The broadcast audio signal has great nonlinearity in the processing. For the audio processing equipment, it not only includes the processing methods of compression, limitation, clipping, expansion, etc., but also the position and lead of the audio processing equipment. There are strict requirements on the length and the anti-interference ability in the high electromagnetic field and strong radiation environment.
1. Requirements for audio processing:
(1) The audio processing method for broadcast transmission is relatively complicated, mainly to compress or limit the frequency range that can be heard by the human ear, and to prevent it from being over-modulated, and to ensure that the audio obtains the best signal-to-noise ratio and Audio bandwidth. The audio signal is processed on the basis of maintaining the characteristics of the original program material, so that it becomes a brand new and characteristic tone for the audience to receive and enjoy.
(2) In a broadcast program, the loudness of an audio signal is increased by reducing the ratio of the "peak" to the "average value" (peak/flat ratio value) in the dynamic audio. Adjust the relationship between peak value and average value within the allowable modulation range to avoid harmful side effects such as clipping caused by clipping during audio signal processing, and process the audio to meet the limits of peak modulation. Try to increase the loudness effect of subjective feelings.
2. The basic principles of audio processing:
Audio processing equipment primarily suppresses noise by means of reduced dynamic range, including compression of program signals, peak limiting and clipping, multi-band compression and frequency selective limiting and equalization. The main purpose of compression is to reduce the dynamic range of the program, increase the density of the sound, and try to make the peak amplitude of the audio signal uniform. The peak limit is an extreme form of compression, but it has a high compression ratio and a fast start-up and recovery time. The main purpose is to protect the transmission of the rear channel from over-loading. The peak clipping process is a process of preventing the distortion caused by the overload of the channel processing circuit and instantaneously "cutting off" the high-level peak portion exceeding the threshold. Peak matching and clipping, if perfectly matched, will handle harmonic distortion and intermodulation distortion and the negative effects of signal bandwidth between the density and loudness of the audio program signal.
The audio spectrum is divided into several frequency bands during audio processing, and each frequency band is separately compressed and limited. That is, "multi-band compression and selectable limits", if the settings are correct and reasonable, will effectively eliminate the intermodulation of spectral gain. For equalization in audio processing, the effect is to use an equalizer to change the balance of the relevant frequency in the overall frequency band of the audio signal, and on the other hand to create an acoustic feature by changing the loudness of the "sensitive frequency" to increase the program. It can also be used as a frequency response correction in transmission systems.
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