Develop high-fidelity digital power amplifier based on TI audio IC
1 Introduction
The audio world has entered digitalization. Only the power amplifier, the heart of the entire audio system, It has been lingering outside the door of digitalization for a long time. Many well-known semiconductor manufacturers are focusing on this and developing digital power amplifiers. Here we briefly introduce the principle and structure of digital power amplifiers, and give the design ideas and methods of high-fidelity digital power amplifiers based on TI series audio ICs.
2 Principle and structure of digital power amplifier
2.1 Principle of digital power amplifier
The basic principle of digital power amplifier: using pulse signal PWM to drive high-speed power switch, where the PWM signal The low frequency part contains the modulated signal. This signal is passed through a low-pass filter, which reproduces the modulated signal to the load (speaker). The essential difference from a traditional analog power amplifier amplifying analog signals is that the digital power amplifier directly amplifies the digital audio signal and then performs high-precision D/A conversion, so the efficiency is as high as 85%; the fidelity is excellent, THD+N (total harmonics Distortion + noise) is less than 0.05%.
2.2 Digital power amplifier structure
Figure 1 shows the basic structure of a digital power amplifier, which consists of a PWM modulator, an H-bridge switching amplifier circuit and a low-pass filter. The PWM modulator converts the input digital audio signal into a PWM signal. The Σ-Δ modulation method has high accuracy and good stability. The H-bridge switch amplifier circuit controls the on and off of the DC power supply and uses the PWM signal current to supply power to the load. The low-pass filter can filter out switching components and smooth the output waveform to obtain an audio signal with an analog waveform.
3 Introduction to TI digital audio series ICs
Texas Instruments (TI) has long been committed to the research and development of digital audio products and has achieved breakthrough results. Among them, TAS55xx, TAS50xx, TAS30xx, and TAS-5lxx series products are the most mature and cost-effective. The TAS55xx series is a highly integrated digital audio device that integrates PWM modulation and DSP sound processing functions. It has 8 built-in channels and the channel THD+N is lower than 0. 05%; TAS50xx series is a high-precision audio PWM modulator. It has only 2 channels, channel THD+N is less than 0.0l%, and has extremely high fidelity; TAS30xx series is a dedicated digital audio DSP that can achieve volume, Various sound effects such as heavy bass adjustment: The TAS51xx series is a dedicated audio H-bridge amplifier circuit.
4 Digital Audio Power Amplifier Design
4.1 Design Solution
Here are two design solutions, the former is TAS55xx+TAS51xx, which can get an 8-channel digital power amplifier; the latter The latter is TAS50xx+TAS30xx+TAS51xx to get a 2-channel digital power amplifier. Because the latter design solution has higher fidelity, better DSP effect, lower cost, and the devices used are easier to buy in China, the latter design solution is introduced in detail below.
The actual design uses a combination of TAS5015+TAS3002+TAS5100. Among them, TAS3002 comes with A/D and D/A converters, which allows the system to receive analog input signals and expands the product's applicable scope.
4.2 Workflow
For the specific workflow of digital power amplifier, the sound source comes from two ways: ①S/PDIF signal obtained through high-quality CD player (a kind of signal widely used by high-end audio manufacturers The digital audio format used), which is converted by the format converter DIRl703 and then enters the TAS3002; ② The analog signal is acquired by the TAS3002 through a microphone and other pickup equipment. After the digital audio signal (represented by SDATA) in the system is processed by DSP sound effects in TAS3002, it enters the PWM modulator TAS5015 and is converted into a PWM signal. Then it enters the TAS5100 for power amplification, and the analog signal is reconstructed through a low-pass filter to drive the speaker. During the amplification process, if there is an abnormality in the circuit, TAS5100 can automatically send out an alarm signal to reset the system, or it can be reset manually to avoid accidents.
4.3 I?S bus
TI's audio devices are connected by the I?S (Inter-IC Sound) bus. The I?S bus is a bus standard developed by Philips for audio data transmission between digital audio devices. This bus is widely used in various multimedia systems for data transmission between audio devices. The hardware interface specifications and digital audio data format are specified in the I?S standard. I?S has three main signals: ① Serial clock SCLK, also called bit clock (BCLK), that is, corresponding to each bit of digital audio data, SCLK has one pulse. The frequency of SCLK = 2x sampling frequency × number of sampling bits. ②Frame clock LRCLK, used to switch the data of left and right channels. LRCLK is "l" indicating that the left channel is transmitting data, and "0" indicates that the right channel is transmitting data. The frequency of LRCLK is equal to the sampling frequency. ③Serial data SDATA is audio data represented by two’s complement. Sometimes in order to achieve better synchronization between systems, it is necessary to transmit an additional signal MCLK, called the main clock, also called the system clock (SysClock), which is 256 times or 384 times the sampling frequency.
4.4 Core Function Module Circuit
4.4.1 Digital Audio Input
The digital audio input module mainly uses TI’s digital audio interface device DIRl703 to achieve the following functions: ① Convert the digital audio signal in CD format into serial audio data that can be recognized by the PWM modulator TAS5015, which can be directly transmitted on the I2S bus; ② Generate the serial clock BCLK, frame clock LRCLK, and system clock SCLK of the entire I2S bus .
The SPDIF-IN signal input from pin 20 of DIRl703 is the PCM signal obtained from the CD player through optical fiber or coaxial cable; after processing by DIRl703, the SDATA signal I2S-DIR1703-OUT is output; the pin of DIRl703 Pins 7 and 8 are connected to the external crystal oscillator X120, and pin 4 (SCK0) outputs the system clock SCLK signal 256XFS; pin 10 (LRCK0) outputs the serial clock BCLK signal FS; pin 11 (BCK0) outputs the frame clock LRCLK signal 64XFS .