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The basic concept of microphone distortion

Author:admin   AddTime:2021-03-14

In film and television recording, microphones can be said to be a very important part. The same period recording network has also carried out related basic introductions in the past. If you are interested, you can read the link at the end of the article. Film and television recording has high requirements for microphones in all aspects. Especially from the perspective of "recovering reality", the distortion of the microphone is a very important part. Let's learn about the distortion of the microphone today.

We don't want any accidental distortion in every part of the entire recording process. Unfortunately, although many distortions are below the threshold of audibility, it is usually difficult to avoid some form of distortion in the final recording. When setting up the recording chain, each link of the audio chain may have distortion in each pair, which in turn affects the total distortion, and the final distortion may become audible.

So before considering the amount of distortion, we must define the various forms of distortion that we encounter in the device-in this case-this article pays special attention to the distortion of the microphone. We must also consider the auditory system (ears and brain), which set the real limits of audibility.

Distortion-linear or non-linear?

First define distortion, the entire transmission process of a signal (sound) from input to output. Any change in the signal (that is, the change in the waveform of the signal) can be regarded as distortion. In general, all distortions are non-linear. According to the definition of signal change, we generally classify the change of sound amplitude as "linear distortion" because it can be corrected at a later stage. The delay of the signal is sometimes regarded as linear distortion, because the waveform is complete and only passed later.

Under this definition, any deliberately applied equalization, any change in frequency response bandwidth and any sound limitation will cause a kind of distortion. However, we should be more aware of some unconscious distortion and how it affects the perceived sound quality.

Distortion-where does it come from?

Distortion occurs due to technical limitations and other non-linear changes in the system.

Let's first look at sound transmission. Even in the air, sound has its "quantity" limitation. When the sound pressure level exceeds 194 dB SPL, no more air molecules form a sound wave, because it has reached the level of a complete vacuum. Looking at the part below 0Pa in the figure below, clipping has already occurred.

After 60 seconds, you can hear severely distorted sound. That is distortion in the air. If you look at the waveform (as shown above), you can see that the positive part of 0 is higher than the negative part.

Microphone distortion

The main basic component of the microphone is the diaphragm. If the microphone is a condenser pickup type, the diaphragm is installed in front of a backplane with a certain distance between the two, usually in the range of 20-50μm.

When the microphone is placed at a high SPL sound pressure level (from hearing threshold to pain threshold, the * value of sound pressure differs by 1,000,000 times. Obviously, it is inconvenient to use the * value of sound pressure to indicate the size of the sound. In order to facilitate the application, people follow The characteristics of the human ear's response to changes in sound intensity, leading to a logarithm to indicate the size of the sound, this is the sound pressure level, usually in decibels dB.) When inputting, it is obvious that at least the sound waves are pushing along the direction of the backboard. When the diaphragm is used, the deviation of the diaphragm is limited, so the diaphragm produces distortion. Moreover, the diaphragm material itself has limitations on how it can be "stretched" in any direction, and once the limit is exceeded, distortion will also occur.

Any condenser microphone needs an electronic interstage that converts the high impedance of the microphone transducer to a relatively low impedance in order to adapt to a long cable line. The electronic design may be asymmetric, which can cause distortion. (DPA®'s CORE microphone is a successful improvement). Although manufacturers have been working hard to improve microphones, the microphone system has limitations that may eventually lead to distortion.

How to quantify distortion?

According to the IEC standard (the International Electrotechnical Commission referred to as IEC, it is the earliest established non-governmental international electrotechnical standardization organization in the world, and is the Class A consulting organization of the United Nations Economic and Social Council), the nonlinearity amplitude is represented by three measures: total harmonics Distortion, Total Harmonic Distortion (THD), N-order distortion and second-order difference frequency distortion.

Let's take a look at what these terms mean?

Total Harmonic Distortion: Harmonic distortion refers to the more harmonic components of the output signal than the input signal, and the sum of all additional harmonic levels is called total harmonic distortion.

