index previous next

Signal arithmetic

[Subtraction]   [Division]   [Interpolation]   [SPECTRUM for Mac OS8]   [Spreadsheets]   [Matlab]   [Octave]  [Getting data into Matlab/Octave]

The most basic signal processing functions are those that involve simple signal arithmetic: point-by-point addition, subtraction, multiplication, or division of two signals or of one signal and a constant. Despite their mathematical simplicity, these functions can be very useful. For example, in the left part of Figure 1 (Window 1) the top curve is the optical absorption spectrum of an extract of a sample of oil shale, a kind of rock that is is a source of petroleum.

Figure 1. A simple point-by-point subtraction of two signals allows the background (bottom curve on the left) to be subtracted from a complex sample (top curve on the left), resulting in a clearer picture of what is really in the sample (right). (X-axis = wavelength in nm; Y-axis = absorbance).

This optical spectrum exhibits two absorption bands, at about 515 nm and 550 nm, that are due to a class of molecular fossils of chlorophyll called porphyrins. (Porphyrins are used as geomarkers in oil exploration). These bands are superimposed on a background absorption caused by the extracting solvents and by non-porphyrin compounds extracted from the shale. The bottom curve is the spectrum of an extract of a non-porphyrin-bearing shale, showing only the background absorption. To obtain the spectrum of the shale extract without the background, the background (bottom curve) is simply subtracted from the sample spectrum (top curve). The difference is shown in the right in Window 2 (note the change in Y-axis scale). In this case the removal of the background is not perfect, because the background spectrum is measured on a separate shale sample. However, it works well enough that the two bands are now seen more clearly and it is easier to measure precisely their absorbances and wavelengths. (Thanks to Prof. David Freeman for the spectra of oil shale extracts).

In this example and the one below, I am making the assumption that the two signals in Window 1 have the same x-axis values - in other words, that both spectra are digitized at the same set of wavelengths. Subtracting or dividing two spectra would not be valid if two spectra were digitized over different wavelength ranges or with different intervals between adjacent points. The x-axis values must match up point for point. In practice, this is very often the case with data sets acquired within one experiment on one instrument, but the experimenter must take care if the instruments settings are changed or if data from two experiments or two instrument are combined. (Note: It is possible to use the mathematical technique of interpolation to change the number of points or the x-axis intervals of signals; the results are only approximate but often close enough in practice. Interpolation is one of the functions of the iSignal function described later).  

Sometimes one needs to know whether two signals have the same shape, for example in comparing the signal of an unknown to a stored reference signal. Most likely the amplitudes of the two signals, will be different. Therefore a direct overlay or subtraction of the two signals will not be useful. One possibility is to compute the point-by-point ratio of the two signals; if they have the same shape, the ratio will be a constant. For example, examine Figure 2.

Figure 2. Do the two signals on the left have the same shape? They certainly do not look the same, but that may simply be due to the fact that one is much weaker than the other. The ratio of the two signals, shown in the right part (Window 2), is relatively constant from 300 to 440 nm, with a value of 10 +/- 0.2. This means that the shape of these two signals is very nearly identical over this x-axis range.

The left part (Window 1) shows two superimposed signals, one of which is much weaker than the other. But do they have the same shape? The ratio of the two signals, shown in the right part (Window 2), is relatively constant from x=300 to 440, with a value of 10 +/- 0.2. This means that the shape of these two signals is the same, within about +/-2 %, over this x-axis range, and that top curve is very nearly 10 times more intense than the bottom one. Above x=440 the ratio is not even approximately constant; this is caused by noise, which is the subject of the next section.

On-line calculations and plotting. Wolfram Alpha is a Web site and a smartphone app that is a computational tool and information source, including capabilities for mathematics, plotting data and functions, vector and matrix manipulations, statistics and data analysis, and many other topics. can perform a huge range of statistical calculations and tests. There are several Web sites that specialize in plotting data, including Plotly, Grapher, and Plotter. All of these require a reliable Internet connection, and they can be useful when you are working on a mobile device or a on computer that does not have the required software. 

SPECTRUM is a 90s era freeware signal-processing application for Macintosh OS8 that includes the following signal arithmetic functions: addition and multiplication with constant; addition, subtraction, multiplication, and division of two signals, normalization, and a large number of other basic functions (common and natural log and antilog, reciprocal, square root, absolute value, standard deviation, etc.) in the Math menu. It also performs many other signal processing functions described in this paper, including smoothing, differentiation, peak sharpening, interpolation, fused peak area measurement, Fourier transformation, Fourier convolution and deconvolution, and polynomial curve fitting. It runs only in Macintosh OS 8.1 and earlier and on Windows 7 PCs and various specific Linux distributions using the Executor emulator. No native PC version is available or planned.

