RME HDSPe MADI FX [83/87] Ds double speed

User's Guide HDSPe MADI FX © RME

Because the HDSPe MADI FX is a completely digital interface, and the delays introduced by

external AD/DA-converters or other digital interfaces are unknown to unit and driver, the drivers

include the digital offset values (3 / 6 / 12 samples). Therefore the delays caused by external

converters have to be taken care off in the record software, which usually means that the user

has to enter specific offset values manually.

Note

: Cubase and Nuendo display the latency values signalled from the driver separately for

record and playback. The current driver includes a safety offset of 32 samples for the playback

side only, which will be included in the shown value.

Core Audios Safety Offset

Under OS X, every audio interface has to use a so called safety offset, otherwise Core Audio

won't operate click-free. The HDSPe MADI FX uses a safety offset of 32 samples. This offset is

signalled to the system, and the software can calculate and display the total latency of buffer

size plus offset plus safety offset for the current sample rate.

30.4 DS - Double Speed

When activating the Double Speed mode the HDSPe MADI FX 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 HDSPe MADI FX 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

HDSPe MADI FX 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.