ALSA Audio Architecture What does audio AES mean-Geffen Maxtrix Control System Manufactory CN

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ALSA Audio Architecture What does audio AES mean

2020-08-29

1. Summary

The three standard sample rates for AES are 32kHz, 44.1kHz, and 48kHz, and studio applications prefer 48kHz sampled audio with a clock synchronized to the video signal, in this case 48kHz synced audio. It provides 2 channels of audio data at the same time, and the quantization accuracy is mostly 20 bits or 24 bits, which can not only transmit mono or stereo audio information alone, but also can also carry clock reference signals and related auxiliary information.

2. Standards

The AES structure is shown in the following diagram. In AES, audio exists in pairs of channels. A subframe containing 32bits of data is arranged in the format below. The 2 subframes make up one frame, and the 192 frames make up another block. A subframe is the smallest unit in the process of audio encoding and packaging, and the composition of subframes is analyzed below.
There are a total of 32 time slots in the subframe, and the first four time slots 0-3 contain 3 prefixes, which are precodes X,

Prefix Y and Prefix Z.

(1) Precode X: indicates the start of transmission of frames other than the first frame.

(2) Precode Y: indicates the start of the subframe transmission of the second channel in a channel pair.

(3) Precode Z: Indicates the start flag of the transmission of the first frame in an audio block consisting of 192 frames, with each block appearing only once.

The role of the precode is to identify and synchronize audio blocks, audio frames, and subframes. AES audio is encoded with biphasic flags, and the precodes are encoded in a special way that is different from the biphasic flags encoded to eliminate the presence of pre-coded data. This eliminates confusion such as data mispacking caused by similar data instead of prefixing. The stability and security of data transmission have been improved.
The four slots of 4-7 bits contain some additional information that is used in 24-bit quantized encoding. Auxiliary information can contain a variety of content, such as some additional audio call channels that are not of high quality. If you don't want to use it, just set it to 0.

There are 20 time slots from 8 to 27 bits, from the lowest bit LSB to the highest bit MSB, and these 20 bits are pure audio data and no other types of information.

28-31 bits are the last 4 time slots, and one contains 4 different state bits.

User Data Bit U: It is a reserved auxiliary bit, which can be used by users according to their own design requirements, and the biggest advantage is that it can be occupied at will without affecting audio transmission. This bit defaults to 0 initially, and users can also display their company information in U.

Channel data status bit C: AES can transmit mono or stereo audio. In the case of binaural, the channel state of subframe 1 and subframe 2 in any single audio block is different. C generally includes the following information: the number of left and right channels, sampling frequency, time coding, length of sampling words, etc. C is usually used to express some state information about the channel

Polar bit P: Also known as the parity bit, when there is a bit error in the audio transmission, it will check all data except the prefix, and is generally set to indicate even polarity, indicating the number of bits with an even number of 0 or even 1.

Valid Data Bit V: It is a judgment indication flag. The system determines whether the received digital audio data can be converted into a PCM-encoded analog audio signal. If it can be converted, the position is 1, and vice versa, it is set to 0. When set to 0, the receiver will automatically block it.

A complete subframe is shown below.

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