RME HDSP 9652 [73/75] Ds double speed

User's Guide HDSP System HDSP 9652 © RME

30.3 DS - Double Speed

When activating the Double Speed mode the HDSP 9652 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 limitation

of the interface hardware), the HDSP 9652 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

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.