Popular spreadsheets, such as Excel or Open Office Calc, have built-in functions for all common math operations, named variables, x,y plotting, text formatting, matrix math, etc. Cells can contain numerical values, text, mathematical expression, or references to other cells. A vector of values such as a spectrum can be represented as a row or column of cells; a rectangular array of values such as a set of spectra can be represented as a rectangular block of cells. User-created names can be assigned to individual cells or to ranges of cells, then referred to in mathematical expression by name. Mathematical expressions can be easily copied across a range of cells, with the cell references changing or not as desired. Plots of various types (including the all-important x-y or scatter graph) can be created by menu selection. See for a nice video demonstration. Both Excel and Calc offer a form design capability with full set of user interface objects such as buttons, menus, sliders, and text boxes; these can be user to create attractive graphical user interfaces for end-user applications, such as in The latest versions of both Excel (Excel 2013) and OpenOffice Calc (3.4.1) can open and save either spreadsheet file format (.xls and .ods, respectively). Simple spreadsheets in either format are compatible with the other program. However, there are small differences in the way that certain functions are interpreted, and for that reason I supply most of my spreadsheets in .xls (for Excel) and in .ods (for Calc) formats.  See "Differences between the OpenDocument Spreadsheet (.ods) format and the Excel (.xlsx) format". Basically, Calc and do everything Excel can do, but Calc is free to download and is more Windows-standard in terms of look-and-feel. Excel is more "Microsoft-y".

If you are working on a tablet or smartphone, you could use the Excel mobile app, Numbers for iPad, or several other mobile spreadsheets. These can do basic tasks but do not have the fancier capabilities of the desktop computer versions. By saving their data in the "cloud" (e.g. iCloud or SkyDrive), these apps automatically sync changes in both directions between mobile devices and desktop computers, making them useful for field data entry.

Matlab is a commercial numerical computing environment and programming language. In Matlab (and in Octave), a single variable can represent either a single "scalar" value, a vector of values (such as a spectrum or a chromatogram), or a matrix (a rectangular array of values, such as a set of spectra). This greatly facilitates mathematical operations on signals. For example, if you have signal amplitudes in the variable y, you can plot it just by typing "plot(y)". And if you also have a vector t of the same length containing the times at which each value of y was obtained, you can plot y vs t by typing "plot(t,y)". Two signals y and z can be plotted on the same time axis for comparison by typing "plot(t,y,t,z)" .  (Matlab automatically assigns different colors to each line, but you can control the color and line style yourself by adding additional symbols; for example "plot(y,y,'r.',y,z,'b-')" will plot y with red dots and z with a blue line.  You can divide up one figure window into multiple smaller plots by placing subplot(m,n,p) before the plot command to plot in the pth section of a m-by-n grid of plots. Type "help plot" or "help subplot" for more options. (You can Copy and Paste any of these code examples into the Matlab or Octave command line and press Enter to execute it).

The function max(y) returns the maximum value of y. Individual elements in a vector are referred to by index number; for example, t(10) is the 10th element in vector t, and t(10:20) is the vector of values of t from the 10th to the 20th entries. You can find the index number of the entry closest to a given value in a vector by using the downloadable val2ind.m function; for example,  t(val2ind(y,max(y))) returns the time of the maximum y, and  t(val2ind(t,550):val2ind(t,560)) is the vector of values of t between 550 and 560 (assuming t contains values within that range). The units of the time data in the t vector could be anything - microseconds, milliseconds, hours, any time units.

A Matlab variable can also be a matrix, a set of vectors of the same length combined into a rectangular array. For example, intensity readings of 10 different optical spectra, each taken at the same set of 100 wavelengths, could be combined into the 10x100 matrix S.  S(3,:) would be the third of those spectra and S(5,40) would be the intensity at the 40th wavelength of the 5th spectrum. The Matlab/ Octave scripts plotting.m and plotting2.m show how to plot multiple signals using matrices and subplots.

The subtraction of two signals a and b, as in Figure 1, can be performed simply by writing a-b. To plot the difference, you would write "plot(a-b)". Likewise, to plot the ratio of two signals, as in Figure 2, you would write "plot(a./b)". So, "./" means divide point-by-point and ".*" means multiply point-by-point. The * by itself means matrix multiplication, which you can use to perform repeated multiplications without using loops. For example, if x is a vector


creates a matrix A in which each column is x multiplied by the the numbers 1, 2,...10. It is equivalent to (but more compact and faster than) the loop

for n=1:10;

Typing "b\a" will compute the "matrix right divide", in effect the weighted average ratio of the amplitudes of the two vectors (a type of least-squares best-fit solution), which in the example in Figure 2 will be a number close to 10. The point here is that Matlab doesn't require you to deal with vectors and matrices as collections of numbers; it knows when you are dealing with matrices, or when the result of a calculation will be a matrix, and it adjusts your calculations accordingly. See Probably the most common error you'll make in Matlab/Octave is getting the rows and columns mixed up (full disclosure: I still make that kind of mistake all the time): here's a text file that gives examples of common vector and matrix operations and errors in Matlab and Octave. If you are new to this, I recommend that you read this file and play around with the examples there.

