CD-quality wireless digital audio transmission system based on nRF24Z1

At present, with the growth of living and office space, audio wiring in large conference rooms, cars and other places is becoming increasingly difficult and costly, and there is an urgent need for wireless transmission of high-quality audio. The transmission rate of CD-quality audio reaches more than 1.5Mbps, so higher bandwidth and distance requirements are put forward for wireless systems.

ISM 2.4GHz (Industrial Scientific Medical 2.4GHz-2.4835GHz) frequency band is a globally open public frequency band with the advantages of high bandwidth and low cost. The ISM2.4GHz transmission system with high bandwidth is more suitable for the transmission of CD-quality audio. Other 2.4GHz systems, such as monitoring and WLAN, have disadvantages such as high cost or limited distance, so this system uses a dedicated ISM audio wireless transceiver chip nRF24Z1.

nRF24Z1 provides standard industrial audio I2S interface and S/PDIF digital audio interface, which greatly reduces the transmission cost of audio. Moreover, the communication rate is up to 4Mbps, and the actual data transmission rate is 1.536Mbps, which ensures lossless transmission of audio with 48kbps sampling rate and 16bit sampling.

1 Chip Introduction

nRF24Z1 is a CD-quality wireless digital audio transmission transceiver chip launched by Nordic Company of Norway, which works in the ISM 2.4GHz frequency band. The maximum output power of the chip is +0dBm, and the receiving sensitivity is -83dBm. The chip integrates PLL, clock control and recovery module, TDM QoS module, GFSK module, I2C interface, SPI interface, RF LNA and PA, etc., and integrates two industrial audio standard interfaces, I2S and S/PDIF. The I2S interface can be directly connected to various audio A/D and D/A, and the S/PDIF can be directly connected to various surround sound equipment.

The chip's RF working mode is GFSK, Gaussian frequency shift keying. This method is widely used in point-to-point wireless communications and has a low bit error rate.

To ensure low bit error rate in communication, the chip also adopts QoS service quality strategy, including two-way communication mechanism and response strategy (time division duplex), data integrity strategy and CRC error detection, adaptive frequency hopping, and offline search and reconnection strategy.

The bidirectional communication mechanism and response strategy can be seen in Figure 1. The communication from ATX to ARX is an audio channel, while the communication from ARX to ATX is a control channel. The information of the control channel includes simultaneous transmission information, register information, and pin status information.

The QoS part includes data integrity strategy and CRC error detection, which are completely implemented by hardware. The frame sent in the audio channel includes multiple packets, each of which consists of RF address, valid audio data, and several CRC bits. When the CRC of the Packel received by the receiving end is verified, the information will be sent back to ATX through the control channel. If the CRC check is incorrect, the sending end will retransmit the incorrect one or several packets in the next frame.

Adaptive frequency hopping is an important means of anti-interference, which is discussed in detail in Section 2.4 of this article.

Disconnected search and reconnection is a measure to ensure connection reliability. When the connection is lost, the transmitter automatically searches according to the RF pattern, searching each channel for a period of time. Similarly, the receiver also monitors each channel. Once a connection is established, the channel is locked and the frequency hopping pattern is followed.

The initial configuration of the chip can be completed by EEPROM or MCU through SPI, I2C interface. Whether the chip is in transmit mode or receive mode is determined by the level of MODE pin.

nRF24Zl uses QFN36 package. The full pin list can be found in the chip documentation. The pins related to operating the chip are shown in Table 1.

2 System composition

2.1 System composition diagram

This system ensures "transparent" wireless transmission of digital/analog audio, that is, the audio signal output from the receiving board to the speaker/headphone is undistorted compared to the audio signal input from the sound source to the transmitting board. For digital audio, it is a serial digital signal that meets the S/PDIF standard; for analog audio, it is a two-channel analog signal.

The system is mainly composed of nRF24Zl, AD/DA, MCU, RFPA, etc. The transmitter composition is shown in Figure 2.

The receiving end composition diagram is shown in Figure 3.

2.2 System Description

The analog audio source of this system is sampled from AD and transmitted to nRF24Z1 through the I2S audio interface for transmission. After receiving the audio data, the nRF24Z1 at the receiving end recovers the clock to MCLK (I2S master clock), and performs D/A conversion and amplification of the audio at the same time, and finally outputs it through the speaker.

Another digital audio source is taken out through the coaxial/optical interface of the DVD/CD player and transmitted through the S/PDIF audio

The interface transmits the audio to nRF24Z1 for transmission, and the nRF24Z1 at the receiving end transmits the audio to the 5.1 digital amplifier speakers after receiving the audio data. Both of these channels realize lossless "transparent" transmission of audio.

