Cars are becoming more and more popular, there are more and more people with cars, and entertainment equipment for cars is becoming more and more abundant. Audio and video equipment such as CD, DVD, MP3, MP4 are everywhere. However, the radio has always occupied its right position, no matter how the entertainment equipment changes, the radio function is indispensable, the most simple car entertainment equipment is only the radio. Why? Because people’s aesthetic fatigue is innate, regardless of CD, DVD or other media, listening to it several times or watching it is tired, and the content of the broadcast is new every time. Finally, listening to the radio. Time is the longest.
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The car is mobile, and the effect of the radio is greatly affected by the environment: the signal is strong and weak (span is very large), there are multi-path interference, there is strong crosstalk, engine interference, electric horn interference, etc. How to get the radio well, listen Comfortable, engineers have developed a number of targeted circuits.
1. High-level circuit. It is reasonable to say that high-level circuits should not belong to special circuits. Some advanced radios also have a power amplifier circuit. In ordinary home radios, there is usually no high-level amplifier. Direct mixing to the center. If the radio is not very good, you can move the antenna position. If it is a portable radio, directly carry the radio. Find a good location for the signal. The car is different, the antenna is fixed, the car is mobile, and it is necessary to continuously receive the radio method in a range from city to suburb. It is the best choice for FM plus a high-tuning with tuning. AM is usually a high-bandwidth broadband. With the development of technology, the choice of low-IF single-chip solution without tuning circuit has also done very well. This is to eliminate the image interference with the new circuit structure, and also eliminate adjacent channel interference and improve selectivity. In the front, will it be surpassed? Hard to say. The trend has been toward low-IF single-chip development, which is better than the price/performance ratio.
2. High-amplitude automatic gain control (RF AGC) with wide-band AGC and narrow-band AGC detection. Gain control is to control the gain of the high-release tube and the on-current of the PIN diode. It is divided into two levels and the signal is strong enough to exceed the IFC control range. When the gain control of the high discharge pipe is started and the signal continues to increase, the PIN diode is turned on to prevent the occurrence of blocking.
3. Soft mute control. When the signal is weak, reduce the output amplitude, that is, reduce the volume and increase the comfort; the early home radio is not available, and it is unclear whether the home radio is available now. Especially in FM, when there is no Taiwan, the noise is full output, which is very irritating, and it is easy to cause irritability and tinnitus.
4. Noise blanker, including ignition noise cancellation and stereo mixing. Ignition noise elimination is divided into AM and FM. FM applications are more common, and AM is less used; now the integration of single-chip solutions is getting higher and higher, and AM noise blanker will be integrated. The principle and effect of FM ignition noise elimination circuit can refer to the specification of Sanyo IC LA2110 or Philips IC TDA1001. The principle and effect of AM ignition noise elimination circuit can refer to the specification of Hitachi IC HA12181FP. The current IC will not introduce these contents anymore; stereo Mixing is when the FM stereo signal is weak, the left and right channels are mixed, the weaker the signal, the higher the degree of mixing, the stronger the signal, the better the separation, which can improve the signal-to-noise ratio of the weak signal. This is an improved circuit for the FM stereo feature.
5, high cut (high cut), when the signal is weak, the signal to noise ratio is reduced, high frequency noise is obvious, high frequency removal can significantly improve the sense of hearing. The early high-frequency ablation circuit was fixed at a frequency point, usually 10 kHz. Now there are multiple frequency points to choose from, and different frequency points can be selected according to the strength of the signal.
6, dynamic IF bandwidth, different companies say different. Philips (NXP) is PACS (Precision Adjacent Channel Suppression); STMicroelectronics is ISS (Intelligent Selective System); Sanyo is called IF variable Filter; signal bandwidth is wider, sound quality is guaranteed, signal When weak, the bandwidth is narrower, improving selectivity and signal-to-noise ratio.
7, multipath interference detection (multipath), did not have this function in the early days, Philips ICs began to have this function in the late 1990s. Not all Tuner ICs have this feature yet. The performance of multipath interference on TV is a ghost image, especially in channels below channel 5. It is especially easy to reproduce. Now it is watching cable TV. Many people have never seen the phenomenon of ghosting, and they don’t know that there is a ghost. thing. Closer to home, this function mainly detects the intensity of multipath interference, and adjusts related parameters such as High cut, Noise Blanker, and Soft mute according to the intensity to improve the sense of hearing and increase comfort.
8, wireless data broadcasting system RDS (US RBDS), the machine sold to Europe must have RDS function. We don't have mainland China, because Hong Kong has it. If China does not, it is not appropriate. RDS has a convenient function called AF. In Europe (possibly outside mainland China), the same station transmits different frequencies in adjacent areas. The AF function can facilitate the car to listen to a station across regions and try to be uninterrupted. When the signal is weak, the highest signal frequency of the same station signal will be automatically switched. The advanced machine has dual tuner, one normal radio, one for RDS reception, and the other tuner when the signal is weak, so that the driver can drive The personnel listened to the radio sound very smoothly, and did not feel the frequency of the jump. We are different here. There are the same frequency phenomenon in the adjacent area. In some places, we will hear the same fight, the same sound in tandem, so that the planning frequency has made a low-level mistake.
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