AC/DC and OFFSET

When we look at a standard waveform or timeline display in our DAW software, we are essentially looking at a more sophisticated and digitized depiction of what an analog oscilloscope depicts when it's measuring electrical voltage. After all, the entire analog path of our signal from microphone to loudspeaker is nothing but a continuous electrical voltage. When we push that signal through our analog-to-digital (A/D) converters and into our DAW software, we are converting the voltages into digital measurements represented by a string of ones and zeros, but those numbers still represent actual analog voltage levels to which they will revert after they leave our DAW, are converted back to analog voltages, and then turned back into sound by our loudspeakers, headphones, or ear buds.

Our timeline displays are just a representation of what that analog signal would look like in terms of voltage levels, with the vertical (y) axis representing the amplitude or amount of voltage and the horizontal (x) axis representing elapsed time. As we move along the timeline (the x axis) from left to right, we are simulating moving through time, with the waveform trace showing the exact voltage of the signal at each moment in time.

The centerline of this waveform display - the horizontal line halfway between the top and the bottom of the display - represents zero (0) voltage. If we were to graph a line showing complete silence; i.e. no signal at all, and therefore no analog voltage, this graph would be a flat line stretching horizontally across and on top of this zero volt centerline (Figure 1).

Any time our waveform traces into the top half of the graph, above the 0V centerline, this indicates a positive voltage. Conversely, the bottom half similarly represents negative voltage. Positive voltages will push the speaker element of our loudspeaker or headphone outwards from it's 0 volt "rest" position, negative voltages will pull the speaker element inwards from it's rest position. It's this push/pull that causes the speaker to create sound by increasing/decreasing the pressure of the air.

In this way, our waveform resembles an alternating current (AC), in that the signal alternates between positive and negative voltages. In fact a pure sine wave, like a pure test tone or signal from an analog synthesizer, will resemble an ideal alternating current (Figure 2). Note that in the real world of sound and music, the positive and negative voltages may not always be absolutely identical or symmetrical like a pure AC current, but the general concept will remain the same.

The difference between alternating current (AC) and direct current (DC) is that direct current does not alternate between positive and negative voltages, it just remains right where it is. Let's say that the AC voltage of the sine wave in Fig. 2 is one volt (1V). A DC voltage of +1V would just look like a horizontal line across the display at a y-axis level equivalent to the top peak level of the AC sine wave (Figure 3).

The DC voltage graph should look familiar; it looks exactly like the graph of silence, no signal whatsoever (Figure 1), except the horizontal line is shifted upwards. In fact, the graph in Figure 1 is sometimes referred to as the 0 DC line, meaning zero volts DC. The amount of difference between the 0 DC centerline and a non-zero DC voltage is referred to (in our world of audio) as the amount of "DC offset". In this case, it would be a DC offset of 1V.

Any AC signal (including our music) that is added to a quiet signal that has such DC offset in it will act like that DC line is the rest voltage. In other words, if we add a 1V sine wave to a DC offset of 1V, the sine wave will now swing between +2V and 0V, instead of the usual +1V and -1V (Figure 4).

This concept of DC offset is important to us for a few reasons. First, it's important to note that DC offset on it's own is inaudible. A flat line is a flat line; neither 0V DC nor 1V DC nor 100V DC will modulate our loudspeaker driver back and forth to create sound. When you pump DC into a loudspeaker, you may hear a small pop at the beginning and end of the signal as the speaker moves from it's rest position to be pulled in or pushed out to it's DC offset position, but other than that, the speaker stays still, and therefore creates no sound. Therefore, if we have DC offset in our signal, it can be difficult to detect by ear alone.

Which brings us to the second reason it's important; while the offset itself is inaudible, the fact that it's causing the speaker to move from it's designed rest position to what is virtually a new rest position, means that the speaker is now working "under stress"; it has less room to move and create sound in the direction that the DC current is forcing it, and it's physical properties will act a bit different when moving in the other direction. So even though DC offset itself is mostly silent, it can have a detrimental effect on your loudspeaker's performance.

Thirdly, just as DC offset limits the speaker's range in the direction in which it is forced, the amount of headroom we have in our DAW in the direction of the offset is reduced. This means that waveforms that have no DC offset that may just barely fill out the amount of headroom we have without clipping, can run out of headroom and clip when DC offset is introduced (Figure 5).

For all these reasons, DC offset is usually not considered a good thing to have in our audio signal. This is why most DAW software has a switch or tool somewhere called "DC Offset" which will eliminate any DC offset that may wind up in our signal. If we don't have a specific DC Offset switch of filter, a high-pass filter set very low (say around 10-15 Hz) is usually a good substitute.

Where does DC Offset come from? It is usually the result of a faulty or inadequate electrical circuit somewhere in the analog recording signal path. These days it's mostly caused by using cheap audio interfaces to our computer, like many (but not all) standard PC audio cards found in home computers or laptops. Laptop audio cards are the most notorious of these for causing DC offset.

The concept of DC offset is also important to a basic understanding of the difference between polarity and phase, which we'll talk about in greater detail shortly. Let's work our way there by talking next about polarity.
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