RME ADI-648 [23/32] Technical background
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RME ADI-648 [23/32] Technical background

RME ADI-648 [23/32] Technical background
User's Guide ADI-648 © RME
23
12. Technical Background
12.1 DS - Double Speed
When activating the 96k FRAME, the MADI output of the ADI-648 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, of
which the 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 being called Double Wire in the professional studio world, and is also
known as S/MUX in connection with the ADAT format.
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 limitation
of the interface hardware), the ADI-648 automatically uses the described Sample Split method
in DS mode. One channel's data is distributed to two channels according to the following table:
Original 1 2 3 4 5 6 7 8
DS Signal
Port
1/2
1
3/4
1
5/6
1
7/8
1
1/2
2
3/4
2
5/6
2
7/8
2
As the transmission of double rate signals is done at standard sample rate (Single Speed) the
word clock output still delivers 44.1 kHz or 48 kHz.
Nearly all devices with ADAT and TDIF ports generate a Single Speed word clock, because
their ADAT and TDIF interfaces work in the same mode. At 96 kHz sample rate, the used
word clock will be no higher than 48 kHz.
Therefore the word clock output of the ADI-648 also will be no higher than 48 kHz. In case a
true Double Speed word clock is needed (88.2 or 96 kHz), press the key 96k FRAME several
times, until the LED WCK is lit.
12.2 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. Therefore in Quad Speed mode the ADI-648 is limited to 2 chan-
nels at the ADAT outputs. As MADI also uses the Quad Wire method at Quad Speed, only 16
channels are available instead of 64.

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