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Someone said to me that Bipolar AMI is not low-pass; it does not include the 0Hz in its frequency domain. I doubt that that is the case.

However, at present I am unable to do Fourier transformations. Does anyone have a diagram of the frequency domain of any Bipolar AMI signal. I will use it to verify the claim mentioned.

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  • $\begingroup$ Can you point us to a reference defining Bipolar AMI so we might know what you're asking about? $\endgroup$ Dec 6, 2023 at 7:07

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Calculating the power spectrum of the AMI code is actually a quite complicated matter, but the fact that the power spectrum has a zero at DC seems intuitively clear, as already mentioned in RBJ's answer: since a "one" is coded alternately as a positive and a negative pulse, the long term average tends to zero.

The figure below shows the power spectrum of the AMI code (without the effect of the transmit pulse). The energy spectrum of the transmit pulse is multiplied with that spectrum, which clearly won't change the zero at DC. $p$ is the probability of a "one" in the data.

enter image description here

[from Digital Communication by E.A. Lee and D.G. Messerschmitt, 2nd ed.]

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Okay, so "Google is my friend" and I found a YouTube.

Bits that are zero contribute nothing to the DC component.

If the number of ones in a message is even, it's as positive as often as it is negative. The average is zero. DC is the average.

If the number of ones is odd, then there is exactly one bit where it's more positive than negative. No matter how many bits there are.

If the length of the message, the number of bits, is very large, then that 50% likelihood that there is one measly bit difference affects the mean or DC value by a vanishingly small amount because that gets divided by the total number of bits.

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