Frequency response is[1]In electronics, it is used to describe the difference in the processing ability of an instrument for signals of different frequencies.Like distortion, this is also a very important parameter.Frequency response, also known as response curve, refers to the curve of gain changing with frequency.Any sound equipment or carrier (object recording sound signal) has its frequency response curve.The ideal frequency response curve should be straight, and the sound signal will not be distorted after passing through.
A "perfect" AC amplifier should have the following qualities in frequency response indicators: it can maintain a stable amplification rate for signals of any frequency, and has the same driving capacity for corresponding loads.Obviously, it is completely impossible. There will be different "prefixes" for different amplifierssignal amplifier(Power amplifier or small signal amplifier), we should also add such a "prefix": audible to the human earfrequency range And "may" affect frequencies within that range.This range has obviously narrowed down a lot. We know that the audible frequency range of the human ear is about 20~20000Hz, that is, as long as the signal within this frequency range can reach the "standard" of the amplifier.In fact, according to research, although some signals higher than this frequency band and some lower than this frequency band are "inaudible", they will still have an impact on people's hearing sense. Therefore, this range should be further expanded. In the modern audio field, this range is usually 5~50000Hz, and some high demand amplifiers may even reach 0.1~hundreds of thousands of Hz.However, the above requirements seem to be much lower than "perfection" on the surface, but they are still "impossible tasks", and we cannot even meet such requirements.Thus, there is the indicator of "frequency response".(Postscript: the indicator itself represents "imperfection". If everything is "perfect", there will be no reason for the indicator to exist.)
Amplifiers have two types of distortion: linear distortion andnonlinear distortion 。We usually call the latter "distortion", and express the former in other ways.We already know what kind of nonlinear distortion is.The linear distortion refers to the "error" in frequency and phase, that is, frequency distortion andPhase distortion。
Frequency distortion
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Frequency distortion is a kind of "linear distortion", which means that when this distortion occurs, the output signal waveform and input waveform of the amplifier are still "similar", and it will not cause the amplifier to produce the signal to be processed“deformation”。A simple frequency distortion can be seen as the amplifier has different amplification for signals at different frequencies. For example, a ten times amplifier has 10 times the amplification for signals at 1KHz, while for AC signals at 10KHz, the amplification may become 9.99 times. Therefore, we can say that this amplifier has frequency distortion.stayElectroacousticsWe call this phenomenon "the irregularity of frequency response curve", and we will talk about the "curve" later.
Distortion cause
For an amplifier, there are many reasons for frequency distortion.The intrinsic characteristics of many amplifiers will affect this parameter, and even the distortion will be plugged in. In general, the following situations will lead to frequency distortion:
1. The natural frequency characteristics of components determine that this is the most fundamental reason, and some of the latter reasons actually come from here.
2. Adoptnegative feedbackThe open-loop characteristic of the technical amplifier and the frequency response characteristic of the negative feedback circuit itself determine.
4. The circuit design of amplifier leads to the non idealization of transmission characteristics
5. The installation and manufacturing process are not perfect, and the introduction of external AC interference signals leads to uneven frequency response.
Speaking of this, we will find that there are many reasons that seem to be related to the "measurement method", so it is necessary to mention how the frequency response is measured and calibrated.
test method
Any "indicator" that can be written in the instruction manual must be measured with the aid of instruments. These indicators must have a common feature, that is, "repeatability", that is, as long as you use the same equipment, you can repeat the same or similar measurement results.We call this kind of indicators "objective indicators", and frequency response certainly belongs to this category.
The frequency response measurement method is very simple. Connect aStandard signal generatorThis signal generator can generate standard sine wave signals, and can adjust the frequency of the output signal of this generator to change without changing the amplitude.Connect a standard pure resistive load at the output end of the amplifier and an AC level meter. By reading the data of the level meter, you can measure the frequency response characteristics of the amplifier.When measuring, in order to ensure the reliability and accuracy of the test results, it is necessary to test as much as possiblefrequency range The "logarithmic sampling method" is usually used to select different frequencies, that is, starting from a standard frequency (such as 1KHz), take points up and down according to the double relationship, such as 2K, 4K, 8K..., 500, 250, 125, 62.5... If the interval is too large, you can reduce the number of times, such as √ 2, √ 2/2, etc.Record the output level (in dB) of these corresponding frequencies and conduct statistical calculation.
Here, we may ignore the question whether the amplification of this amplifier can be adjusted?Amplifieroutput powerHow much should it be?It's not that I want to make a fuss, but the "mystery" here is very big.Due to the imperfect characteristics of the amplifier, the frequency response characteristics of the amplifier will change under different working conditions.This is called "test condition".We often find that there is "no difference" in frequency response indicators between two amplifiers with completely different quality. Is it the amplifier with poor quality that is "lying"?No, the test conditions are fundamentally different.
Dimensioning method
Amplifier in differentoutput powerIn general, the higher the output power, the worse the frequency response index.A more responsible indicator label should refer to "the indicator measured at the maximum undistorted power of the amplifier". In order to avoid the deterioration of the amplifier characteristics under high power output and make the indicator "look good", some manufacturers often use the "standard test method", that is, at a given amplifier magnification (gain)And this magnification is usually 1.Obviously, most amplifiers are used for "amplification", so this test method is actually not comprehensive, but "for commercial purposes and test standards," this test is still considered "correct".In this way, we should pay attention to that when looking at indicators, we should not only care about those values, but also look at them in connection with the test conditions.Indicators without test conditions are meaningless.
