Production and trial experience of several wireless headphones

Now I would like to introduce my experience of several wireless headphones that I have made and used for your reference. The first option is to make an FM wireless transmitter as shown in Figure 1. The total current of the whole machine including the LED is only 5 mA. Any FM radio can receive stably within a range of 20 meters and is not blocked by walls. Impact. Adjusting the transistor emitter resistance can improve the modulation distortion, and adjust the trimmer capacitor so that the signal emitted by the transmitter is within 87~108MHz, while avoiding local FM stations and not interfering with TV programs. The signal source is taken out from the audio output hole of the TV or DVD or VCD with a lotus plug. At the beginning, I used an ordinary head-mounted hand-tuned AM/FM radio to listen, and the effect was very good. Later, I switched to Sony SRF-MH22 head-mounted digital tuner stereo radio for listening. It was extremely convenient to use, the sound quality was greatly improved, and there was no background noise. Option 1 cannot listen to stereo signals, and according to the "Electronic News" introduction, an FM stereo transmitter was made using BA1404. When the machine is powered by 1.5V, the total current is only 4 mA, and it can receive stably within a range of 10 meters. When it was first made, it was made according to the regular circuit diagram, but after successive replacements of four BA1404s, it still failed to work properly. After careful analysis of the circuit, it is estimated that the radio frequency oscillation circuit is short-circuited to ground by the 15pF capacitor on pin 9 because there is no power amplification stage. The circuit can only work normally after disconnecting this capacitor, as shown in Figure 2. This point was made during production. should be noted. The receiver still uses Sony SRF-MH22 for reception, with obvious mono channel effect and good sound quality. However, when receiving stereo signals (that is, the receiver stereo decoder is turned on), there is obvious background noise. I don't know whether it is caused by the 19kHz pilot signal of the transmitter or by the receiver decoder. At the same time, the stereo signal separation of this machine is not complete, and the use effect is average. In order to receive high-quality stereo sound, I purchased a set of Blue House brand infrared wireless stereo headphones. After analysis, the structure of this machine is similar to the IR900 infrared stereo headset introduced in the seventh issue of "Electronic News" this year. The left and right channels of the transmitter each use a CD4046 to modulate the signal and then transmit it through two infrared diodes respectively. The left and right channels of the receiver The sound channels are respectively composed of infrared reception, transistor amplification, frequency selection, and then demodulation and amplification through the chip integrated block A007B to drive the earphones. The diameter of the earphone speaker is 25 mm. After listening to it, the stereo separation is very good, the sound is clear, and the sound quality is very good. The disadvantage is that the machine uses two AA batteries for power supply, and the battery consumes too quickly. After using it, it is believed that if the main purpose is to receive mono-channel signals, the solution in Figure 1 can fully meet the requirements. If you mainly receive stereo signals, you should use an infrared wireless stereo system for listening. However, it is best not to use an FM stereo system due to poor signal-to-noise ratio and insufficient stereo separation. Of course, use two mono-channel FM transmitters respectively. It is also a good idea to transmit the left and right channel signals and then use two FM receivers to receive them respectively.