The BALUN structure in Figure 2 and Figure 3 is a double-ended to single-ended network conversion structure of RF, which is composed of capacitors and inductors. Because the antenna is single-ended and the RF interface of nRF24Z1 is a double-ended balanced input or output, conversion is required.

The function of the radio frequency amplifier (RF PA) is to enable the transmitter to have a larger transmission power when in the transmitting state, so as to achieve a longer transmission distance. The working mode of each part is determined by its own VDD_PA signal. Taking the receiving end as an example (as shown in Figure 3), when receiving audio data, VDD_PA is low level, it controls both RF switches to the bottom, and the RF signal directly enters the nRF24Z1 through the transmission line; when returning control data and register information, VDD_PA is high level, both RF switches are turned to the top, and the RF PA is started at the same time, sending with a larger power to achieve a longer transmission distance.

The transmitter works in a similar way. Generally, the PAs of the receiver and transmitter are turned on and off alternately.

2.3 System Configuration and Workflow

The system configuration method and the system workflow are shown in Figure 4.

2.4 Frequency Hopping Sequences and Patterns

This system uses adaptive frequency hopping, which is part of the QoS strategy.

Frequency hopping communication is a type of spread spectrum communication and is also the most widely used type. The working principle is that the carrier frequency of the transmission signal between the sender and the receiver changes discretely according to a predetermined rule. Frequency hopping communication has the characteristics of good concealment and strong anti-interference ability. The frequency sequence composed of the predetermined rule is called the frequency hopping pattern.

This system does not use the sequential frequency increase sequence given in the reference design, but uses a PN pseudo-random code sequence. This sequence has good anti-interference and has an autocorrelation similar to white noise, making it difficult to be monitored and cause crosstalk.

The characteristics of PN code are as follows: there are enough address codes; the number of different code elements is balanced and equal; there is a sharp white correlation characteristic, that is, it satisfies the following formula:

The m sequence is a PN sequence that meets the above characteristics. Since this system has 38 frequency hopping points, a 5-level m sequence is used as the PN code. The primitive polynomial is x5+x2+1, and the final sequence pattern is: 16, 24, 28, 14, 7, 19, 9, 4, 2, 17, 8, 20, 10, 21, 26, 29, 30, 15, 23, 27, 13, 22, 11, 5, 18, 25, 12, 6, 3, 1. In order to ensure that the frequencies have a certain interval, each of the above sequences is multiplied by 2 to obtain the frequency hopping pattern.

After testing, it was found that when this frequency hopping sequence system coexists with other frequency hopping sequence systems, the number of noise jitters is less than that of the sequential sequence frequency hopping pattern system, and the frequency of audio noise is only half of the latter, so it has a strong anti-interference ability.

2.5 RF Amplifier Design and Circuit Design

When designing an RF amplifier, the following points should be noted:

(1) The amplifier module must meet the gain requirements, including size, stability, power consumption, etc., and must also meet other S parameter requirements. This system uses SiGe's Class A amplifier PA2423L, which has an output peak power of +22.5dbm.

(2) The input and output of the amplifier should be isolated as much as possible. Since the amplifier gain is very high, it is easy to cause positive feedback oscillation where the output returns to the input. Therefore, the components at the input and output ends should be as close to the pins as possible, and the routing should avoid sharp corners to prevent the antenna effect of long leads and sharp corners, and impedance matching should be done well. See Figure 5.

(3) For EMI/EMC considerations, it is necessary to drill holes every λ/20 (or smaller) on the PCB board.

(4) PA from AD/

There must be a certain distance between analog parts such as DA and key digital parts. Control signals and RF signals should not cross as much as possible. They can be crossed if necessary, but it is best to be orthogonal. Try to avoid damaging the integrity of the copper on the bottom surface of the RF signal.

3 System Software Design

The software design of this system includes transmission mode selection, address selection, address code design, frequency hopping pattern design, etc. When designing, you first need to choose whether the system transmits digital audio or analog audio, which can be determined by the pin level status outside the MCU. Secondly, you need to select the appropriate address and address code and write them into the internal register to distinguish between two different transmission systems. Frequency hopping pattern design is the focus of software design.

Since the SPI interface rate of MCU and nRF24Z1 is high, which can reach 1Mbps, the timing needs to be accurately designed in the software. In addition, in order to eliminate the influence of power-on POP noise, this system performs a delay operation when powering on, which can detect the status of the wireless connection and take corresponding measures.

Wireless transmission of digital and analog audio is a hot topic. This system has achieved the above system functions well. Through software design, frequency hopping design, amplifier design, etc., the digital/analog audio transmission system has reached the CD high-quality transmission index, and the transmission distance has reached more than 80 meters outdoors and more than 30 meters indoors. The radiation has also reached the relevant standards of the FCC. In addition, this system has a strong commercial prospect and will be widely used in PC multimedia, home theater, automotive electronics, etc.