FM radio broadcast

Combined Application of Audio Processor and FM Transmitter

The loudness of an audio program on broadcast is increased by reducing the peak-to-“average” ratio in dynamic audio. Within the allowed modulation range, the average can be increased if the peak is reduced, but this is unavoidable. It is easy to bring harmful side effects such as clipping, so how to solve the “peak/flat ratio” and to what extent is it good?

Principles of Audio Processing

  1. Compression The compression action is formed by reducing the gain of the signal when the average or root mean square value of the program signal level exceeds the compression threshold, so the dynamic range of the program is compressed, and the automatic gain control (AGC) The amplifier is the compressor. The compressor reduces the level difference between the soft and loud sounds, and makes full use of the allowable peak level range, so that the sound of the soft part is subjectively increased, but it does not make the loudness of the loud part louder. louder.

Peak limit and clipping Peak limiting is an extreme form of compressor, which is characterized by a high compression ratio and fast attack and recovery times. In modern audio processing, peak limiting itself usually limits not individual instantaneous peaks in the waveform, but the entire segment Packet waveform peaks, these instantaneous peaks are often clipped; clipping and clipping, will reduce the ratio of short-lived peaks to the average value (peak/flat ratio) of the audio signal, the main purpose of which is to protect the FM transmitter from As for overloading, compression is different. The main goal of compression is to reduce the dynamic range of the program, achieve multi-band compression and selectable frequencies. Multi-band compression and frequency selection is to divide the audio frequency spectrum into several frequency bands, and compress or limit each frequency band separately, which is a more advanced audio processing technology in the contemporary era. The unified adjustment of audio processor and FM transmitter

Static adjustment Connect the FM comprehensive tester, the audio processor and the FM transmitter, and turn the output knob of the audio processor counterclockwise to the end to turn off the output signal.

1. According to the audio signal level sent by the main control room of the radio station, set the built-in attenuator of the audio processor and determine the output signal size of the FM comprehensive tester. For example, the normal broadcast level of the radio station is +10dB, the audio processor must be connected to a +20dB attenuator, and the output signal of the FM comprehensive tester is at most +10dB.

2. Select the output frequency of the FM comprehensive tester as 400Hz or 1kHz, adjust the output level of the comprehensive tester to +10dB, add the output signal of the comprehensive tester to the audio input terminal of the left channel of the audio processor, and then adjust the audio processing The audio input of the device controls the potentiometer, and observes that the gain on the limiter meter is reduced to 10-15dB. ③Adjust the left channel output OUTPUT control potentiometer clockwise, observe that the frequency deviation indication of the FM comprehensive tester should be slightly lower than or equal to 55%, and observe that the frequency deviation indication of the left channel of the FM stereo decoder should be slightly lower than or equal to 45%.

3. Dynamic adjustment On the basis of static adjustment, add the program signal according to the normal broadcasting level, turn off the transmitter, and observe the left and right signals respectively. The input control potentiometers of the channel and right channel signals can be individually fine-tuned. Note that the left and right channel signals must be kept in balance when adjusting.

4. Turn on the transmitter, and observe the left and right modulation frequency deviation values on the frequency deviation indication of the FM stereo decoder. If the frequency deviation is too large or too small, you must adjust the output control potentiometers of the left and right signals. Individual fine-tuning. Note that when adjusting, the frequency deviation of the left and right channel signals must be kept the same.

5. It is necessary to observe more about different types of program content. Adjusting the audio processing of broadcast programs according to the worst case is a relatively complicated technology, which requires both engineering and artistic considerations. The engineering goal is to During the transmission of FM radio, it is necessary to obtain the best signal-to-noise ratio and audio bandwidth while preventing overmodulation. The artistic goal is determined by the user of the audio processor. In short, the use of audio processors in the broadcasting system has greatly improved the quality of broadcasting programs, greatly improved the loudness and timbre of the received broadcasting programs, and the broadcasting effect of the broadcasting can satisfy the audience.