With the rapid development of electronic technology and digital technology, digital audio has been widely used in various application fields such as recording, broadcasting and transmission of broadcast television. In many cases, analog audio has been unable to accommodate the most basic requirements of the entire sound reinforcement system. The key issue in the design of large stadium sound reinforcement systems is how to solve the long-distance high-quality transmission of weak audio signals. For large venues, the distance that needs to be transmitted is usually several hundred meters. Using traditional analog transmission methods, it is difficult to solve the problems of signal loss, electromagnetic interference and grounding interference. The performance of digital audio is far superior to the analog mode, so the digitization of broadcast television equipment has become an inevitable trend. The advantage of using digital signals for transmission and processing is that digital signals are not sensitive to interference. The signal-to-noise ratio and distortion of the whole system are independent of the transmission distance. For long-distance transmission, its excellent performance index is unmatched by analog transmission.
At present, the studios of radio and television stations are developing in the direction of digitization. As the main function of digital TV production, the basic theory, interface mode, audio format and system design of digital audio have also become major issues in the field of radio and television program production. However, most of the current high-performance digital broadcast and transmission equipment are imported equipment and are expensive. This article is designed to design a high-performance digital audio transmission system for this field.
2 digital audio interface standardAt present, the commonly used digital audio interface standards mainly include AES/EBU (AES3-1992) interface, S/PD IF interface, MAD I interface, and the like. The S/PD IF is mainly used as a civil digital audio format standard. The MAD I interface is based on the dual-channel AES/EBU interface. The AES/EBU interface standard is mainly used in the professional digital audio field.
The full name of AES/EBU is Audio Engineering Society/Eu2ropean B roadcast Union, which has become a popular standard for professional digital audio, a large number of civilian products and professional audio digital equipment such as CD player, DAT, AES/EBU is supported by MD machines, digital mixers, and digital audio workstations.
The AES/EBU standard is a digital audio transmission standard developed by AES and EBU. It is a digital device interface protocol for transmitting and receiving digital audio signals. It is required that audio data must be encoded in 2's complement. The transmission medium is a cable that allows for high bandwidth capacity and serial transmission of parallel data bytes generated by the A/D converter. When serially transmitting 16 to 20 bit parallel bytes, the least significant bit is transmitted first. The byte clock flag must be added to indicate the start of each sample. The last data stream is bi-phase code code encoding, and the clock information is also included. Embedded in the AES/EBU signal stream.
AES/EBU transmits digital audio data based on a single twisted pair, using a serial bit transfer protocol to transfer data over distances up to 100 m without equalization. It provides audio data for two channels (up to 24-bit quantization), and the channels are automatically timed and self-synchronized.
It also provides a method of transmission control and a representation of status information (chan2nel status bit) and some error detection capabilities. Its clock information is controlled by the transmission side, and the bit stream from AES/EBU.
The common physical connection medium of AES/EBU is: (1) Balanced or differential connection, three-core mic shielded cable using XLR (card) connector, the parameter is impedance 110Ω, the level range is 0. 2 ~ 5 Vpp, the jitter is ±20 ns. (2) Single-ended unbalanced connection, audio coaxial cable with RCA plug. (3) Optical connection using a fiber optic connector.
Since the revision of AES/EBU in 1992, the standard has been widely used in the recording production, digital cinema and broadcast television industries, becoming the most common digital audio format, with related equipment, interfaces, cables, accessories, etc., and at a low price.
3 system circuit design3. 1 overall plan of the system
The entire digital audio transmission system is divided into three parts: the transmitting end, the receiving end and the transmission medium (cable), as shown in Figure 1. Transmission media mainly include twisted pair shielded cable, coaxial cable, optical fiber and wireless transmission (such as PDH or SDH digital microwave), which are selected according to the specific occasion and transmission distance.
Figure 1 Block diagram of the digital audio transmission system.
The transmitting end mainly performs the operations of accessing signals, A/D conversion, format coding, and clock generation. In order to increase the dynamic range of the signal while preventing aliasing distortion in the A/D conversion, a signal conditioning circuit and an anti-aliasing filter should be provided in the analog input channel.
The receiving end mainly completes the receiving and decoding of the AES/EBU format data, recovers the main clock signal, the synchronization signal, and then performs D/A conversion on the audio data.
3. 2 Transmitter circuit design
According to the system scheme described in the previous section, we use C5241 and CS8406 of Cirrus Logic to complete the A/D conversion of analog signals and the transmission of AES/EBU format codes respectively. The circuit principle is shown in Figure 2.
Figure 2 Schematic diagram of the transmitting end.
The CS5381 is CirrusLogic's 120 dB, 192 kHz high performance 24-bit stereo analog-to-digital converter. The CS5381 can operate in both master and slave modes. Mode selection is done via pin 2 (M / S) and the design works in master mode. The CS5381 sample rate can be controlled by the three pin logic levels of MD IV, M0 and M1. The master clock selection can be selected based on the selected sampling frequency and the MD IV pin. The 48 kHz single-speed sampling rate is selected in this design, and the 12.288MHz active crystal oscillator is used as the clock source. The CS5381 conversion result is 24-bit complement form serial data, and the left and right channels are alternately output, which can be distinguished by LRCK high and low levels. There are two formats for output data, left-justified and I2S format. This design uses the I2S format.
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