1. AES/EBU interface standard
The digital audio transmission interface standard developed by AES and EBU:
AES/EBU standard, namely AES3-1992, ANSI S4.40-1992, or IEC-958 standard.
It is a digital device interface protocol for transmitting and receiving digital audio signals:
1) It is stipulated that audio data must be encoded with 2's complement.
2) The transmission medium is a cable, which allows serial transmission of high-bandwidth capacity and parallel data bytes. The least significant bit is transmitted first when serial transmission of 16 to 20-bit bytes.
3) The serialized data flows through the formatter and adds a byte clock mark to indicate the beginning of each sample value.
4) The formatted serial data stream is encoded by a two-way mark code encoder and output, and the final transmitted data stream is a two-phase mark code code stream.
2. AES/EBU interface data structure
An audio frame includes two 32-bit sub-frames (sub-frame 1 and sub-frame 2), and one sub-frame includes only one sample data of one audio channel: 20 bits, synchronization data (sub-frame header): 4 Bit, additional data: 4 bits, valid bit (V): 1 bit, user bit (U): 1 bit, channel status bit (C): 1 bit, parity bit (P): 1 bit. (1 subframe = 1 sample)
Every 192 audio frames constitute a block. Mark the beginning of each block with the marker Z in the data stream. In a 48kHz sampling system, the time of each audio frame is 20.83s. The time of an AES/EBU block is 20.83s×192=4000s.
The meaning of three kinds of 4-bit synchronous data:
Z: Indicates the beginning of the first frame of each audio block.
X: Represents the beginning of each remaining frame in a block.
Y: Indicates the start of subframe 2 of each frame.
The length of these synchronization data is 4 bits, which is different from other data structures in the subframe and does not need to be encoded with a bi-phase mark code.
Audio frame structure in AES/EBU signal format Audio frame = subframe 1 + subframe 2, from the same audio source
Synchronous data waveform of AES/EBU digital audio frame (8-bit encoding sequence) The DC component of this synchronous data structure is the smallest, which is conducive to clock recovery and identification of sub-frames in the serial data stream.
Each subframe has 4 additional bits:
Valid bit (V): The sample data is audio and can be D/A converted, then this bit is set to 0. Otherwise, there is a problem with the sample, and the receiving device will output muting.
User bit (U): sent to a 28×8bit memory. There are 192 subframes per channel in an audio block, so there are 192 user bits in the memory.
Channel bit (C): sent to a 28×8bit channel state memory. This bit is very important for the identification of the audio data content.
The channel state memory describes the bit allocation and its meaning in the AES/EBU data stream channel. For example, the bit 0 of byte 0 indicates whether it is household or professional. If the channel is used for consumption, bit 0 in byte 0 is set to 0; for professional use, it is set to 1.
Parity bit (P): usually even parity. The even check ensures that the number of 1s in the 64 bi-phase flag symbols in a subframe is even. The parity bit can detect an odd number of errors that occur during transmission. Some devices ignore this bit or do not handle this identification correctly.
3. AES/EBU data characteristics
When the sampling frequency is 48kHz, the total data rate is 32×2×48000=3.072Mbps. After the bi-phase mark code is encoded, the data transmission rate is doubled, 6.144Mbps. The spectral energy of the bi-phase mark code is 0 at a multiplier of 6.144 MHz.
The sync word consists of three low units and three consecutive high units that follow. Occupies a low fundamental frequency in the AES/EBU signal spectrum, 3.072/3=1.024MHz.
Each audio frame includes 64bit, and one frame is sent out every 20.83s.
The duration of a data bit in the frame is 325.5ns
The bit unit time of a bi-phase mark code is 163ns. The eye width time of the eye pattern generated by the superposition of some data stream bits is 163ns.
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