The standard frequency response labeling method is XHz~YHz ± ZdB, where X refers to the low-end frequency, Y refers to the high-end frequency, that is, the range of test frequency, and Z refers to the range of test frequencyfrequency range Difference in amplifier magnification.
Unfortunately, the frequency response characteristics of this amplifier can not be fully understood just by looking at the frequency response indicators, so the manufacturer gave another representation - frequency response curve.
Frequency response curve
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concept
The frequency response curve is used in the above test circuit toSignal generatorThe output signal frequency of the amplifier changes continuously (commonly called "sweep frequency") and keeps the amplitude unchanged. At the output end, record the output level corresponding to this continuous change of the amplifier through an oscilloscope or some other recorders, and then draw a curve of the level corresponding to the frequency on a coordinate.Of this coordinateOrdinateIs the level, and the abscissa is the frequency.The unit of ordinate is dB, and the unit of abscissa is Hz (or KHz).For the convenience of recording, the scale of abscissa is logarithmic, and the ordinate is linear.
features
We can look at the frequency response curves of different equipment provided by different manufacturers. We will find that even if two equipment with identical frequency response indicators appear, their frequency response curves are very different.Here we will not discuss the different pairs of frequency response curvestone qualityOnly those important characteristics of the frequency response curve should be noticed.Here we should pay attention to two characteristics: flat and straight.Flat means the amplifier is workingfrequency range The maximum difference of internal frequency response.Here we need to pay attention to the "operating frequency". For audio equipment, we should pay attention to the 20~20000Hz section. If the requirements are very high, we can expand the range to 5~40000Hz, which is enough.
influence
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Frequency distortion fortone qualityThe influence of voice quality is very huge. Most of the time, it will completely affect one's evaluation of sound quality.Due to the frequency response to subjectiveSound quality evaluationThere are too many influencing factors, so it is impossible to list them one by one. Just select some aspects that have the greatest impact.
timbre
In a broad sense, timbre is also a component of sound quality.We know that different musical instruments have different sound characteristics, pitchOvertoneThe resonance interaction constitutes the timbre characteristics of an instrument. The timbre is the frequency and proportion of these pitches, overtones, and resonances.If a system is not straight enough in frequency response, it may change the proportion of each component in the timbre. Some overtones may be enhanced, while others may be weakened or even hard to hear, which changes the timbre characteristics of instruments.Since we don't have the opportunity to compare the sound of the original instrument for many times, this change is not extremely important. However, since the "pleasant" of the instrument is almost synonymous with the timbre, excessive destruction of the timbre characteristics may cause the sound of the instrument to become unpleasant. Therefore, it is best not to change the timbre characteristics for people with high requirements.Because the frequency response will affect the timbre, some equipment designers will skillfully use this phenomenon to make up for the shortcomings of recording.For sound recorders, this adjustment is also "common", because they cannot "invite" those "famous zithers" to every record.
Sound field, positioning
Sound field is a very complex electroacoustic phenomenon, in which the frequency response characteristics will also affect the sound field performance to some extent.Due to the influence of frequency response, some sound details related to sound field performance will be weakened or strengthened, which will lead to the so-called "distortion" of sound field.This is a very subtle influence, which cannot be fully explained in this limited text, and will be discussed later.For positioning, the situation is also very complicated, especially thosefrequency range A wide range of musical instruments will have a greater impact.This is easy to understand. The sense of distance is closely related to the size of the sound. If the frequency response is not straight, the instrument will feel farther or closer when it emits a sound of a certain frequency than when it emits other sounds. In this way, we will feel that the instrument has been stretched longitudinally and its shape has changed.When the irregularity of frequency response is serious, we will feel the instrument shaking back and forth.
Overall voice
This topic can be very old. I won't talk about it any more here.The cold (high frequency) and warm (low frequency) of equipment, the density and intensity of sound are mainly from this (of course, there are other factors that will be discussed in the advanced chapter).
Frequency response index
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We should pay enough attention to the frequency response indicators of manufacturers.But we should also remember that this indicator is not "the higher the label", the better. Because our ears have some characteristics of their own, we need to have a clear understanding of frequency response.
1. The frequency response index we need should be the whole system, not a single equipment.The straight frequency response of a single equipment does not mean that we will hear the "straight" sound, but also depends on the condition of other equipment in the system.
2. Even when the frequency response of all equipment in the system is straight, we may not be able to hear straight sound.This is because our ears are not "straight".We know that the sensitivity of human ears to high frequencies will change in life, reaching its peak at the age of 20, going downhill at the age of 35, and losing more than half at the age of 60. In addition, it is also related to physical health and genetics.Therefore, when we consider straightness, we must take ears into consideration.In this regard, there seems to be an unspoken agreement in the industry, which is mainly completed by speakers, headphone manufacturers and sound recorders.
3. We have limited ability to distinguish frequency response fluctuation. Some experiments show that 0.2dB is the limit of very few people (about less than one in 100000), and most people are between 1 and 3dB.That is to say, it is almost meaningless to have a flat frequency response less than 1dB. If you abandon some other elements in order to pursue a flat frequency response, you will lose more than you gain.This principle is the same for other indicators.
4. As I said before, we can't ignore certain frequency bands because they can't be heard, because they may imply some other characteristics of the equipment.
5. Any indicator should be viewed in combination with others, rather than in isolation.
