Serial Multichannel Audio Digital Interface
October 15, 2008
The standard AES3 signal can carry up to two channels of digital audio, either on 110 ohm twisted pair or 75 ohm coaxial cable. However, a single coaxial, and for that matter fiber-optic cable, is capable of carrying even more channels.
So why not multiplex a bunch of AES3 channels together to make it easier to wire among studios, control rooms, and central equipment rooms for digital audio consoles and routing switchers, and between stage boxes and mixing consoles?
What a great idea. The AES standards committee thought so too. Their efforts resulted in the "AES Recommended Practice for Digital Audio Engineering—Serial Multichannel Audio Digital Interface (MADI)," which is currently AES10-2003.
THE CURRENT STANDARD
The MADI standard describes the serial digital transmission over coaxial or fiber-optic cable "of 32, 56, or 64 channels of linearly represented digital audio data at a common sampling frequency within the range of 32 kHz to 96 kHz, having a resolution of up to 24 bits per channel."
At 48 kHz sampling rate, MADI can accommodate the multiplexing of 16, 28 or 32 AES3 channels including their associated validity (V), user (U), channel status (C) and parity bits (P). At 96 kHz sampling rate, MADI can carry up to 16 AES3 channels.
MADI originally allowed for up to 28 multiplexed AES3 signals (56 audio channels), so be aware that older equipment wasn't designed to handle the greater channel capacity allowed in the current 2003 standard.
MADI specifies asynchronous simplex single-point to single-point connections, that is, from one transmitter to one receiver. The standard specifies a maximum data transfer rate of 100 Mbps with a link channel rate of 125 Mbps no matter the sampling rate or the quantity of active audio channels.
The type of cable called for in the MADI spec is either a 75 ohm coaxial cable (terminated in BNC connectors) or a fiber-optic cable (terminated in STI connectors).
For coax, the standard specs that the peak-to-peak voltage of the output when terminated by a 75 ohm resistor shall be between 0.3 and 0.6 V.
The fiber-optic interface must follow ISO/IEC 9314-3 specifications. The fiber cable, the MADI standard says, "should be graded-index fiber within a core diameter of 62.5 mm, a nominal cladding diameter of 125 mm, and a numerical aperature of 0.275 at a wavelength of 1300 nM." According to AES10-2003, this can provide a range of up to 3 km.
A MADI frame consists of a sequence of 64 or less subframes, with each subframe holding one audio sample plus the V, U, C, and P bits from one audio channel. The audio channels transmitted in a MADI frame occur sequentially and in order.
|Fig. 1: MADI frame for 48 kHz sampling rate with 56 active audio channels (56 subframes per frame). AES3 A channel refers to AES3 subframe 1. AES3 B channel refers to AES3 subframe 2. Reference: AES10-2003. |
Fig. 1 shows the MADI frame construction for 56 active audio channels, i.e. 56 subframes (28 AES3 channels) and the timing for 48 kHz sampling rate.
(In the figures, the bit on the left side of the diagram is the one transmitted first.)
Each MADI subframe consists of 32 bits as shown in Fig. 2.
Bits in timeslots 4–31 are easily recognized as the standard AES3 audio sample word bits plus the V, U, C and P bits. According to the MADI standard, any unused audio bits must be set to zero.
The first four bits of a MADI subframe (zero–3) are mode bits used for frame synchronization and to indicate whether a channel is active or not. When active, the channel active bit is set to one for each active channel.
If a channel is inactive, all of its bits must be set to zero as well as the channel active bit.
|Fig. 2: MADI channel subframe data format. Reference: AES10-2003. |
The mode bits also indicate AES3 block start, and AES3 channel identification—the A channel corresponding to AES3 subframe 1, and the B channel corresponding to AES3 subframe 2.
Like AES3, MADI is polarity-insensitive and DC-free.
Unlike AES3, sample timing information is not intended to be carried on a MADI link.
According to the MADI standard, "each transmitter and receiver shall be provided with an independently distributed master synchronizing signal. This signal shall be in accordance with AES11. Alternatively, a SMPTE or EBU video reference signal may be used in accordance with the timing reference point provisions of AES11."
(AES11 refers to the AES standard "AES11-2003: AES recommended practice for digital audio engineering—Synchronization of digital audio equipment in studio operations.")
The standard further states that the exact timing of connected equipment shall be controlled by the independently distributed master synchronizing signal, not by MADI.
In addition, a 96 kHz coded link "shall be capable of being synchronized by either a 48 kHz or 96 kHz reference signal."
When designing an audio facility, don't overlook MADI for an interconnection scheme. With major console manufacturers offering MADI interfaces, this could be a good solution for managing multiple audio feeds.