to two channels according to the following table:
ADAT Ch. 1 2 3 4 5 6 7 8
DS Channel
Samples 1
1a
1
1b
2
2a
2
2b
3
3a
3
3b
4
4a
4
4b
As the transmission of double rate signals is done at standard sample rate (Single Speed), the
ADAT outputs still deliver 44.1 kHz or 48 kHz.