I routinely talk about a signal having the value zero. This comes from more than fifty years of engineering experience. However, some claim that zero is not a signal value; it means there is no signal at all. This is one of those disputes where engineers, physicists, and users of everyday language are talking past each other, even though everyone thinks they are being precise.
From an engineering point of view, a “signal” is a value defined on some independent variable such as time, space, or sample index. Zero is simply one value in that range. An input identically equal to zero is therefore a perfectly valid signal. In fact, it is often a very important one. Zero input is the reference against which offset, bias, drift, and noise are measured. In linear systems theory, the zero signal is the additive identity of the signal space. We rely on it implicitly whenever we talk about superposition, impulse response, or stability. A system that does something nontrivial when driven by a zero input is either nonlinear, unstable, or broken, and in any case deserves close scrutiny.
The claim that “zero is no signal” usually comes from a different conceptual frame, one that mixes physical intuition or communications jargon into the discussion. In everyday speech, “no signal” means “nothing is happening” or “nothing is being transmitted.” A disconnected cable, an open circuit, or a powered-down transmitter produces no defined input at all. That is a different situation from a defined input whose value happens to be zero. Engineers normally distinguish these cases carefully. An open input is undefined, high-impedance, or floating. A zero input is defined, bounded, and testable.
Communications theory adds another layer of confusion. In some modulation schemes, “no signal” is used loosely to mean the absence of a carrier or the absence of energy, whereas a logical zero may be represented by a nonzero physical waveform. That linguistic shortcut makes sense in context, but it does not generalize. Once you step back into system theory, estimation, control, or measurement, the distinction collapses. Zero is a signal. It may carry information, such as “hold your state,” “remain idle,” or “there is no excitation,” and it is indispensable in defining noise, SNR, and dynamic range.
Zero is a signal with a well-defined value. “No signal” means no defined input at all. Confusing the two leads to sloppy reasoning, especially when you are trying to characterize noise, offsets, or system behavior around a reference point. Engineers are right to insist on that distinction, even if everyday language sometimes blurs it.
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