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, while preventing it from being over-modulated, and ensuring that the audio obtains a better signal-to-noise ratio and The audio bandwidth allows the audio signal to be processed on the basis of the characteristics of the original program material, making it a new and distinctive tone for the listener to receive.
(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 one type of compression and its form, 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 matched well, 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.
3. Method of rational use of audio processor
The success of audio processing of a broadcast program is judged by its actual effect, that is, the auditory effect. For example, the broadcast effect of the broadcast can be accepted by the audience. This kind of processing is considered to be successful, otherwise it is a failure. In this regard, I will proceed from the principle of the audio processor, combined with the actual use situation, fully tap its potential advantages, and more effectively and rationally play its performance, should start from the following three aspects:
(1) Keep the signal undistorted
At the front end of the mid-wave broadcast transmitter, the audio signal highly processed by the audio processor will contain many flat-top waveforms similar to square waves. The square wave's waveform requires a relatively high amplitude and phase response to the transmission path it passes through. In principle, in the frequency range of the main energy of the program, if the flat amplitude and the group delay are deviated, the flattened top of the processed audio signal will be tilted, thereby increasing the peak modulation voltage, but the average level is not increased. . From the peak/flat ratio value, the average level of the path is reduced, and the loudness is correspondingly reduced. In this regard, we want to maintain the prototype of the processed signal waveform. The first method adopted is to select a transmission cable with high quality and excellent performance in the use of the transmission signal cable, and to require a distribution parameter small, a frequency bandwidth, a thick copper wire, a small attenuation, and a shielded copper core transmission line. This is very important and very effective. In addition, in the transmission connection, try not to add any additional equipment and branch components, such as intermediate amplifiers, distributors, etc., to reduce signal waveform distortion and ensure good transmission quality.
(2) Audio processing system settings
j For the audio processor, it consists of two circuits, one is a slow moving AGC, and the other is a compressor with moderate motion and recovery time. The frequency constant is adjusted according to the needs of each frequency band. We conclude in actual use that the time constant of the low-frequency band is set to be slower than the high-frequency band (about 200 μs), which has a greater effect on increasing the signal density of the program.
k The audio processor also adds some auxiliary components in the basic system, and enables the frequency equalization processing component of the audio processor installed between the slow moving AGC and the multi-band compressor to compensate for the typical sound of the medium wave broadcast signal. Properly increasing the distribution of 600 Hz-1.2 KHz sound energy throughout the audio spectrum makes this sound audibly "larger" (the most sensitive range of human hearing is 2KHz-8KHz). It can make the audience feel the sound is really beautiful.
l We also used the “Attenuation Distortion” device on the audio processor to provide negative peak control to prevent audio signal overflow to eliminate distortion in some frequency bands that the listener is most likely to hear.
(3) The location of the audio processor in the system
In the system, the location where the audio processor is placed is also very particular. In order to effectively protect the peak-limited waveform processed by the audio processor so that it does not change during transmission to the transmitter, the audio should be the processor is placed close to the transmitter and the shorter the distance, the better. In order to avoid the variation of the distribution parameters during the transmission process, the parasitic modulation peak is caused, and the waveform whose peak limit has been processed is changed, resulting in distortion of the waveform of the audio signal.
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