MADI transceiver

Project: MADI transceiver
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Members	Danny Witberg
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Description	This project descibes a MADI transceiver in VHDL
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Introduction

The Multichannel Audio Digital Interface or MADI protocol is a way to send multiple digital audio channels over a single transmission line. The protocol describes up to 64 channels of up to 24 bit audio at a sample rate of 48kHz. The sample rate can be higher at the expense of the number of channels, so up to 32 channels of 96kHz sampled audio is also a possibility. With the S-MUX protocol, even higher sample rates can be achieved with putting consecutive samples in adjacent channels. This project describes an implementation of the MADI protocol with the use of an FPGA in the VHDL language.

MADI protocol

The MADI protocol is based on the not-so-popular-anymore FDDI protocol, which was a networking protocol. MADI was invented by the AES or Audio Engineering Society back in 1991, and became known as the AES10 standard. The line speed or symbol speed of MADI is 125Mbps, while the transmission speed is 100Mbps. The difference is because of a line encoding scheme known as 4B5B, which converts 4 incoming bits into 5 outgoing bits. Also, for synchronisation purposes, there is a special bit sequence. A MADI datastream consists of a continuous flow of frames, each carrying up to 64 channels. The maximum length of a frame is equal to the sampling rate, because between frames, the synchronisation symbol is sent. Each channel has 4 mode bits, 24 bits representing the audio data, and 4 other bits used for other purposes. So, a total of 32 bits is needed for a channel. The maximum effective transmission speed is 32 bits x 64 channels x 48000Hz is 98304000 bits per second or 98.304Mbps. This leaves a minimum of 1.696Mbps for synchronisation.