User Manual

User's Guide Fireface 800 © RME
58
30.3 DS - Double Speed
When activating the Double Speed mode the Fireface 800 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 in connection with the ADAT format. The DTRS recorder DA-98HR by Tas-
cam also uses this technique, which is called Dual Line here.
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 Fireface 800 automatically uses the described Sample Split
method in DS mode. One channel's data is distributed to two channels according to the follow-
ing table:
Analog In 1 2 3 4 5 6 7 8
DS Signal
Port
1/2
ADAT1
3/4
ADAT1
5/6
ADAT1
7/8
ADAT1
1/2
ADAT2
3/4
ADAT2
5/6
ADAT2
7/8
ADAT2
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.
30.4 QS – Quad Speed
Due to the small number of available devices that use sample rates up to 192 kHz, but even
more due to a missing real world application (CD...), Quad Speed has had no broad success so
far. An implementation of the ADAT format as double S/MUX would result in only two channels
per optical output. Devices using this method are not known to us, so we decided to do without
this format.
The AES outputs provide 192 kHz as Single Wire only.