MADI

MADI interface-box (RME MADIface) equipping both optical and coaxial interfaces, each can handle 64ch digital audio per link.

Multichannel Audio Digital Interface (MADI) or AES10 is an Audio Engineering Society (AES) standard electronic communications protocol that defines the data format and electrical characteristics of an interface that carries multiple channels of digital audio. The AES first documented the MADI standard in AES10-1991, and updated it in AES10-2003 and AES10-2008. The MADI standard includes a bit-level description and has features in common with the two-channel format of AES3. It supports serial digital transmission over coaxial cable or fibre-optic lines of 28, 56, or 64 channels; and sampling rates of up to 96 kHz with resolution of up to 24 bits per channel. Like AES3 or ADAT it is a Uni-directional interface (one sender and one receiver).

Transmission format

MADI links use a transmission format similar to Fiber Distributed Data Interface (FDDI) networking (ISO 9314), which was popular in the mid-'90s for backbone links between LAN segments. Since MADI is most often transmitted on copper links via 75 ohm coaxial cables, it more closely compares to the FDDI specification for copper-based links, called CDDI.

AES10-2003 recommends using BNC connectors with coaxial cables (see part 7.1.4) and ST1 connectors with optic fibres (see part 7.2.1). The specifications about fibres can provide a range of up to 2 km.

The basic data rate is 100 Mbit/s of data using 4B5B encoding to produce a 125 MHz physical baud rate. Unlike AES3, this clock is not synchronized to the audio sample rate, and the audio data payload is padded using "JK" sync symbols.

The audio data is almost identical to the AES3 payload, though with more channels. Rather than letters, MADI assigns channel numbers from 0–55 or 0–63. The only difference is that frame synchronization is provided by sync symbols outside the data itself, rather than an embedded preamble sequence, and the first four time slots of each sub-channel are encoded as normal data, used for sub-channel identification:

• Bit 0: Set to 1 to mark channel 0, the first channel in each frame
• Bit 1: Set to 1 to indicate that this channel is active (contains interesting data)
• Bit 2: notA/B channel marker, used to mark left (0) and right (1) channels. Generally, even channels are A and odd channels are B
• Bit 3: Set to 1 to mark the beginning of a 192-sample data block

Sync symbols may be inserted at any subframe boundary, and must occur at least once per frame (0.45% minimum overhead.)

Sampling frequency

The original AES10-1991 specification allowed 56 channels at sample rates from 32 to 48 kHz with a tolerance ±12.5%, in part 4.1. It leads to a total range of 28 to 54 kHz. This produced a total of 56×32×54 = 96768 kbit/s, leaving 3.232% of the channel for synchronization marks and transmit clock error.

The 2003 revision specifies different relations between sampling frequency and number of channels. The next list is from the part 5.1 of the AES10-2003 specifications:

• 32 kHz to 48 kHz ± 12.5%, 56 channels;
• 32 kHz to 48 kHz nominal, 64 channels;
• 64 kHz to 96 kHz ± 12.5%, 28 channels.

If the data rate is limited to 48 kHz, then 64 channels take 64×32×48 = 98304 kbit/s. Adding the minimum 8×58 kbit/s of framing produces 98688 bit/s, leaving 1.312% free for timing variation or additional channels.

Both versions of the standard accommodate higher sampling frequencies (for example, 96 kHz or 192 kHz) by using two or more channels per audio sample on the link.

Improvements

The original specification (AES10-1991) defined the MADI link as a 56 channel transport for linking large-format mixing consoles to digital multi-track recording devices. Large broadcast studios also adopted it for routing multi-channel audio throughout their facilities. The 2003 revision, called AES10-2003, adds a 64 channel capability and support for "double-rate" sampling at 96 kHz by removing vari-speed operation. The latest AES10-2008 standard includes minor clarifications and updates to correspond to the current AES3 standard.

Use

MADI was developed by AMS Neve, Solid State Logic, Sony and Mitsubishi^[1][2] and is widely used in the audio industry, especially in the professional sector. It provides advantages over other audio digital interface protocols and standards such as AES/EBU (AES3), ADAT (Alesis Digital Audio Tape), TDIF (Tascam Digital Interface), and S/PDIF (Sony/Philips Digital Interconnect Format). These advantages include:

• Support for a greater number of channels per line
• Use of coaxial and optical fiber media that support transmission of audio signals over 100 meters, up to 3000 meters over Multi Mode and 40,000 meters over Single Mode fiber

Main providers of interfaces and computer cards for MADI include:

References

• AES10-1991 AES Recommended Practice for Digital Audio Engineering — Serial Multichannel Audio Digital Interface (MADI) (PDF), Audio Engineering Society, 1991, retrieved 2010-03-03 

External links

• Audio Engineering Society standards, in print
• Gruszka, by Mary C. (2008-10-15), "Serial Multichannel Audio Digital Interface", TV Technology, retrieved 2009-03-24