In the electrical channels of sound-broadcast the signal undergoes various changes: it is amplified or attenuated (change level), adjusts the dynamic range, spectrum and time structure, is mixed with other signals, shifted frequency (transpose), converted from analog to discrete form and back, reduces increases the duration of sound. These changes are made both to solve artistic (creative) problems and to adapt the signal parameters to the properties of electric channels and paths. A significant part of audio signals processing is performed by automatic control devices, which are installed directly in the communication channels and at the input of the radio transmitter. The use of automatic control devices is due to the fact that most communication channels are not designed to transmit sound-broadcast signal with its natural dynamic range. For example, such devices include noise reduction systems that compensate channel noise, volume control systems that allow you to align the volume of the broadcast signal with the same peak level, but different power.
Dynamic processing associated with changes in the dynamic range of signals is usually implemented by automatic level controls. As a result of dynamic processing, the entire range of sound-broadcasting levels should be within the
limits established by the norms and rules of technical operation. The minimum signal levels should be significant (by 10...20 dB) above the noise floor of a communication channel, and the maximum level should not exceed the values at which nonlinear distortions are unacceptably increasing.
Reducing the dynamic range of the sound-broadcast signal leads to an increase in its average power and, as a consequence, the average modulation coefficient of the transmitter. This increases the service area of the transmitter.
The above tasks are relevant and for their solution the device of automatic gain control with the algorithm of its operation has been developed, the block diagram of which is shown in figure 1.
In figure 1 blocks ADC-analog digital converter, DBF- digital band pass filter, Preamp-preamplifier, CG- controlled gain, DPU- digital processing unit, DAC-digital analog converter, AGC - automatic gain control.
The device operates as follows; the original signal is subjected to an analog digital conversion, then the received signal is filtered by a digital filter and amplified(us ing a pre-amplifier) thus increasing the signal-to-noise ratio, in turn this allows more efficient digital processing. After pre-filtering and amplification, the signal is fed to an amplifier with an adjustable transmission ratio and to a digital processing unit. Digital processing unit regulates the transmissions characteristics of the amplifier. The output signal is controlled by a feedback loop to prevent negative effects.
The main characteristics of automatic gain control devices are:
-the degree of compression or ekspedirovanie "Ratio»;
- threshold of operation " Threshold»;
- the response time of the "Attack»;
- recovery time (Release).
Developing the device model comparisons with classical solutions of the tasks have been made.
The figures below show the time characteristics of the classical automatic control device and the developed on.
Comparing the characteristics one can see automatic adjustment with extrapolation correction has a better time characteristic, and it should be noted that the amplitude characteristic changes in time depending on the input signal, which provides better regulation.
The following example demonstrates the case when the harmonic input signal is affected by a multiplicative noise in the form of a video pulse, the noise level is 6dB.
Figure 4 - Input action
Harmonic distortion when exposed to such interference equals Kh=0.64.
The output signal of the classic automatic gain control is shown below.
Figure 5 — output signal
In this case, the harmonic distortion after automatic gain control is decreased to Kp=0,5229.
Figure 6 below shows the output of the developed automatic gain control device.
Figure 6 — Output signal
The harmonic distortion is decreased to Kg=0.188. In addition to a fairly low harmonic distortion, the developed device at the end of transients fully compensated the level of multiplicative noise.
Studying the algorithm we have found that it has advantages over the classical device both in time characteristics and in nonlinear distortions.
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