How to use audio processor for FM transmitter?

The loudness of audio programs in broadcasting is improved by reducing the ratio of the peak value to the "average value" in the dynamic audio. Within the allowable modulation range, if the peak value decreases the average value, it can increase, but this is inevitable. It is easy to bring harmful side effects such as clipping, so how to solve the "peak/average ratio" and how far should it be handled?



Audio processing principle

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, automatic gain control (AGC) The amplifier is a compressor. The compressor reduces the level difference between the soft sound and the loud sound, and makes full use of the allowable peak level range, so that the sound of the soft sound part is subjectively increased, but it does not make the loudness of the loud sound part. Louder.

2. Peak limit and clipping

Peak limit is an extreme form of compressor, which is characterized by high compression ratio and fast start-up and recovery time. In modern audio processing, peak limit itself usually limits not individual instantaneous peaks in the waveform, but the entire segment The peak value of the packet waveform, these instantaneous peaks are often clipped; limiting and clipping will reduce the ratio of the short-term peak to the average value (peak/average ratio) in the audio signal, and the main purpose is to protect the FM transmitter from As for overload, compression is different. The main goal of compression is to reduce the dynamic range of the program, achieve multi-band compression and frequency selection. Multi-band compression and frequency selection means that the audio frequency spectrum is divided into several frequency bands, and each frequency band is compressed or restricted separately. This is the most advanced audio processing technology in the contemporary era.

Integrated tuning of audio processor and FM transmitter

1. Static adjustment

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

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

②Select 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 of the left channel of the audio processor, and then adjust the audio processing The audio input of the amplifier controls the potentiometer, and observe that the gain on the limiter 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%. If you observe the frequency deviation indication of the left channel of the FM stereo decoder, it should be slightly lower than or equal to 55%. 45%.

2. Dynamic adjustment

On the basis of static adjustment, add the program signal according to the normal broadcast level, turn off the transmitter, and observe that the left and right signals are transmitted separately when the gain on the limiter table is reduced to 10-15dB. If it is too large or too small, the left The input of the channel and the right channel signal controls the potentiometer for individual fine-tuning. Note that the left and right signals must be balanced when adjusting.

①Turn on the transmitter and observe the left and right modulation frequency deviation values ​​on the frequency deviation indicator of the FM stereo decoder. If the frequency deviation is too large or too small, the output control potentiometers of the left and right signals must be adjusted. Fine-tune individually. Note that the frequency deviation of the left and right signals must be kept the same when adjusting.

②It is necessary to observe more different types of program content. Adjusting the audio processing of broadcast programs according to the worst-case scenario is a more complex technology. It must take into account both engineering and artistic aspects. The engineering goal is to In the transmission process of FM broadcasting, it is necessary to prevent overmodulation while obtaining the best signal-to-noise ratio and audio bandwidth. The artistic goal is determined by the user of the audio processor.

In short, the use of audio processors in the broadcast system has greatly improved the quality of broadcast programs, greatly improved the loudness and timbre of the received broadcast programs, and the broadcast effect can reach the audience's satisfaction.

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