How to Use Audio Processors Rationally in Broadcasting Systems

As we all know, public broadcasting plays an important role in our daily life, such as various natural disasters, geological disasters, etc. Therefore, effective maintenance of the system is also an important guarantee for the safety of people's lives and property. The audio processor is one of the key equipment in the broadcast transmission system, and it has a great impact on the broadcast quality. In daily work, there are three issues that radio station technicians are most concerned about: one is the matching problem between the transmitting antenna and the network. The second is the three major technical indicators of the broadcast transmitter. The third is the noise and attenuation of the audio signal during the transmission process. Few people care about the working status and setting of the audio processor. Let's take a look at the reasonable application of the audio processor in the broadcast system.

There is a lot of nonlinearity in the processing of broadcast audio signals. For audio processing equipment, it not only includes signal compression, limitation, clipping, expansion and other processing methods, but also has strict requirements on the installation location of audio processing equipment, the length of lead wires, and the ability to resist interference in high electromagnetic field radiation environment. In view of this situation, I explored the working principle, setting method, placement, additional functions, etc. of audio processors, so as to adopt a variety of effective measures to make audio processors play a greater role in future work.

1. Requirements for audio processing

(1) The audio processing method used for broadcast transmission is relatively complex, mainly compressing or limiting the frequency range audible to the human ear, preventing it from being overmodulated while ensuring that the audio obtains the best signal-to-noise ratio and audio bandwidth. The audio signal is processed to a greater extent while maintaining the characteristics of the original program material, making it a brand new and characteristic timbre for the audience to receive and appreciate.

(2) In broadcast programs, the loudness of audio signals is increased by reducing the ratio of the "peak" to the "average" in dynamic audio (peak/average ratio). Within the allowable modulation range, the relationship between the peak and average values ​​is adjusted to avoid harmful side effects such as distortion caused by clipping during audio signal processing. The audio is processed to meet the limits of peak modulation and maximize the subjective loudness effect.

2. Basic principles of audio processing

Audio processing equipment mainly suppresses noise by reducing the dynamic range, including compression of program signals, peak limiting and clipping, multi-band compression, and frequency selectable limiting and equalization. The main purpose of compression is to reduce the program dynamic range, increase the density of sound, and try to make the peak amplitude of the audio signal uniform. Peak limiting is an extreme form of compression, but it has a high compression ratio and fast start-up and recovery time. Its main purpose is to protect the transmission of the subsequent channels from overload. Peak clipping processing is to prevent distortion caused by overload of the channel processing circuit, and instantly "cut off" the high-level peak part that exceeds the threshold. If peak limiting and clipping can be perfectly matched, it will be able to deal with the negative effects of harmonic distortion, intermodulation distortion and signal bandwidth between the density and loudness of the audio program signal.

In the process of audio processing, the audio spectrum is divided into several frequency bands, and each frequency band is compressed and limited respectively. That is, "multi-band compression and optional limitation". If the settings are correct and reasonable, the intermodulation of spectrum gain will be effectively eliminated. For equalization in audio processing, its role is to use the equalizer to change the balance of related frequencies in the overall frequency band of the audio signal on the one hand, and to create a certain sound feature by changing the loudness of the "sensitive frequency" in it on the other hand, so as to increase the influence of the program. In addition, it can also be used for frequency response correction in the transmission system.

3. How to use the audio processor properly

The success of audio processing of radio programs is judged by its actual effect, that is, the auditory effect. If the broadcasting effect of the radio can be accepted by the audience, we consider this processing method to be successful, otherwise it is a failure. In this regard, I will start from the principle of the audio processor, combine it with the actual use situation, fully explore its potential advantages, and more effectively and reasonably exert its effectiveness. We should start from the following three aspects:

(1) Maintaining signal transmission without distortion

At the front end of the medium wave broadcast transmitter, the audio signal that has been highly processed by the audio processor will contain a lot of flat-top waveforms similar to square waves. The square wave waveform has relatively high requirements for the amplitude and phase response of the transmission path it passes through. In principle, in the frequency range of the main energy of the program, if the flat amplitude and group delay deviate, the flat top of the processed audio signal will be tilted, thereby increasing the peak modulation voltage, but the average level has not increased. From the peak/flat ratio, the average level of the channel is reduced, so the loudness will be reduced accordingly. In this regard, we must maintain the original shape of the processed signal waveform. The first method is to try to choose high-quality and excellent transmission cables when using transmission signal cables. It is required that the distribution parameters are small, the frequency band is wide, the wire diameter is thick, the attenuation is small, and the shielding is good. The copper core transmission line. This is very important and 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

a. For the audio processor, it consists of two circuits, one is the slow-motion AGC, and the other is the compressor with moderate action and recovery time. The optimal time constant is set for each frequency band according to the needs. We have concluded in actual use that it is appropriate to set the time constant of the low-frequency band slower than that of the high-frequency band (about 200μs). This method plays a greater role in increasing the density of program signals.

b. The audio processor also adds some additional components to the basic system, and uses the frequency equalization processing component installed between the slow-motion AGC and the multi-band compressor to compensate for the poor audio frequency response typical of medium-wave broadcast signals. The distribution of 600HZ-1.2KHZ sound energy in the entire audio spectrum is appropriately improved, making this sound "larger" in hearing (the most sensitive hearing range of the human ear is 2KHZ-8KHZ). This can make the audience feel that the sound becomes real and beautiful.

c. We also use what is called an "anti-distortion" device on the audio processor, which provides absolute negative peak control to prevent the audio signal from overflowing and eliminate distortion in some frequency bands that the listener is most likely to hear.

(3) The position of the audio processor in the system The position of the audio processor in the system is also very particular. In order to effectively protect the peak-limited waveform processed by the audio processor and prevent it from changing during transmission to the transmitter, the audio processor should be placed close to the transmitter, and the shorter the distance, the better. This is to avoid parasitic modulation peaks caused by changes in distributed parameters during transmission, which will change the peak-limited waveform and cause waveform distortion of the audio signal.

4. Benefits of using audio processors properly

In addition to the advantages of reducing peak level, improving average modulation, and increasing sideband wave power to increase loudness, the use of audio processors also has the following four benefits: it can solve the problem of large differences in program levels caused by recording, switching, different program content, and the use of different recording and playback equipment, which is difficult to solve well when manually adjusting the program level; it creates conditions for establishing a volume balance between language programs and music programs; it can amplify a certain amount of weak signals at the low ebb of the program so that they are not drowned by noise components. Usually, the level of the treble components in the program is low, and it can be appropriately improved after processing. The treble components not only determine the clarity of the program, but also have a complex psychological and physiological relationship with the loudness. Due to the improvement of clarity, the audience feels that the loudness has also increased; the coverage radius of stereo broadcasting is about half of that of mono broadcasting in the same power level transmission state. After using an audio processor, due to the increase in sideband wave power, it is possible to receive higher quality signals in places where the receiving carrier field strength is slightly lower than the requirement, thereby expanding the coverage range.

Relevant data show that the average power of processed music is 8.2dB higher than that of unprocessed music. Compared with the ordinary limited release with a compression of 15dB, the value obtained is 5.9dB, which is actually equal to 4 times the increase of the transmission power. Using audio processors to increase coverage can save a lot of electricity, equipment investment and maintenance costs compared with other methods. It is a very economical and feasible method. Of course, the audio processor should compress the program content appropriately. Excessive compression will damage the program content, affect the artistic effect of music, and make stereo broadcasting lose its due characteristics. In summary, high-quality audio processors should be used at the program input end of the transmitter to compress the dynamic range of the program, increase the loudness, expand the coverage of the program, and ensure high-quality stereo broadcasting. In terms of the current technical level of audio processors, it can fully meet the requirements of high-fidelity broadcasting, so that the processed sound can not hear the processed sound at all, that is, without any trace of processing, the high quality of the original sound is still retained, and it is even more pleasant, clear and pleasant than the original sound.