RME Audio Fireface 802, User Manual

Page: 104 / 112

104 User's Guide Fireface 802 © RME
To use FireWire as efficiently as possible, the Fireface allows to reduce the number of trans-
ferred channels.
Limit Bandwidth
provides four options, limiting the transmission internally to 30,
22, 14 or 8 channels. This limitation is independent from the sample rate. As can be seen in the
following table, in 96/162 kHz mode there is no difference between the setting
All Channels
and
An.+AES+ADAT1
as the Fireface 802 offers only 18 channels in Quad Speed mode anyway.
Limit Bandwidth 48 kHz (30) 96 kHz (22) 192 kHz (18) FW-
Channels
All Channels x / / 30
An.+AES+ADAT1 x x / 22
AES x x x 14
Analog 1-8 x x x 8
35.5 DS - Double Speed
When activating the
Double Speed
mode the Fireface 802 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 (Sample Multiplexing)
in connection with the ADAT format.
Because the ADAT interface does not allow for sampling frequencies above 48 kHz (a limitation
of the interface hardware), the Fireface 802 automatically uses
Sample Multiplexing
in DS
mode. One channel's data is distributed to two channels according to the following 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.
35.6 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 results in only two channels per
optical output.
In Quad Speed mode the Fireface 802 automatically uses
Sample Multiplexing
. One channel's
data is distributed to four channels according to the following table:
Analog In 1 2 3 4 5 6 7 8
QS Signal
Port
1/2/3/4
ADAT1
5/6/7/8
ADAT1
1/2/3/4
ADAT2
5/6/7/8
ADAT2
- - - -
In any way the ADAT output physically always delivers 44.1 kHz or 48 kHz.