Where do we draw the limits of what is signal processing? Do we include biological, chemical, electrical, mechanical, or other related signals? Do we include non-linear signals? Do we include things that while they might not be thought to be signals in the first place, but might have properties similar to signals?
To facilitate discussion, I'll include a couple excerpts from the classic version of Oppenheim and Schafer:
A signal can be defined as a function that conveys information, generally about the state or behavior of a physical system...Signals are represented mathematically as functions of one or more independent variables.
In almost every area of science and technology, signals must be processed to facilitate the extraction of information...These techniques usually take the form of a transformation of a signal into another signal that is in some sense more desirable than the original.
And a couple from Proakis for good measure:
A signal is defined as any physical quantity that varies with time, space, or any other independent variable or variables. Mathematically, we describe a signal as a function of one or more independent variables.
A system may be defined as a physical device that performs an operation on a signal...When we pass a signal through a system, as in filtering, we say that we have processed the signal...Such operations are usually referred to as signal processing.
Those quotes paint a pretty broad definition of the field. What we have to decide is what we're interested in talking about here. People may not want to talk about, for example, biological systems here, but when it comes to modeling those systems to transform a signal, then it becomes much closer to the domain suggested by the references I cited.
If we open ourselves up to too broad of a definition, it would defeat the purpose of the site, but if we restrict the definition too much, we may never get out of beta, and there won't be a site at all.
In my humble opinion, it's probably reasonable, while we're still in beta, to favor a more permissive definition of signal processing--with the caveat that we try to use apparently off-topic questions to help define where we want to draw the line(s). In other words, rather than completely rejecting the idea of say, a question about the saturation properties of the human eye, we encourage the OP to massage the question until there's consensus that it finally falls just within the boundary of what we're interested in talking about here.
Or, more colorfully, we have to allow (or even encourage) people to paint outside the invisible lines in order to precisely determine where they are. People working in different sub-fields of signal processing will naturally draw those lines in different places, so it's potentially a learning experience for all of us.
I'm going to say that we should include any form of a signal, and any form of processing. A signal can be defined as any measured quantity which varies over time or space, over any number of dimensions. There must be a method of measuring the signal, but the methods could vary significantly.
Examples could include, but are not limited to:
- Photon count
- Membrane voltages
- Amounts of chemicals in a particular area
- Number of cars passing over a particular patch of road.
Processing could include any of the following, but again, is not limited to:
- Performing some kind of a filter, linear or non-linear.
- Looking for frequencies
- Trying to understand a particular phenomena of a signal.
- Characterizing the original system (IE, finding noise, resolution, etc)
Especially for non-traditional signals, the signals must be doing something interesting in order to qualify. Basically, don't just set them up and let it run (IE, radioactive decay type problem), but rather, the input signal must vary in some method, and the output signal must have some kind of processing done to it that is non-trivial (Not strictly a multiplier or something like that)