The general measurement method is to generate a pure sine wave with one and only one frequency. When distortion (typically through clipping) occurs, additional frequencies-integer multiples of the fundamental tone are generated. If the microphone is placed in the sound field of this pure sine wave (for example, 1kHz), the distortion products-the generated harmonics-including frequency components such as 2kHz, 3kHz, 4kHz, etc., will usually drop in level. (In the case of symmetric clipping, only non-uniform harmonics are generated, the third order, the fifth order, the seventh order, etc.).

THD measurement is performed by measuring the amount of all higher harmonics (RMS) as a percentage of the fundamental frequency level. *A good result is that the part in the dynamic range is smaller than <1%. 1% harmonic distortion means that the unwanted frequency components are 40 decibels below the fundamental frequency. This is the *standard of many microphone manufacturers. This is also a "seeable" indicator because the amount of distortion becomes visible on the waveform, such as when monitoring on an oscilloscope or recording and viewing in a DAW workstation.



In practice, THD is difficult to measure at low-to-medium SPL because speakers usually exhibit higher distortion compared to microphones. It should also be pointed out that when approaching the microphone maximum sound pressure level limit level, THD may increase rapidly.

Distortion of the nth order: In principle, it is the same as THD, except that the harmonics are measured and quantified separately. Generally, the distortion in the third harmonic is usually the most obvious, and the highest level of harmonics generated by all harmonics is represented by symmetric clipping.

Difference frequency distortion: The measurement principle is that two pure sine wave tones are applied to the same level. The IEC standard defines the distance between frequencies as 80 Hz. Examples of frequencies are 1000 Hz and 1080 Hz. However, a wider range of scanning can also be applied. Differential distortion produces sum and difference frequencies (that is, 1000 Hz and 1100 Hz produce 100 Hz difference frequencies). These frequency components are measured and quantified as a percentage of the fundamental frequency.

It must be pointed out that the application of standardized methods to quantify distortion is *correct. However, this standard can only take into account a small number of possible forms of distortion. In real life, sound signals are much more complicated, and distortion is even more so.

What can I hear?

To find what is important for distortion, we must also study psychoacoustics. The following are some factors that affect the perception of sound.

Threshold of hearing: Our hearing system has a natural lower limit. This limit varies with frequency. At low frequencies, the threshold level is quite high. In the frequency range of 2-4kHz, the auditory threshold level is low. See below.

Masking effect: refers to the shielding of surrounding frequencies when the ear is exposed to sound energy in a specific frequency range. This shielding is especially suitable for higher frequencies.

The figure below shows the shielding curve of 1kHz pure audio under various SPLs.

This means that the sound distortion that occurs usually becomes inaudible due to the shielding effect.

For beat frequency distortion, frequency components below the shielding frequency become *audible. Especially in music recording, this is not necessarily related to the difference in pitch, so it is considered more annoying.

For all bit-rate compressed audio formats, there will be a shielding effect. In this case, the distortion may be high and the signal-to-noise ratio is low. However, we (general listeners) often accept the way it sounds.

Distortion of the ear: The human ear itself also produces distortion. This phenomenon is especially present at higher SPL sound pressure levels, and ears have the best resolution at lower SPL.

This phenomenon is heard when two tones that are almost equally loud are heard. According to the frequency interval between the two tones, the third tone can be heard. For example, if you hear two tones separated by a fifth, C3 and G3 (131 Hz and 196 Hz), your ears will produce a 65 Hz difference tone (C2, one octave below C3.

The following are examples of intermodulation or difference frequency distortion. Two tones are produced, and then the ears produce different tonal distortions. However, due to the shielding effect, only tones below the true tone are perceived.
in conclusion

Generally, the microphone should be free of distortion. However, in reality there will definitely be some. However, measurements like THD and difference frequency distortion cannot account for the full range of distortions. However, these standardized index numbers can be regarded as indicators of the "health" of the design microphone. What the ear perceives is quite complicated, because the ear also produces distortion. Despite these digital indicators, the more distortion of the microphone, the more mixed and unclear the sound in the recording.

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