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 must maintain the original shape of the processed signal waveform. The first method is to select the transmission cable with high quality and excellent performance in the use of the transmission signal cable, and require small distribution parameters, frequency bandwidth, and wire diameter. Thick, low-loss, 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
(1) 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 optimal time constant is adjusted for each frequency band as needed. 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.
(2) The audio processor also adds some additional components to the basic system, enabling the audio processor to install a frequency equalization processing component between the slow AGC and the multi-band compressor to compensate for the typical presence of the mid-wave broadcast signal. The audio frequency response is poor. Appropriately increase the distribution of 600HZ-1.2KHZ sound energy throughout the audio spectrum, making this sound audible "larger" (the most sensitive range of human hearing is 2KHZ-8KHZ). It can make the audience feel the sound is really beautiful.
(3) The position of the audio processor in the system is also exquisite. In order to effectively protect the peak-limited waveform processed by the audio processor, it will not change during transmission to the transmitter, and 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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