Special Circuit of Car Audio Radio

     The car is moving, and the radio reception effect is greatly affected by the environment: the signal is strong and weak (very large span), there is multipath interference, strong channel crosstalk, engine interference, electric horn interference, etc. How to do a good job of radio reception and listen comfortably, engineers have developed many targeted circuits.

    1. High level amplifier circuit. It is reasonable to say that high amp circuits should not belong to special circuits. Some advanced radios also have power amplifier circuits. In ordinary household radios, there is usually no high level amplifier. They are directly mixed to the medium level amplifier. If the reception is not good, you can move the antenna position. If it is a portable radio, you can directly carry the radio to find a good signal position. The car is different. The antenna is fixed, and the car is mobile. The way to continuously receive the radio from the city to the suburbs is to improve sensitivity and selectivity. FM plus a tuner is the only choice. AM is usually a broadband amplifier. With the development of technology, the low intermediate frequency single chip scheme without tuning circuit has also been well chosen. This is to eliminate mirror frequency interference with a new circuit structure, and also to eliminate adjacent frequency interference and improve selectivity. Does NXP go ahead in this regard, and will it be surpassed? It‘s hard to say. The trend has been towards low IF single chip development, which is better than cost performance.

    2. High level amplifier automatic gain control (RF AGC) has broadband AGC and narrowband AGC detection. The gain control is to control the gain of the high level amplifier and the conduction current of the PIN diode. It is controlled in two stages. When the signal is strong enough to exceed the control range of the IF AGC, the gain control of the high level amplifier is started. When the signal continues to increase, the PIN diode is conduction to avoid blocking.

    3. Soft mute control. When the signal is weak, reduce the output amplitude, that is, reduce the volume to increase comfort; There was no home radio in the early days, but it is unclear whether there is a home radio now. Especially in FM, when there is no radio, the noise output is full range, which is very nerve stimulating, and is easy to cause irritability, tinnitus, etc.

    4. Noise blanker, including ignition noise cancellation and stereo mixing. Ignition noise elimination is divided into AM and FM. FM is widely used, while AM is rarely used; Now the integration of single chip solutions is getting higher and higher, and AM noise blanker will also be integrated. The principle and effect of FM ignition noise elimination circuit can refer to the specifications of Sanyo IC LA2110 or Philips IC TDA1001, and the principle and effect of AM ignition noise elimination circuit can refer to the specifications of Hitachi IC HA12181FP. The current IC will not introduce these contents; Stereo mixing refers to mixing left and right channels when FM stereo signal is weak. The weaker the signal is, the higher the mixing degree is. The stronger the signal is, and the better the separation degree is. This can improve the signal-to-noise ratio of weak signals. This is an improved circuit for FM stereo characteristics.

    5. High cut: when the signal is weak, the signal-to-noise ratio decreases, and the high-frequency noise is obvious. High cut can significantly improve the hearing sense. In the early days, the high frequency cutoff circuit was to fix a frequency point, usually 10kHz. Now there are multiple frequency points to choose from, and different frequency points can also be selected according to the strength of the signal.

    6. Different companies have different opinions on dynamic IF bandwidth. Philips (NXP) is PACS (Precision Adjacent Channel Compression); STM is ISS (intelligent selectivity system); Sanyo is called IF variable Filter; When the signal is strong, the bandwidth is wider and the sound quality is guaranteed. When the signal is weak, the bandwidth is narrower to improve selectivity and signal-to-noise ratio.

    7. Multipath interference detection did not have this function in the early days. Philips IC began to have this function in the late 1990s. Not all Tuner ICs have this function yet. The manifestation of multipath interference on TV is ghosting, especially in channels below Channel 5, which is particularly easy to ghosting. Nowadays, most people watch cable TV. Many people have not seen the phenomenon of ghosting and do not know about it. To get to the point, this function is mainly used to detect the intensity of multipath interference, and adjust the High cut, Noise Blanker, Soft mute and other related parameters according to the intensity to improve the hearing sense and increase the comfort.

    8. RDS (American RBDS), a wireless data broadcasting system, is required for all machines sold to Europe. We don‘t have one in mainland China, because Hong Kong has one. It‘s not appropriate to say that China doesn‘t have one. RDS has a convenient function called AF. In Europe (possibly outside the mainland of China), the transmission frequencies of the same station in adjacent areas are different. The AF function can facilitate the car to listen to a certain station across regions without interruption. When the signal is weak, it will automatically switch to the strongest frequency of the same station. Advanced computers have two tuners, one for normal reception, the other for RDS reception, and when the signal is weak, it will switch to another tuner, In this way, the driver and passenger can listen to the radio very smoothly, without feeling that the frequency has changed. We are different here. There is the phenomenon of the same station and the same frequency in the adjacent area. In some places, you can hear the same station fighting, and the same voice one after the other. This makes a low-level error in frequency planning.