# Is an analogue audio signal DC or AC?

**URL:** https://boards.straightdope.com/t/is-an-analogue-audio-signal-dc-or-ac/474740
**Category:** Factual Questions
**Created:** [November 27, 2008, 9:41pm UTC](https://boards.straightdope.com/t/is-an-analogue-audio-signal-dc-or-ac/474740 "2008-11-27T21:41:22Z")
**Posts on this page:** 1
**Showing post:** 4

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### Author: ![erislover](https://avatars.discourse-cdn.com/v4/letter/e/71e660/32.png) [@erislover](https://boards.straightdope.com/u/erislover)
#### Post date: [November 27, 2008, 10:25pm UTC](https://boards.straightdope.com/t/is-an-analogue-audio-signal-dc-or-ac/474740/4 "2008-11-27T22:25:10Z")

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The speaker perspective is a good one. You are correct that the alternating current moves the cone in and out. An AC signal with a DC component would also move the cone in and out, but the DC portion would permanently move the cone a bit in one direction, making a new zero point, as it were. As you might imagine, this would affect the sound quality.

DC components can cause other problems. Some uses of scrambling in transmitting a digital signal over radio waves are there to remove DC components. There’s no such thing as a DC offset of a radio wave, but the DC component might appear at the receiver or transmitter prior to sending or receiving the radio waves, so the scrambling function attempts to remove significant DC offsets for the amplifiers before the digital signal is recovered (at which point the DC offset is no longer a concern).

I quote a patent to explain this:

> [@](#):
>
> Another aspect of digital communications related to bit transition density or maximum run length is cumulative DC offset, which reflects the sum of the low frequency voltage components of the transmitted data stream experienced at the receiver. In binary systems, discrete logic values for zero and one are typically assigned opposite polarity voltages. As a result, without periodic adjustment, the cumulative DC offset experienced at the receiver can migrate toward the positive or negative power supply limit, which may lead to an overload condition at the receiver. The cumulative DC imbalance can be expressed as the number of bit values required to be inverted to produce a balanced bit stream. If the cumulative DC offset experienced at the receiver can be effectively balanced, the DC voltage swing experienced at the receiver can be reduced. In this regard, a balanced DC offset can be exploited to reduce overall signal to noise ratio (SNR) at the receiver because low frequency noise may be more effectively filtered out.

I didn’t read [the whole patent](http://www.freshpatents.com/Forward-error-correction-encoding-for-multiple-link-transmission-capatible-with-64b-66b-scrambling-dt20080508ptan20080109707.php) but this point is interesting to think about.

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