The adjustment method of dividing point of audio frequency divider

- Apr 10, 2020-

Frequency divider

What is a frequency divider?

The frequency divider can be defined as: the input electrical signal is separated into two separate signals, and the bandwidth of each signal is smaller than the bandwidth of the original signal. This device composed of one or more pairs of filters is called the frequency divider. It can also be called "frequency distribution network".

The divider usually consists of a high pass (low cut) filter (HPF) and a low pass (high cut) filter (LPF). A filter is a frequency selective device that can block the passage of other frequencies through the selected frequency. The filter usually has three parameters: cut-off frequency, network type and slope. The cut-off frequency refers to the frequency at which the response of the filter drops to a point when it is lower than its maximum level, usually 0.707 or 0.5 times of the maximum level, or 3dB or 6dB lower.

Design framework of triplicate frequency divider

The network type refers to the shape of the frequency response curve of the filter near the cut-off frequency. In recent years, many types of filters have been designed. The common filter types are Butterworth, linkwitz, Bessel, etc. Figure 1 shows the frequency response curve of various filters. The slope is defined as the slope of the frequency response curve of the filter when it drops to the cut-off frequency Degree, in dB / octave, usually with slopes of 6, 12, 18, and 24 dB per octave. It can also be called "filter slope" or "filter order". When the filter order is increased by one order, the slope will increase by 6dB / octave, that is, the first-order filter has a slope of 6dB / octave, and the second-order filter has a slope of 12dB / octave. Then, a 24 dB / octave Butterworth filter is equivalent to a fourth-order Butterworth filter.

Two way horn frequency divider

Since the speaker unit will not have the same sound level and full band output, the frequency divider must be used in the speaker system with full frequency range. The low frequency unit is used to reproduce the low frequency signal, the high frequency unit is used to reproduce the high frequency signal, and the frequency divider transmits the appropriate frequency signal to the appropriate horn unit.

Generally speaking, the frequency divider is divided into active type and driven type. Generally speaking, the audio signal (speaker level) after the power amplifier is separated by the driven frequency divider is often used in the speaker. The active frequency divider separates the audio signal (line level) before amplifier amplification, which is usually an independent electronic device between the signal source and amplifier. After passing through the frequency divider, the signal finally flows into the corresponding horn unit, which is used to reproduce the appropriate part of the sound spectrum. When the frequency divider is designed, the signals of each horn unit can be superimposed and the original input signals can be accurately reproduced. The divider will also affect some other parameters, such as power, bandwidth, which must be considered in the design.

Two divider

phaseAt a certain frequency, if the frequency response of two signals has similar amplitude and slope, the signals will be added together to form a new signal. We can use the phase response to explain the difference in phase or time between two signals.If the phase responses of the two filters are similar, their output signals will be added; otherwise, they will be cut off from each other. The different types and slopes of filters we discussed above all have their own unique phase response curves, as shown in Figure 1.The frequency division point is usually defined as the frequency at the intersection of the response of two frequency dividers (generally composed of an LPF and an HPF), which may be the frequency division point on the electrical characteristics of two electronic frequency dividers (driven or active), or the frequency division point on two acoustic filters. Any loudspeaker unit is essentially a filter, each of which has its own inherent high-pass and low-pass filters, as well as its own cut-off frequency, slope and network type.

Frequency divider

People often ask, "what is the frequency division point for a system?" in fact, what they want to know is where is the overall acoustic frequency division point for the system? The overall acoustic frequency division point of a system depends on the mathematical combination of the frequency response of the electronic filter and the horn unit in the system. When an electronic filter is added to an acoustic filter system, their frequency The response will be superimposed to form a new response curve. As shown in the example in Figure 4.

The difference of sound level / sensitivity between two different units and the phase lag of high frequency devices are obvious. The high frequency part is likely to be fixed on the horn of a long larynx, so the delay compared with the low frequency speaker is generated. In order to make the system reproduce the signal better, the newly developed frequency divider is required to be able to smooth the frequency response curve. After processing as shown in Fig. 6, the result shown in Fig. 7 is obtained.

Audio three dividers

We can notice that in the whole frequency response curve, the flat part is from 50Hz to 20kHz (- 3dB), the phase response of the high frequency part and the low frequency part has similar slope near the frequency division point, and the phase difference is not more than 90 degrees. This is to calibrate the low-frequency part to the high-frequency part by delaying the low-frequency part. We should realize that this is only a feasible scheme of frequency division, and many other schemes are also feasible. It can be seen that there is no correlation between the acoustic frequency division point of 1.3khz in Figure 7 and the low-pass filter with a cutoff frequency of 944hz in the low-frequency part and the high-pass filter with a cutoff frequency of 2053hz in the high-frequency part.

In addition, it does not correspond to the frequency division point of the unit in the original state, nor to the frequency division point of the electronic filter