Here is a plot of a bell curve EQ applied and the phase shift that occurs. Since EQ is usually applied at a number of different frequencies and as a number of different types, such as a bell curve, notch filter, or Hi Shelf (these will be explained), you will end up with practically the entire spectrum of sound a mess in terms of the timing of the signals at various frequencies. Having this occur takes the saxophone note a bit away from sounding “real”. What happens is that the saxophone harmonic at 1,048 Hz will occur slightly before the fundamental tone at 262 Hz (because it occurs on the left side of the bell EQ, see explanation below). OK, so let us also assume that a bell curve at 1.5 kHz has been applied as EQ. The amplitude of these harmonics are what make the saxophone sound distinguishable. It is part of the music, the sound of the instrument. So, there is a harmonic at 524 Hz, at 786 Hz, at 1,048 Hz, and so on. The harmonics of that note distinguish the saxophone sound so you can tell it from a trumpet playing the same note. Let’s call it 262 Hz for the sake of this discussion. Suppose a saxophone player is playing a middle C note which is 261.63 Hz. To make a seemingly complicated story shorter, slowing down or speeding up various parts of the music in relation to the rest of the music disturbs the sound that the listener hears. The middle waveform has positive phase shift (it is moved to the left, earlier in time), and the right-side waveform has negative phase shift (it is moved to the right, later in time). The first waveform (on the left end of the diagram) is the original signal, with no phase shift. Here is a diagram showing positive and negative phase shift. The phase-shifted signal is added to the original non-phase-shifted signal, and depending on the amount of phase shift and whether it is positive or negative, the audio signal is amplified or attenuated at the desired frequency (frequencies). The irony is that phase shift is what makes EQ work. Smaller amounts of phase shift will occur at farther distances from the bell. Half-way down the left side of the bell there will be maximum positive phase shift, and halfway down the right side of the bell there will be maximum negative phase shift. However, the phase shift extends beyond this. In other words, if a bell curve is introduced at, say 1.5 kHz, as shown below, the effect of the curve extends to 1 kHz at the leading edge (left hand side of the bell) and 2 kHz at the trailing edge (right hand side of the bell). Altering the phase means that the timing of the signal is moved backward or forward in relation to the frequencies in between the front and rear ends of the frequency modification. Modifying the frequency response alters the phase, with the most alteration occurring at the front end and rear ends of the frequency response modification. The basic flaw with EQ has been that it exhibits group delay, which is a frequency-dependent phase shift of a group of frequencies.
And, when the final tape was prepared for cutting the lacquer for making LPs, an RIAA pre-emphasis EQ curve was also applied. In the old days of analog recording, analog EQ was usually applied during the recording stage too. It is given that digitally recorded music is EQ’d in the editing stage, using digital EQ plug-ins to the DAW (Digital Audio Workstation). “EQ” stands for equalization and is used to modify the frequency response of music, in the recording stage, editing stage, or playback stage.
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