Embedded wireless audio transmission system solution based on CC8520

In recent years, wireless technology has received more and more attention in the field of audio transmission, including Bluetooth, WIFI and 2.4GHz technology, which have developed rapidly. Among the many wireless audio transmission technologies, the 2.4GHz technology is currently favored and is most likely to be used in ordinary audio devices and on a large scale. 2.4GHz, the full name is "2.4GHz non-networking solution". Like Bluetooth and WiFi, it works on the 2.4-2.485GHz ISM wireless frequency band. And this frequency band is almost free for use all over the world. Therefore, there are some inherent advantages in product cost, which is conducive to the widespread popularization of products. At present, Bluetooth technology is the most used in wireless audio products and the technology is the most mature, but it has inherent disadvantages, such as: narrow bandwidth, which cannot meet the requirements of transmitting high-quality audio signals; short transmission distance, about 10 meters; and the widely criticized anti-interference problem. WIFI technology has the advantages of wide bandwidth and long transmission distance, and has a very good prospect, but its disadvantages are still anti-interference and relatively immature technology. The advantages of 2.4GHz technology over Bluetooth and Wifi are: 1) wide bandwidth, capable of transmitting CD-quality wireless signals. 2) strong anti-interference, 2.4G technology uses automatic frequency modulation technology, when the device is working, if it finds that the frequency band is occupied, it will automatically jump to an unused frequency band. 3) low power consumption, 2.4GHz technology does not need to work continuously when transmitting and receiving.

本文提出了基于TI 公司CC8520 系列芯片无线音频传输方案，该方案采用目前最热门且有发展前景的2.4GHz 短距离无线传输技术；无需进行软件开发，只需通过TI 提供的免费PurePath 无线配置器设置目标系统的期望功能及参数，显著减少了开发时间和开发难度，并且提高了系统的可靠性。CC8520内部集成了微控制器，无需额外的控制器或 DSP 即可完成对系统的控制，如音量调节、网络配对等操作，并且提供数据旁路通道，即可以在传输音频的同时，对接受端发送额外数据，降低了整体的成本。

After actual testing, the solution can work continuously for 22 hours when using a 9V dry battery; it can transmit high-quality stereo with multi-channel sampling rate (44.1/48KHz) and sampling bit number (16/24 bits); the barrier-free open area transmission distance is up to 130 meters, and the transmission radius can still reach 35 meters in a crowded and barrier-free environment.

1 System Overview

1.1 CC8520 Chip Introduction

The CC8520 chip uses TI's "PurePath Wireless" proprietary technology1. The chip can provide seamless and reliable audio streaming in various complex environments. It uses an embedded audio network protocol with advanced error correction and concealment technology, CD-quality uncompressed audio quality sampling, and uses I2S and I2C interfaces to achieve seamless connection and control with audio codecs, DAC/ADCs, and digital audio amplifiers. The wireless data transmission rate is 5Mbps, the audio delay is less than 16ms, and it has a programmable power output of up to +4dBm and a sensitivity of -83dBm). Few peripheral devices are required, so CC8520 is fully suitable for the design of wireless audio system transmission.

1.2 Working Principle

As shown in Figure 1, at the transmitting end, the sound or audio signal is converted into an analog electrical signal through a microphone (commonly known as a microphone, microphone) or an analog audio interface; the analog electrical signal is transmitted to the audio interface IN1_L and IN2_R ends of the audio codec chip TLV320AIC3204, and the analog electrical signal is pre-amplified inside the chip and converted into a digital audio signal in I2S format after A/D processing, and transmitted to the I2S interface of CC8520; CC8520 transmits the received digital audio signal as a carrier, and transmits the audio signal through the chip and the antenna by connecting to the RF_N and RF_P ends of the RF extender CC2590.

Figure 1: Block diagram of the transmitter system.

As shown in Figure 2, at the receiving end, another CC8520 chip demodulates the received carrier signal through CC2590, transmits the demodulated digital audio signal to the audio codec chip TLV320AIC3204 for D/A conversion, and outputs the analog audio signal. Because the audio amplifier gain inside the chip is small, the analog signal is amplified by the directional amplifier and output through the power amplifier or headphones. [page]

Figure 2 Schematic diagram of the system receiving end.

2 System Hardware Design and PurePath Wireless Adapter Settings

2.1 Launch Part Design

The wireless audio transmitter can be designed as a portable handheld device, which is mainly composed of low-power micro-chips such as the wireless digital audio chip CC8520, the audio codec chip TLV320AIC3204, and the RF expansion chip CC2590. All of them can be assembled in the circuit board of the small space of the portable device. The transmitter circuit principle is shown in Figure 1. When CC8520 and TLV320AIC3204 are used as the transmitter, first select "CC85XXDKdemo AIC3204" in the PurePath wireless adapter Projects. After opening it, select "Analog Inputmaster (AIC3204)" as the transmitter configuration.

Considering that it is used indoors and the high-quality CD sound quality of CC8520 is to be brought into play, the main signal source of the audio transmission system 1) selects a microphone with good acoustic performance and connects it to the IN1_L and IN1_R terminals of TLV320AIC3204 for preamplification and A/D conversion:

2) Any other standard stereo sound source such as built-in CD player or music played through computer can be pre-amplified and A/D converted by IN2_L and IN2_R of TLV320AIC3204, because TLV320AIC3204 supports both microphone input and stereo input modes.