Getting data into Matlab/Octave.
You can easily import your own data into Matlab or Octave by using the load command. Data can be imported from plain text files, CSV (comma separated values), several image and sound formats, or spreadsheets. Matlab has a convenient Import Wizard (click File > Import Data). It is also possible to import data from graphical line plots or printed graphs by using the built-in "ginput" function that obtains numerical data from the coordinates of mouse clicks (as in DataTheif or Figure Digitizer). Matlab R2013a or newer can even read the sensors on your iPhone or Android phone via Wi-Fi. To read the outputs of older analog instruments, you need an analog-to-digital converter or a USB voltmeter.
GNU Octave is a free alternative to Matlab that is "mostly compatible". Everything I said above about Matlab also works in Octave. In fact, the most recent versions of almost all of the Matlab functions, scripts, demos, and examples in this document will work in the latest version of Octave without change. (The exceptions are the keystroke-operated interactive iPeak, iSignal, and ipf.m). If you plan to use Octave, make sure you get the current versions; many of them were updated for Octave compatibility in 2015 and this is an ongoing project. There is a FAQ that may help in porting Matlab programs to Octave. See Key Differences Between Octave & Matlab. There are Windows, Mac, and Unix versions of Octave; the Windows version can be downloaded from Octave Forge; be sure to install all the "packages". There is lots of help online: Google "GNU Octave" or see the YouTube videos for help. For signal processing applications specifically, Google "signal processing octave". 

Octave 3.8.2 has a trial graphical user interface (GUI), pictured on the left, which seems to work well enough, but it still does not currently work with my interactive programs ipf.m, ipeak.m, and isignal.m. (GNU has released version 4, which has an official GUI, but it does not currently work under Windows 8).

Installation of Octave is somewhat more laborious than installing a commercial package like Matlab. Here is what I did to install Octave 3.8 and its GUI onto Windows 8:

    1. Run the install script at This installs Octave 3.8 into C:\Octave\Octave-3.8.2
    2. Download the bat file "octave-gui.bat" from onto C:\Octave\Octave-3.8.2
   @echo off
   set PATH=%CD%\bin\
   start octave --force-gui -i --line-editing
    3. Launch Octave 3.8 by double-clicking that bat file.
    4. Install the packages like so:
         >> cd src
    >> build_packages

        (This last step takes 10-15 minutes to complete).

More seriously, Octave is also slower than Matlab computationally - about 2 to 20 times slower, depending on the task (for some specific example, see TimeTrial.txt). Also, the error handling is less graceful and it's less stable and tends to crash more often (in my experience). I am working to make all my Matlab scripts and functions compatible with Octave; make sure you have the latest versions of my functions (a number of functions were made more Octave-compatible in March 2015). Bottom line: Matlab is better, but if you can't afford Matlab, Octave provides most of the functionality for 0% of the cost.

Spreadsheet or Matlab/Octave? For signal processing, Matlab/Octave is faster and more powerful than using a spreadsheet, but it's safe to say that spreadsheets are more commonly installed on science workers' computers than Matlab or Octave. For one thing, spreadsheets are easier to get started with and they offer flexible presentation and user interface design. Spreadsheets are better for data entry and are easily deployed on portable devices (e.g. using iCloud Numbers or the Excel app). Spreadsheets are concrete and more low-level, showing every single value explicitly in a cell. Matlab/Octave is more high level and abstract, because a single variable, punctuation, or function can do so much. An advantage of Matlab and Octave is that their function and script files (“m-files”) are just plan text files with a “.m” extension, so those files can be opened and inspected even on devices that do not have Matlab or Octave installed. Also, user-defined functions can call other built-in or user-defined functions, which in turn can call other functions, and so on, allowing very complex high-level functions to be built up in layers. Fortunately, Matlab can easily read Excel .xls and .xlsx files and import the rows and columns into matrix variables.

The bottom line is that spreadsheets are easier at first, but in my experience sooner or later the Matlab/Octave approach is more productive. This point is demonstrated by the comparison of both approaches to multilinear regression in multicomponent spectroscopy (RegressionDemo.xls vs the Matlab/Octave CLS.m), and particularly by the dramatic difference between the spreadsheet and Matlab/Octave approaches to finding and measuring peaks in signals (i.e. a 250Kbyte spreadsheet vs a 7Kbyte script that's 50 times faster).

Both spreadsheets and Matlab/Octave programs have a huge advantage over commercial end-user programs and compiled freeware programs such as SPECTRUM; they can be inspected and modified by the user to customize the routines for specific needs. Simple changes are easy to make with little or no knowledge of programming. For example, you could easily change the labels, titles, colors, or line style of the graphs - in Matlab or Octave programs, search for "title(", "label(" or "plot(". My code contains comments that indicate places where specific changes can easily be made: just use Find... to search for the word "change". You are invited to modify my scripts and functions as you wish. 

index previous next

This page is part of "A Pragmatic Introduction to Signal Processing", created and maintained by Prof. Tom O'Haver , Department of Chemistry and Biochemistry, The University of Maryland at College Park. Comments, suggestions and questions should be directed to Prof. O'Haver at Updated February 2016.
Unique visits since May 17, 2008: