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Abstract: This article discusses the basic structure, basic working principles and advantages of pure digital DDX audio amplifiers, and how to use DDX audio amplifiers for product design and development.
Key words: DDX modulation; digital-to-analog converter; DDX audio amplifier
Foreword
With the rapid development of digital audio and digital audio technology, digital audio amplifiers that directly power digital audio signals without the need for analog conversion (DAC) have been rapidly developed, and they are highly efficient and can be directly interfaced with digital audio sources. To achieve end-to-end pure digital audio processing and amplification. This DDX audio amplifier accepts digital audio-encoded signals from the DSP's direct input and uses patented DDX signal processing to control high-efficiency power devices without the need to prepare D/A converters for each channel, reducing the intermediate The necessary conversion level, the sound quality is significantly improved, and the cost is reduced as the number of parts is reduced, bringing high sound quality, low power consumption and low manufacturing cost to popular high-speed applications such as flat-panel TVs. Machines, wireless products and personal audio systems.
The basic structure of the DDX audio amplifier
The DDX audio amplifier consists of two main parts: the first part is a modulator using patented DDX technology, which converts the PCM digital audio data obtained by the digital audio interface or A/D conversion into a tri-state modulated signal output; the second part is A power output stage that includes a three-state drive logic circuit and a full bridge circuit. The three-state modulated pulse signal controls the on and off of the transistors in the full-bridge circuit, and generates pulse signals of opposite polarities at both ends of the load. The frequency components of the pulses include the restored audio signal and the high-frequency components associated with the modulation process. Therefore, it is usually necessary to insert a low-pass filter between the output stage and the speaker to prevent the high-frequency component from directly driving the speaker, thereby obtaining a restored and amplified audio output on the speaker (as shown in Figure 1).
Figure 1 DDX basic function block diagram
DDX audio amplifier driving method and modulation method
The output stage of the DDX audio amplifier uses a full-bridge circuit that includes two half-bridge output stages. Each half-bridge circuit includes two output transistors, one being a high-side power transistor connected to a positive supply and the other being a low-side power transistor connected to a negative supply. The full-bridge circuit can be powered from a single supply. At the same supply voltage, the output signal swing of the full-bridge circuit is twice that of the half-bridge circuit, and the theoretical maximum output power is four times. The traditional class D amplifier uses a differential operation mode. The switching signal controls the conduction and turn-off of the power transistors in the two half-bridge circuits. The output polarity of the half bridge A must be opposite to the output polarity of the half bridge B, so that the load current is from one. The half bridge flows in and flows out from the other half bridge, providing the filter with a pulse signal of opposite polarity, so there are only two differential operating states, normal and negative.
Figure 2 DDX driver status
The DDX audio amplifier's modulator uses DDX's patented three-state modulation technique to add a common mode operation, ie the two half-bridge outputs have the same polarity (both low), allowing both ends of the filter to be connected to ground. . This common mode state is called the negative state, and the differential working state cooperates to generate DDX tristate modulation, as shown in Figure 2. The negative state is used to represent the low power level, replacing the switch between the normal and negative states in the two-state scheme. When the audio signal is at a low power level, the traditional two-state scheme still keeps the output transistor in a switching state, outputting a positive and negative offset unwanted signal to the filter and the speaker, which not only increases the switching loss and energy overhead, but also reduces the audio. The efficiency and signal-to-noise ratio of the amplifier, and constantly in the switching state, inevitably generate EMI. The DDX tri-state modulation scheme utilizes the negative state to represent low power levels, and the normal and negative states are used to provide high power to the speaker. Under the same test conditions, the DDX tri-state modulation scheme is 16 dB lower than the high-frequency carrier component produced by a conventional class D amplifier using a two-state modulation scheme, and the amplifier efficiency is increased by 20% at low power levels. The unique characteristics of the DDX tri-state modulation scheme also improve the power supply rejection ratio (PSRR), because at low power levels, the differential action of the filter is very small, and the cathode state grounds both ends of the speaker, thus making the noise of the power supply not Was heard.
Many Class D amplifiers use a PWM output to the negative feedback loop of the device input to improve the linearity of the device and correct the output through the control loop to reduce distortion and power issues. The advantage of closed-loop design is at the expense of possible stability issues, which is a common problem for all feedback systems. The DDX audio amplifier uses a digital open-loop design that does not create amplifier stability even when driving low-impedance speakers. At the same time, advanced digital signal processing (DSP) is used to pre-compensate or correct the expected output stage error, which also improves the linear output characteristics of the amplifier. And each channel audio signal can be independently programmed in the digital domain, such as segmented EQ control, bass/treble control and volume control, etc., which can be realized by programming the internal registers through the I2C digital interface, which is convenient The development and use of the user, and added value to the user.
DDX audio amplifier type
DDX audio amplifier chips are mainly divided into two categories, one is a completely independent design, that is, the DDX control chip and the audio power amplifier chip are separate, can handle up to eight audio channels, the maximum output power is single channel 350W; the other is The single-chip design integrates DDX control and audio power amplifier functions, and has 2.1 channels of DDX control and audio amplifiers with a total output power of 40W to 160W. Users can flexibly select and match combinations according to the actual needs of product development.
Reference design plan - flat panel TV speaker
Let's take the example of a new DDX audio amplifier STA328 from STMicroelectronics (STM) to understand the structure and function of DDX audio amplifiers and how to design and develop products using DDX audio amplifiers.
The main features of the solution:
* The output of the audio amplifier is 2.0 channels (2 × 80W) or 2.1 channels (2 × 40W + 1 × 80W);
* 32 preset audio EQ curves;
* Four choices of HDMI selection switch controller;
* Receive analog stereo audio signals;
* Receive true digitally encoded audio signals (stereo PCM) for fiber and coaxial interfaces;
* Infrared remote control.
As flat-panel TVs become thinner, speakers become smaller, and acoustic characteristics of the chassis become less and less desirable, it is important to correct the audio signal. The 2.1 channel dedicated speaker we designed for flat-panel TVs makes full use of the high integration of DDX single chip, combined with the pure digital stream processing capability from sound source to speaker, providing low cost, high performance and high sound quality for flat panel TV. External sound system. The circuit structure of this special speaker reference scheme is shown in Figure 3.
Figure 3 Schematic diagram of the circuit structure of the flat panel TV speaker reference scheme
This set of speakers can be operated by infrared remote control. STMicroelectronics (STM)-ST72324 acts as the human-machine interface control MCU, accepts commands from the infrared remote control, and sends corresponding control commands to the DDX audio amplifier STA328.
In addition, the ASAHI KASEI MICROSYSTEMS (AKM)-AK4113 is a 24-bit stereo digital audio receiver that accepts high-fidelity digitally encoded audio signals from fiber optic interfaces and coaxial interfaces, which are then converted to PCM audio signals for output via the I2S bus. Supports sampling rates up to 216KHz; the AKM-AK5358A is a cost-effective 24-bit stereo A/D converter that converts stereo analog audio signals into PCM audio signals for output via the I2S bus. The AK4113 and AK5358A can receive audio sources from the digital and analog interfaces, respectively, and provide PCM digital audio signals to the DDX audio amplifier STA328. Set the output level of STA328 to 2.1 channel (2×40W+1×80W), and use the corresponding speakers to restore and amplify the audio signal from the front digital source or analog source.
Because it is a display and playback platform for flat-panel TVs, when faced with multiple high-definition content input options, the HDMI interface of most flat-panel TVs is inconvenient to use, so we joined Intersil. -ISL54100. It is a four-choice HDMI selector switch controller that not only switches digital video and audio signals, but also has a refresh function. It can be re-synchronized and balanced through a built-in phase-locked loop, which can effectively restore the physical properties of the wires. The signal decay caused by the problem can extend the HD signal transmission distance by 15 meters.
Conclusion
The DDX audio amplifier directly processes and amplifies the audio signal output from the digital audio source, which can easily realize high-fidelity, high-efficiency and low-cost audio amplifiers, providing complete end-to-end integration of digital audio, audio processing and power amplification. Digital solution.
references:
1. Apogee DDX enterprise documents
2. STM DDX solution seminar documents
3. STM STA328 datasheet
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