MADI

For other uses, see Madi (disambiguation).

Multichannel Audio Digital Interface, MADI or AES10 is an industry-standard electronic communications protocol that defines the data format and electrical characteristics of an interface carrying multiple channels of digital audio. The Audio Engineering Society (AES) standard for MADI was originally documented in AES10-1991, which was updated by AES10-2003. The MADI standard includes a bit-level description and has features in common with the two-channel format of AES3. Serial digital transmission over coaxial cable or fibre-optic lines of 28, 56, or 64 channels is supported, with sampling rates of up to 96 kHz and resolution of up to 24 bits per channel.

Transmission format

MADI links use a transmission format that is similar to the Fiber Distributed Data Interface (FDDI) networking technology (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 is more closely related 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. 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, although with more channels. Rather than letters, they are assigned 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 4 time slots of each subchannel are encoded as normal data, used for subchannel 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 say that higher sampling frequencies may be accommodated (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 the purpose of linking large-format mixing consoles to digital multi-track recording devices. Large broadcast studios adopted it for use routing multi-channel audio throughout their facilities as well. The 2003 revision, called AES10-2003, adds a 64 channel capability as well as 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.

Usage

MADI was developed by AMS Neve, Solid State Logic, Sony and Mitsubishi and is widely used in the audio industry, especially in the professional sector. Its 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) are: first, support of a greater number of channels per line; and second, the use of coaxial and optical fibre media that enable the transmission of audio signals over 100 meters and up to 3000 meters. Some main providers of interfaces and computer cards for MADI are:

• Allen & Heath
• AMS Neve
• Avid Technology
• Calrec Audio
• Cobalt Digital
• Deubner Hoffmann Digital
• DiGiCo
• DirectOut
• Euphonix
• Evertz
• Fairlight
• Ferrofish
• Harrison Audio Consoles
• Harris Corp.
• Innovason
• Klotz Digital
• Lab X Technologies
• Lawo
• Lynx Studio Technology, Inc.
• Merging Technologies
• Miranda Technologies
• Optocore
• PESA Switching Systems
• RME
• SADiE
• Sierra Video Systems
• Solid State Logic
• Sonic Core
• Soundcraft
• Stagetec
• Studer
• Sydec Audio
• Yamaha Commercial Audio

References

• AES10-1991 AES Recommended Practice for Digital Audio Engineering — Serial Multichannel Audio Digital Interface (MADI), Audio Engineering Society, 1991, http://www.urbancom.us/aes10-1991.pdf, retrieved 2010-03-03 
• AES10-2003 AES Recommended Practice for Digital Audio Engineering — Serial Multichannel Audio Digital Interface (MADI), Audio Engineering Society, 2003, http://www.iis.ee.ethz.ch/~felber/DataSheets/AES-EBU/aes10-2003.pdf, 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, http://www.tvtechnology.com/article/68328, retrieved 2009-03-24