BNC cables for MADI Digital Audio Standard: RG59 and RG179

75-Ohm BNC cables for Digital Audio

Our BNC cables made with RG59 and RG179 are compliant with the MADI digital audio standard that is capable of supporting multiple channels of digital audio.

MADI-compliant BNC Digital Video Cables: 75-Ohm: RG59 and RG179

BNC cable assemblies are used for Multichannel Audio Digital Interface (MADI). MADI is a digital audio standard that is capable of supporting multiple channels of digital audio. These standards were developed by the Audio Engineering Society (AES), a body comprised of manufacturers and professionals in the audio industry including Sony, Phillips, and Mitsubishi.

The quality of the cable is critical to the overall performance of an audio system.

As MADI carries up to 56 audio channels a dysfunctional or failed cable will affect all the channels and potentially prevent the audio equipment from functioning. Data Alliance’s 75-Ohm BNC cables meet such standards.

Coax cable used for MADI should match the attenuation of RG59U with a loss of 12 dB at 100 MHz and 18dB at 200MHz for a 100 meter (368 feet) length of cable. For longer cable runs (>100 meters), a coaxial cable that exceeds these parameters (i.e. low loss or double-shielded cable) should be used. Testing will determine the absolute maximum length that can be used without excessive noise.

MADI digital audio data transfer over copper links is capable of signal transmission over distances greater than 100 meters (328 feet). 75 Ohm BNC connector cables are specified for use by the AES standards. MADI has a data rate of 100 Mbits per second. This produces a symbol or baud rate of 125 MHz. Data transfer is over channels numbered 0 to 63.

MADI audio installation may be permanent building installations or external patch systems. BNC cable assemblies must be fully compatible and impedance matched with MADI systems, mixers, and consoles, or the audio quality will deteriorate. The standard cable used is usually 75 Ohm RG59U cable which is also used for digital video.

The MADI standard was first released in 1991 with the most recent revision published in 2008. MADI provides specifications and bit-level descriptions for serial digital transmission over either coaxial cable or fiber-optic lines. Transmission along the lines is unidirectional and can have between 28 and 64 channels each with an audio bit depth of 24 bits.

Multichannel Audio Digital Interface (MADI): a standard for Audio Over Coax

Multichannel Audio Digital Interface or MADI is an advanced industry standard for the transmission of multi-channel digital audio. It supports simultaneous transmission over up to 64 individual audio channels via a single coaxial cable with limited loss over hundreds of meters.

MADI has emerged as an audio interface technology that is capable of superseding the cumbersome audio multicore snakes (composite cables that carry up to 64 individual cables in a stiff outer jacket ) that are used in audio engineering. The simplicity of using a single coaxial cable means that common problems encountered with using multicore cable are bypassed including:

  • Stiffness
  • Weight
  • Signal losses
  • Awkward handling and routing

This makes MADI a convenient solution for pro audio requirements, as coax is cheaper, more flexible and cable to deliver the same overall functionality as a snake cable. MADI also delivers sample-accurate transmissions over long distances without the troubleshooting and maintenance needs of multicore.

Audio industry insight and expertise at the forefront of this digital audio technology.

These standards were developed by the Audio Engineering Society (AES), a body comprised of manufacturers and professionals in the audio industry including Sony, Phillips, and Mitsubishi. The MADI standard was first released in 1991 with the most recent revision published in 2019.

The leading audio manufacturers led the development of MADI as a common interface for multichannel digital audio transfer. This agreed specification describes the data organization for this digital audio technology and specifies the simplex transmission format over coaxial cable with a 75 Ohm impedance. This collaborative effort aims to meet the key mission objectives of:

  1. Simple installation, engineering, and use.
  2. Creating a complete functional system
  3. Being cost-effective and simple, so the uptake and implementation of this technology are easily justified.
  4. The implementation of this technology not being dependent on specific hardware or software.

The development and refinement of MADI as a leading audio interface has taken place in response to industry demand, building on the established two-channel interface outlined in AES3.

