For different digital audio subsystems, several microprocessor or DSP (Digital Signal Processor) and audio devices are used for digital conversion interfaces. This article will introduce several audio interface specifications currently in the market.
PCM specifications
One of the simplest audio interfaces is the so-called PCM (Pulse Code Modulation) interface. Strictly speaking, all digital signals are transmitted through PCM, and it is necessary to carefully refer to the monophonic mechanism used for digital phones. The PCM interface is composed of a clock pulse (BCLK), a frame synchronization signal (FS), and a data queue. Each PCM corresponds to a piece of data to be received or sent.
On the rising edge of the FS signal, data transmission starts from the MSB (Most Significant Bit) word, and the FS frequency is equal to the sampling rate. After the FS signal, the transmission of the data word starts, the single data bits are transmitted in sequence, and one data word is transmitted in one clock cycle. When sending the MSB, the signal level is first reduced to the lowest to avoid the loss of the MSB when using different data schemes on the interfaces of different terminals. In addition to the RJ (Right-JusTIfied) format that is fading in applications, this method has been used in most audio interfaces.
The PCM interface is easy to implement. In principle, it can support any data scheme and any sampling rate, but each audio channel needs to obtain an independent data queue. This attribute will make PCM very popular in primary target application systems such as digital phones. s Choice.
I2S specifications
The I2S interface (Inter-IC Sound) was first used by Philips to consume audio in the 1980s and undergoes multiple conversion in a signal mechanism called LRCLK (Left / RightCLOCK) to convert two audio signals into a single data queue. When LRCLK is high, the left channel data is transmitted; when LRCLK is low, the right channel data is transmitted. Compared with PCM, I2S is more suitable for stereo systems. For multi-channel systems, it is also possible to execute several data queues in parallel under the same BCLK and LRCLK conditions.
However, Hi-Fi audio requirements in portable systems are higher than stereo. First, more complex audio ICs are usually controlled by writing to internal registers. Since I2S, PCM and similar audio interfaces cannot provide register entries, an independent control interface is required, such as increasing the number of audio IC pins on the controller. Second, the ability to perform audio at different sampling rates is critical. Both 44.1kHz (the standard frequency of audio CDs) and 48kHz (the standard frequency of computer audio) are very widely used frequencies.
For I2S and its derivative series, the system either generates Low-jitterBCLK and LRCLK at different frequencies (or FS in the case of PCM), or converts all audio streams to a single sampling rate in the software environment . The first case requires at least one analog phase-locked loop (PLL) and two simultaneous feedbacks, and recording on different frequencies. And when evaluating the efficiency of the interface, the increased power consumption must be taken into account. Although the second case enhances the computing power, it also significantly increases the power consumption of the processor. And when this processor executes user applications at the same time, the entire system will run slower, and even stop running when the audio is turned on.
Now more and more consumers want to get better hearing effects and more functions from digital cameras, digital video cameras, MP3 players, portable DVD players, portable multimedia players and multimedia processors. Wolfson has expanded the scope of the decoder for this purpose. The recently launched WM897x system I2S audio protocol not only improves the integration of the system, but also improves the audio quality of the system.
Taking stereo WM8980 and mono WM8982 as examples, it can be directly connected to the TV through the screen display. In a portable system, the same video function as the high-quality audio function requires an additional video amplifier to realize it. The WM8978 system is an upgrade to the full stereo function of the WM8974 system launched in October 2004.
The three products use the DSP microprocessor as the core, which can filter wind sounds to improve the recording function of the audio system, especially in the visualization system. In addition, the new product also uses the equalization of the 5 band and 3D audio system to improve the audio output and the programmable resistance filter to eliminate noise. These systems usually also support microphones with clock frequencies between 12MHz and 19MHz, as well as driver parts for mobile phone speakers, which can further reduce the number of components in the product. In order to meet the power consumption of high-quality audio speakers and piezoelectric speakers can reach 900mW, the digital recording playback limiter prevents excessive output of the speakers, the analog part of the three decoder products requires a power supply voltage as low as 2.5V, and the digital part The supply voltage is as low as 1.6V.
With the rapid increase in demand for mobile products, by using two interface technologies in one, it is possible to integrate a simple form of mono audio transmission with an expandable standard such as Hi-Fi function. This combination greatly improves The service life of the battery, and can integrate resources, such as handling incoming calls easily during MP3 playback.
WM8753L integrates the stereo Hi-Fi analog-to-digital converter in IIS protocol and independent mono PCM digital-to-analog conversion technology in a single chip, and has the digital-to-analog conversion function of both IIS protocol and PCM interface, which makes MP3 , Dialogue and other audio functions can work together.
The MAX8753L is one of the few decoders that incorporates the PCM / Hi-Fi function in the analog part with a working voltage of less than 1.8V and the digital part with a working voltage of less than 1.42V. Under the working voltage of 1.8V, the minimum power consumption of the decoder in stereo playback is 7mW, and the minimum power consumption in the PCM working state is less than 6mW. The system integrates dual interface technology for connecting different amplifiers, including the driving part of speakers, headphones and earpieces. External devices no longer need separate headphone or headphone amplifier parts. Cap-less interface can connect all loads. The embedded digital signal processing system can control the tone, bass enhancement, automatic adjustment of the headphone volume or analog-to-digital converter. These two analog-to-digital conversion methods can perform noise cancellation or stereo storage for dual DSP systems.
The WM8753LHi-Fi analog-to-digital converter can be used either as a control part or as a system with a USB interface in a main clock frequency of 12MHz to 24MHz, a 19.2MHz mobile system, and a standard 256fs ratio of 12.288MHz and 24.576MHz. The charged part. The internal phase-condensation loop system can generate the clock frequency required for PCM and Hi-Fi conversion. If the required clock frequency in the audio system already exists, the PLL can be used for other purposes.
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