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However, ideal filters - are not present. How make struggle with this unpleasant effect?īefore digitizing a signal, by means of the analog filter of low frequencies (which passes through itself only low frequencies) delete all ultrasonic components. And now imagine, that you will hear, if the baritone continues to sing in A-major, and tenor starts to sing in G-sharp, moreover and words sings on the contrary. Ultrasound spectrum are mirrored: top frequencies appear below, and bottom - above.Īs example, you can imagine a duet of a baritone and the tenor. The ultrasound are moved by frequency to area below half of sample rate.
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And instead of a clean sound from 0 to 22 kHz (half sampling rate) we have a mix pure sound and the deformed («mirrored») ultrasound (spectrum's part, located above 22 kHz). On digital processing's slang it refers spectrum «mirrored». What occurs now at analog-digital conversion of a signal with spectrum from 0 Hz to infinity without acceptance of special measures? All ultrasonic sinusoids with frequencies above half of sampling rate are moved to heard range. And on sound card's input it is necessary to understand as ultrasound not only a signal of acoustic wave which practically loses all the ultrasonic components directly in microphone or in guitar's pickup, but also various noises (high frequencies) had added to ultrasound components of initial signal at passage of amplifier's electric circuits, ADC and e.t.c (even noises from your computer). The sinusoids which are being above 20 kHz we have named all ultrasound (in this article). Our initial (analog) signal has the acyclic form (its waveform never repeats) and, hence, it have an infinite continuous spectrum. Let's assume, that we really do not hear ultrasound above 20 kHz, but it plays the pernicious role in black business of distortion initial (analog) sound. Having accepted for true that average person does not hear above 20 kHz, we confidently reject all sinusoids which have frequency above 20 kHz (ultrasound), and we consider, that our perception of a sound loses nothing. Not-periodic or not-repeating signal (it is any audiorecord) has an infinite continuous spectrum (the infinite amount of sinusoids «nestled to each other» with level, generally, falls down with growth of frequency). This frequency depends on duration of a cycle of repetition.Ģ. The periodic or cyclically repeating signal of any form can be presented, as the sum of sinusoids (harmonics) with different phases (speaking roughly, time delays) and frequencies, multiple (to increase on 2, 3 and so on) the basic frequency of this signal.