Alesis Hammerfall DSP System manual DS Double Speed, Core Audios Safety Offset

Note: Cubase and Nuendo display the latency values signalled from the driver separately for record and playback. While with our digital cards these values equal exactly the buffer size (for example 3 ms at 128 samples), the HDSP MADI displays an additional millisecond – the time needed for the AD/DA-conversion.

Core Audios Safety Offset

Under OS X, every audio interface has to use a so called satety offset, otherwise Core Audio won't operate click-free. The HDSP MADI uses a safety offset of 32 samples. This offset is sig- nalled to the system, and the software can calculate and display the total latency of buffer size plus AD/DA offset plus safety offset for the current sample rate.

30.4 DS - Double Speed

When activating the Double Speed mode the HDSP MADI 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 HDSP MADI 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 HDSP MADI 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.

72 User's Guide HDSP MADI © RME