RME OCTAMIC XTC [56/62] Madi basics

User’s Guide OctaMic XTC © RME

22.8 MADI Basics

MADI, the serial Multichannel Audio Digital Interface, has been defined already in 1989 as an

extension of the existing AES3 standard following several manufacturers' wish. The format also

known as AES/EBU, a balanced bi-phase signal, is limited to two channels. Simply put, MADI

contains 28 of those AES/EBU signals in serial, i. e. after one another, and the sample rate can

still even vary by +/-12.5%. The limit which cannot be exceeded is a data rate of 100Mbit/s.

Because an exact sampling frequency is used in most cases, the 64 channel mode was intro-

duced officially in 2001. It allows for a maximum sample rate of 48 kHz + ca. 1%, corresponding

to 32 channels at 96 kHz, without exceeding the maximum data rate of 100 Mbit/s. The effective

data rate of the port is 125 Mbit/s due to additional coding.

Older devices understand and generate only the 56 channel format. Newer devices often work

in the 64 channel format, but offer still no more than 56 audio channels. The rest is being eaten

up by control commands for mixer settings etc.. The OctaMic XTC shows that this can be done

in a much better way, with an invisible transmission of 16 MIDI channels plus serial RS232 data

stream, and the 64-channel MADI signal still being 100% compatible.

For the transmission of the MADI signal, proved methods known from network technology were

applied. Most people know unbalanced (coaxial) cables with 75 Ohms BNC plugs, they are not

expensive and easy to get. The optical interface is much more interesting due to its complete

galvanic separation, but for many users it is a mystery, because very few have ever dealt with

huge cabinets full of professional network technology. Therefore here are some explanations

regarding 'MADI optical'.

• The cables used are standard in computer network technology. They are thus not at all

expensive, but unfortunately not available in every computer store.

• The cables have an internal fibre of only 50 or 62.5 µm diameter and a coating of 125 µm.

They are called network cables 62.5/125 or 50/125, the former mostly being blue and the lat-

ter mostly being orange. Although in many cases not clearly labelled, these are always (!)

glass fibre cables. Plastic fibre cables (POF, plastic optical fibre) can not be manufactured in

such small diameters.

• The plugs used are also an industry standard and called SC. Please don't mix them up with

ST connectors, which look similar to BNC connectors and are being screwed. Plugs used in

the past (MIC/R) were unnecessarily big and are not being used any longer.

• The cables are available as a duplex variant (2 cables being glued together) or as a simplex

variant (1 cable). The OctaMic XTC’s opto module supports both variants.

• The transmission uses the multimode technique which supports cable lengths of up to al-

most 2 km. Single mode allows for much longer distances, but it uses a completely different

fibre (8 µm). By the way, due to the wave-length of the light being used (1300 nm), the opti-

cal signal is invisible to the human eye.