Design of high quality wireless audio repeater based on VMR6512

Wireless FM audio repeaters can modulate audio signals into high-frequency signals and receive them with FM radios within a certain distance. Wireless audio repeaters are widely used in wireless headphones, car MP3 repeaters, conference broadcasts, building and park broadcasts, etc. The simplest wireless audio repeater can be constructed with only one triode, as shown in the following figure:

Figure 1 Single-tube audio FM repeater

The figure above uses a transistor 9018 to form an oscillator. The audio signal is input from the base of the transistor, and the frequency modulation is achieved by using the varactor characteristics of the BE junction. Although the circuit is simple, the sound quality is very poor, and the stability of the oscillation circuit is very poor. The change of voltage, the presence of human body or metal objects near the antenna will change the oscillation frequency and cause instability at the receiving end. Moreover, the circuit can only transmit mono audio, and users cannot listen to high-quality stereo music.

More complex stereo repeaters use integrated circuits, the most commonly used is RoHM's BA1404. This circuit set integrates a stereo encoder, which can generate a 19kHz stereo pilot signal to be transmitted along with the audio. If the radio has a stereo decoding function, it can demodulate stereo music. The application circuit of BA1404 is shown in the figure below:

Figure 2 BA1404 application circuit

Compared with the single-tube repeater, BA1404 can not only transmit stereo signals, but also has significantly improved sound quality. However, the separation, frequency response and other indicators are still not very ideal. The sound quality of BA1404 is still far from enjoying high-quality music. Since BA1404 still uses LC oscillator, the frequency stability is still not good enough and it is easy to produce frequency drift. There are quite a few car audio repeaters or wireless headphones using BA1404 on the market.

To create an FM radio transponder with broadcast-grade sound quality and very stable frequency, complex technologies such as digital audio preprocessing and frequency synthesis are required. Due to cost and complexity, these technologies are usually only used in radio stations and are difficult for ordinary users to accept.

Does that mean that ordinary users will not be able to get high-stability, Hi-Fi quality audio repeaters? The wireless audio repeater module VMR6512 developed by Beijing Ruiyufei Technology Co., Ltd. solves the contradiction between cost, sound quality and frequency stability, allowing everyone to enjoy broadcast-level sound quality at a very low cost. The figure
below is the internal block diagram of the VMR6512 wireless audio modulation module.

Figure 3 VMR6512 wireless audio repeater block diagram [page]

The VMR6512 wireless audio forwarding module has the following features:

•  Broadcast-grade sound quality
•  Fully integrated package, works without any external components
•  Audio is processed by DSP to ensure high sound quality
•  Using frequency synthesis technology, the oscillation frequency is highly stable
•  Output power is adjustable, up to 115dBuV
•  Frequency range 88.0MHz-108.0MHz, low end can be extended to 76.0MHz upon request
•  Can input analog audio or digital audio
•  With external UART interface, it can be easily controlled by external CPU or PC
•  Provide frequency setting UP/DOWN input for easy independent use

This module can be used for:

•  Hi-Fi wireless headphones
•  Car MP3 audio repeater
•  Wireless microphone
•  Conference broadcasting system
•  Building music playback
•  Park music player
•  Audio-visual entertainment equipment accessories
•  Campus radio station

The VMR6512 has a CPU inside that controls the entire module. An external processor can communicate with the module through the UART serial port to set the module's frequency, transmit power, interface mode, etc. In addition, the module also provides two pins, UP and DOWN, so that the frequency can be adjusted without an external CPU or control serial port. The module's default power-on transmit frequency is 100.0MHz. Each low-level pulse on the UP or DOWN pin increases or decreases the transmit frequency by 0.1MHz. If the low level is maintained continuously on these two pins, the transmit frequency changes every 0.3 seconds.

The audio signal interface can be a common analog audio input or a digital audio input. The analog audio is first converted into a digital signal by an audio A/D converter and then enters the DSP for processing. Digital audio provides three commonly used interface modes: I2S, left-justified and DSP, which can be seamlessly connected to almost all codecs or DSPs.

After the audio signal is digitized, DSP will perform a series of processing on the signal, such as filtering, pre-emphasis, pilot generation, etc. Since the processing is performed in the digital domain, it has advantages that analog circuits cannot achieve, providing a prerequisite for high sound quality.

The module also integrates the automatic gain control (AGC) function. When the input audio exceeds a certain threshold, the AGC will automatically attenuate the audio signal to avoid distortion. When the audio signal is lower than a certain threshold, the AGC circuit will amplify the audio signal to a certain extent, thus ensuring that the receiving end always has a suitable volume output.

The signal after DSP processing is then converted by D/A and sent to the high-frequency modulator for FM modulation. The high-frequency signal is generated by frequency synthesis. The frequency can be adjusted in steps of 10KHz. The frequency range is continuously adjustable from 88.0MHz-108MHz. If there is a special need, it can be expanded to 76.0-108.0MHz. The frequency deviation of the FM modulated signal can also be accurately adjusted, but it is usually not recommended for users to set it themselves. The maximum power of the output FM signal is 115dBuV, and the transmission distance in open areas can reach fifty or sixty meters.
The following figure shows the appearance and pinout of VMR6512:

Figure 4 VMR6512 module appearance and pinout description

The VMR6512 module has a size of only 25mmx15mmx3.8mm (length x width x height), which can be easily embedded in small devices such as wireless headsets and car MP3 audio repeaters.

The following figure shows the component symbol and pin description of VMR6512:

Figure 5 VMR6512 component symbol
Table 1 VMR6512 pin description

Table 2. Related parameters of VMR6512

The following figure is the reference design circuit diagram of VMR6512:

Figure 6 VMR6512 reference design circuit