The I2S interface of the audio codec chip TLV320AIC3204 is connected to the main chip CC8520, and is set in the PurePath wireless adapter according to the characteristics of audio transmission in this system, as shown in Figure 3.

Figure 3 PurePath wireless adapter audio interface settings

Interface type: Select I2S; Maximum number of processing bits: Select 16 bits.

In this design, TLV320AIC3204 uses CC8520's MCLK as its own system clock, so "Internal with MCLK" is selected for the clock source. After the selection is completed, the system automatically generates control instructions without user programming.

CC8520 has a built-in microcontroller, so the system does not need an additional microcontroller. The chip CC8520 is connected to TLV320AIC3204 through the I2C interface to initialize TLV320AIC3204 and send control commands.

Bias voltage setting: When the audio source is input through microphone input or stereo input, the bias voltage needs to be set. In hardware, the bias network is composed of R1 and R2; set it in the "AudioDevice Customization" of the PurePath wireless adapter. According to the data sheet and the characteristics of this design, select 2.5V bias voltage, so enter "W 51 05" in the setting box.

Key input: PurePath wireless adapter provides the following event occurrence modes: 1. click 2. hold 3. repeat 4. click+repeat. The advantage of this is that it allows designers to flexibly choose key styles based on user needs or product requirements, such as push-pull, trigger, self-locking, etc.

In this design, three buttons are used on the transmitter, namely the network pairing button, the remote volume control + button, and the remote volume control - button. The network pairing button adopts the "hold" mode, that is, the "press and hold" mode, and the volume control button adopts the "hold+repeat" mode, that is, "can be pressed and tapped".

Network pairing, volume + and - are connected to the CSN, GIO1, and GIO3 of CC8520 respectively. These can be easily selected on the PurePath wireless adapter.

Status light prompt: The status display mode set in the PurePath wireless adapter. In this design, the flashing light indicates that the network pairing is in progress, and the steady light indicates that the pairing is complete. Figure 4 shows the set I/O mapping diagram. [page]

Figure 4 shows the I/O port mapping of the CC8520 after setup.

2.2 Receiving part design

The receiving end of the wireless audio transmission system is fixed, similar to the transmitting end, and consists of a wireless digital audio chip CC8520, an audio codec chip TLV320AIC3204, a radio frequency expansion chip CC2590, and a power amplifier circuit. The main circuit part of the receiving end is similar to the receiving end. The I2C interface of the chip CC8520 is connected to the I2C end of TLV320AIC3204 to initialize it and send control commands. Select "Analog Output Slave (AIC3204)" as the receiving end configuration in the PurePath wireless adapter.

The I2S interface of the audio codec chip TLV320AIC3204 is connected to the main chip CC8520, and the received analog audio signal is processed by A/D conversion, post-amplification, etc., and the digital audio signal is sent to the power amplifier through 1. output from LOL and LOR 2. headphones or small sound amplification equipment output from HPL and HPR. Considering that the audio amplification function of the audio codec chip TLV320AIC3204 is limited, an amplifier is designed at the output end of the LOL and LOR analog audio signals to amplify the audio signal again, so as to facilitate connection to the power amplifier equipment. The principle is shown in Figure 5. The design uses the cost-effective 4558 power amplifier chip as the main body, which is powered by + and -9 volt power supplies. After amplification, it can be connected to the power amplifier through the XLR interface for further amplification.

Figure 5 Schematic diagram of analog signal amplification circuit.

3 System Configuration Process

Figure 6 shows the configuration process in the PurePath wireless configurator. Engineers can configure according to the characteristics of the hardware circuit and the functions to be designed. Customization mode, when the designer believes that the default settings do not meet the requirements of the design product, such as: needing more advanced functions; needing to change the form of the external audio interface, such as the original audio interface is I2s, and now I2C is used as the audio transmission interface; reducing device power consumption and turning off functions that are not needed in the default settings.

Figure 6 Design configuration flow chart.

4 Network topology and device identification

When establishing network pairing, the CC8520 on the sending end establishes the network and serves as the network host, as shown in Figure 7. It should be noted here that the device ID number of the CC8520 is the ID number of the established network, so when automatic fixed pairing is required, the device ID number must be entered in the "Default network ID" option in the "Netwok pairing" of the PurePath wireless adapter. The device ID number is unique and cannot be changed, which is different from the manufacturer ID and product ID that can be defined by yourself.

In this design, the audio transmitter (host) can be connected to multiple receivers (slaves), and the audio data flow, bypass data channel, and network signal flow are shown in Figure 7.

PurePath wireless adapters provide two custom device identification methods: 1) Manufacturer ID 2) Product ID. The manufacturer ID allows the device to automatically identify devices produced by the same manufacturer and perform network pairing. When other manufacturers also use CC8520 series products, they are filtered and not paired with them. The product ID is used when the same manufacturer has different "CC8520" products, such as: when the manufacturer has both wireless headphones and wireless microphones, to prevent pairing crosstalk between different products.

Figure 7 Network topology.

5 Conclusion

This paper proposes a design scheme for a wireless audio transmission system based on TI's latest chip CC8520. It uses today's popular 2.4GHz wireless technology, does not require software development, and improves the reliability of the system. Actual tests have shown that it can effectively improve sound quality, reduce power consumption, and can be used continuously for 22 hours. The transmission distance is 130 meters, which meets the design requirements and provides a new idea for designing wireless audio transmission systems.