RME Audio MADIface XT, Руководство пользователя

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102
User's Guide MADIface XT
© RME
30.6 DS - Double Speed
When activating the
Double Speed
mode the MADIface XT 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 multichannel
ADAT format. The AES3 specification uses the uncommon term
Single channel double sam-
pling 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.
With MADI, sample multiplexing is often used as well to offer sample rates higher than 48 kHz.
The MADIface XT supports all formats. 96 kHz can be received and transmitted both as 48K
Frame (using S/MUX) and as native 96K Frame. In 48K Frame Double Speed mode, the MADI-
face XT distributes the data of one channel to two consecutive MADI channels. This reduces
the available channel count from 64 to 32.
As the transmission of double rate signals with 48K Frame is done at standard sample rate
(Single Speed), the MADI ports still operate at 44.1 kHz or 48 kHz.
30.7 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 (S/MUX4) results in only two
channels per optical output. Devices using this method are few.
In earlier times the transmission of 192 kHz had not been possible via Single Wire, so once
again sample multiplexing was used: instead of two channels, one AES line transmits only one
half of a channel. A transmission of one channel requires two AES/EBU lines, stereo requires
even four. This transmission mode is being called
Quad Wire
in the professional studio world.
The AES3 specification does not mention Quad Wire.
With MADI, sample multiplexing is used as well to offer sample rates higher than 96 kHz. In
fact, technical reasons require to use this method beyond 96 kHz. A 192K or 384K Frame for-
mat would not be fully compatible to the MADI standard. Therefore 192 kHz is supported as
S/MUX4 only. So in 48K Frame Quad Speed mode, a MADI device distributes the data of one
channel to four consecutive MADI channels. This reduces the available channel count from 64
to 16.
As the transmission of quad rate signals with 48K Frame is done at standard sample rate (Sin-
gle Speed), the MADI ports still operate at 44.1 kHz or 48 kHz.