There has been a steadily increasing uptake of digital audio equipment and rising demand for an increasing number of audio channels to be used in large-scale sound production.

MADI provides specifications and bit-level descriptions for serial digital transmission over either coaxial cable or fiber-optic lines. Transmission along the lines is unidirectional and can have between 28 and 64 channels each with an audio bit depth of 24 bits. MADI digital audio data transfer over pure copper links (versus copper clad steel) is capable of signal transmission over distances greater than 100 meters (328 feet).

How does Multichannel Audio Digital Interface work?

The support of multiple channels by a single cable is achieved by the use of Time Division Multiplexing (TDM) , keeping AES/EBU signals in series. This method of digital signal transmission allows the independent audio signals from each of the channels to use the single coax by using a system of synchronized switches. The switching creates an alternating pattern of sequential short transmissions from each channel on the line.

Unidirectional TDM provides a point-to-point interface between the source and destination for each of the 64 channels it supports. The standard sample rate is between 44.1 and 48 kHz on a coaxial cable with 75 Ohm impedance. Increasing the sample rate reduces the number of channels available for use, with a maximum of 192 kHz attainable if just 16 channels are used.

Data is transferred using frames and subframes. MADI frames consist of 56 subframes, with each subframe containing audio channel, status, and user data for a single channel. Each subframe is made up of 32 bits of which at least 24 bits are digital audio data. The frames are then transferred serially, with the next frame only transmitted once the previous one has been received.

Digital audio is a practical and cost-effective upgrade for analog audio systems:

Because coaxial cable is used, the costs of installing multichannel connections are greatly reduced compared to generic analog multichannel systems that not only use multicore cable but also include:

  • Breakout boxes
  • Circuit connectors
  • Stage boxes
  • Signal splitter

In comparison a MADI system is more cost-effective with a simpler setup and cabling while still including all necessary audio signal processing components and functionality including routing, merging, splitting, redundancy, and remote control. This makes the cost of audio installations in large real estate like theaters or stadia far cheaper and future-forward. MADI installations are also low maintenance in comparison to analog audio, which often requires retrofitting of larger and larger snakes if the number of audio channels is to be increased. The flexibility of MADI audio over coax makes it ideal for a range of settings including multiroom production studios, outside broadcast, live studios, and pop-up installations. MADI hardware such as interface boxes routinely carry fiber optic and female BNC coaxial connector ports.

75-Ohm BNC cables for MADI Digital Audio

The MADI audio standard specifies the use of 75-Ohm coaxial cable terminated by BNC connectors. These higher impedance BNC connector cables are in widespread use for audio applications and are also specified for use by other AES standards.

MADI digital audio data transfer over copper links is capable of signal transmission over distances greater than 100 meters (328 feet). Using this type of coaxial cable, MADI has a data rate of 100 Mbits per second. This produces a symbol or baud rate of 125 MHz

The quality of the cable is critical to the overall performance of an audio system.

As MADI carries up to 56 audio channels a dysfunctional or failed cable will affect all the channels and potentially prevent the audio equipment from functioning.

Coax cable used for MADI should match the attenuation of RG59U with a loss of 12 dB at 100 MHz and 18dB at 200MHz for a 100 meter (368 feet) length of cable. For longer cable runs (>100 meters), a coaxial cable that exceeds these parameters (i.e. low loss or double-shielded cable) should be used. Transmission distances achieved over coax are also affected by the performance of the MADI transmitting hardware that is used to drive the signals down the coax, as well as the sensitivity of receiving equipment that the coax is connected to. Testing will determine the absolute maximum length that can be used without excessive noise.

The quality of the cable assembly also matters. High-quality BNC connectors that correctly terminate the cable with crimping or soldering provide a low-loss connection with discontinuities kept to a minimum. If the cable is self-assembled crimping, with the correct tool, often proves more reliable than soldering. 

Posted by George Hardesty on 22nd Mar 2021


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