What to expect from digital audio
A few years ago, the word “multimedia” entered the computer lexicon, and more recently, the PC is increasingly used as a home entertainment center. In both cases, the computer must reproduce the sound, which, as you might guess, exists on it only in digital form. And if with the advent of the first transistor technology, the phenomenon of “transistor sound” was vigorously discussed and covered with myths and legends; However, it is often believed that computer signal processing, on the other hand, is obviously better. So what is digital audio and how is it inferior to or superior to analog?
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From a human point of view, sound is air vibrations with a frequency of approximately 16 Hz to 20 kHz. A person perceives the lower frequencies (with sufficient amplitude) not as sound, but as vibration. Superiors are not captured at all. The upper limit of the frequency range depends on age: in young children it reaches 22-24 kHz, and gradually decreases to 8-12 kHz over time. Therefore, the human ear can hear signals of a very wide bandwidth. For comparison: the eye can perceive color only in the range that covers the change in frequency of electromagnetic oscillations by less than 2 times. Of course, not all frequencies are equally important. For example, a range of 500 to 3500 Hz is sufficient for speech intelligibility. But to listen to music or the soundtrack of a movie, this is not enough. Ideally, the sound field in the listening area should be indistinguishable from the sound field in the recording area. That is, the entire audio path, from a studio microphone to a home speaker, must not introduce distortions that are within the resolution of the human auditory analyzer.
The sound that our ears perceive when playing a digital recording has previously undergone a series of transformations:
1) electromechanical conversion of air vibrations into an electrical signal;
2) amplification and processing of an analog electrical signal (frequency equalization, addition of reverb, etc.), mixing;
3) analog to digital conversion;
4) digital signal processing: frequency correction, mixing, mastering, etc .;
5) storage or transmission of digitized sound;
6) digital signal processing: frequency correction, volume control, oversampling;
7) digital to analog conversion;
8) Analog signal processing (frequency equalization, mixing, adding reverb, etc.);
9) amplification of the analog signal;
10) electromechanical transformation of electrical current oscillations into sound oscillations.
When processing an analog signal in a studio, devices with an analog interface and digital “fill” are often used, so the chain of analog-to-digital and digital-to-analog conversions can be much longer.
The first four stages are most often carried out on studio equipment, which has incomparably higher performance than home equipment. Therefore, although the distortions are unavoidable, we will assume that they are insignificant compared to the distortions of a similar nature introduced by the household equipment in the last five stages. In amateur audio recording, additional distortion should be considered in the early stages, which will be described below.
Electromechanical conversion is usually done with a studio microphone. This device generates a very weak signal that needs amplification and is also extremely susceptible to mechanical stress. Even under ideal conditions, for example in a concert hall, acoustic noise can cause the dynamic range of the music being played to be less than the maximum dynamic range of a 16-bit sound presentation.
A signal recorded from several microphones is inevitably processed: the required volume levels of the different channels are selected, the noise is cut with filters, etc. Also, the dynamic range of the signal is generally compressed. The last operation leads to a significant increase in the noise level, but without it, the recording would sound unsatisfactory on middle-class consumer equipment, first of all, too quiet.
The distortions introduced by the sound path have a varied physical nature and very different manifestations, but nevertheless they can be divided into three large groups.
