Techniques in Signal Processing

CSC 508

Time and Spatial Domain Analysis

Basics of Time Varying Signals

We have looked at a number of methods to analyze, predict and simulate measurements of a fixed random variable. We have assumed that the quantities being measured were constant and the differences between samples we observed were due to noise or statistical variations about some fixed mean value.

Now we will investigate non-stationary random quantities. We will study signals that change in time or relative position in their domains. A simple example of a time-varying signal is the voltage level used to drive an audio speaker.

AudioSignalSource

Lets look at a sample of the electrical signal used to drive the speaker. We can plot the voltage level as a function of time.

Quickly varying voltages correspond to high frequency sounds and slowly varying voltage levels correspond to low frequency sounds.

The shape of the wave determines the timbre of the sound. Wave shape is the reason a trumpet and a piano playing the same note sound different. This is also the reason you can hear the difference between the sound of a live performance and a recording.

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age

timevolt

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original signal

distorted signal

It is the goal of the audiophile to achieve the perfect acoustic reproduction of recorded sounds. But what does this mean?

Digitization

Digitization is the conversion from analog (continuous) to digital (discrete) samples. Using our example audio signal we will convert it to a series of data values at a specified sample rate.

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The two types of errors we introduce by digitization are called decimation and quantization errors.

Decimation is the sampling of the signal source at discrete moments in time. Usually these samples are separated by a constant interval.

Quantization is the division of the signal levels into a number of discrete values determined by the number of bits in the base-2 representation of the samples.

clipping

Sampling - Time is not directly represented in a digital device. A running comuter jumps from one state to another in a sequence of discrete steps. Any simulation of a signal or process on a digital computer must be quantized in time. Once a minimum time interval t is established we can determine the state of a simulated system only at times T for which T/t is an integer. Unless we change our time quantum the state of the system at any other time, sat T+t/2, never exists in the simulation.

In our analog-to-digital conversion, we must first establish a time between sample dt or equivalently a sample rate 1/dt. This rate must be at a greater number of samples per second than the rate of change of the highest frequency signal we wish to record.

Aliasing - Consider two sine wave signals as shown below. the dots indicate the points at which sampling is performed.

12 samples/cycle

12 samples/ 11 cycles

In this example one sine wave is sampled at 12 samples/cycle, while the other sine wave is 11 times higher in frequency. This higher frequency sine wave is indistinguisable from the lower frequency sine wave at the specified sampling rate. This is a phenomenon called aliasing.

To better understand aliasing, look closely at the samples of the two waves. The first sample is at the peak of both waves. The second sample is 1/12 of a cycle along the lower frequency wave and 1 1/12 of a cycle along the higher frequency wave. The third sample is 2/12 (or 1/6) of a cycle along the lower frequency waves and 2 1/6 cycles along the higher frequency wave. This trend continues to the 13th sample which is actually the same as the 1st sample relative to the amplitudes of the two waves.

If we want to be able to measure a certain frequency fmax, we must make sure that our sampling rate is at least twice as high. This is called the Nyquist interval named after Harry Nyquist, the communications engineer who first defined it.

The selection of a sampling rate establishes the decimation in time of the digitization process. Now we need to decide how many bits we will use to encode the voltage values of the samples. this will define the quantization of the signal amplitude.

The number of bits we select to represent a sample in the digitization process is directly related to the minimum detectable amplitude change in the signal. For example, if we choose 8 bits for our sample word size for a signal whose amplitude is between +/- 10 Volts, our amplitude resolution is 20/28 Volts = 78 milliVolts, with a maximum quantization error of 78/2 = 39 milliVolts. In general the quantization error is given by,

1minmax

2

nquant

VVE

where n is the number of bits in the data sample. Quantization error is a noise created by the digitization process itself and is not an inherent part of the signal source or the recording medium. Quantization noise can be reduced by increasing the number of bits used to encode signal values. Additional errors are introduced when our signal exceeds the range of our quantization limits. This is called clipping.

So whay would we want to add the additional errors of quantization and decimation by digitizing a signal? The reason is simple. Once the sample values have been digitized they can be saved, read, transmitted, or copied repeatedly without additional degradation.

The recording, playing, copying and transmitting of an analog signal all contribute to the noise level in the signal. Consider the quality of a third-generation copy of an analog or video tape recording compared to the perfect (usually) reproduction of a copy of a computer file.

A major task of digital signal processing is dealing with the noise present in the original signal before digitization. Time varying signals are always characterized by some uncertainty in their values. The amount of uncertainty can be quantified by a relationship called the signal-to-noise ratio (SNR). The SNR is defined as the peak signal divided by the root-mean-square (RMS) noise,

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VVSNR

This assumes that the noise sources Ni are not correlation (i.e. they are independent). Both the signal and the noise are measured in the same units so that the SNR is itself a unitless quantity. The mean noise levels for all noise sources Ni are combined as RMS values for the same reason that we used RSS to compute standard deviations. Since we assume noise sources are independent, they can cancel each other reducing the effect.

We will now briefly review some of the more common sources of noise in analog signals.

Types of Noise in Signals

Thermal Noise - This noise is due to random motion o fht charge carriers within any circuit element. Thermal noise is proportional to the square-root of electrical bandwidth of the circuit and the temperature of the components. (When audible, this noise makes a hissing sound.)

Shot Noise - This noise is due to the spontaneous emission of electrons by electrical components. This noise always accompanies the motion of electrons. (If audible, shot noise makes a popping or clicking sound.)

Temperature Noise - Variation in termperature cause variation in electrical conductivity. These changes occur very slowly relative to other kinds of noise and therefore contribute to the overall uncertainty in responsivity over hours or days.

1/F Noise - Also known as current noise, this noise is a characteristic of solid state electronics. The distribution of 1/F noise is mostly at low frequencies and drops off as 1/F with increasing frequency F.

Preamplifier Noise - A source of noise inherent in the design of electronic preamplifiers (these are amplfiers for very low power signals). This noise is a function of the manner in which components are combined to make an amplifier circuit.

Effects of Noise on Signal Processing Performance

Lets look at an example of how the level of noise affects our ability to measure a common feature in a signal. Assume that we wish to determine the location (in position or time) of the peak of a Gaussian-shaped signal pulse.

A popular method for peak detection is to compute the derivative of the signal and then determine the zero-crossing point of this derivative signal.

In the digital world, the derivative is estimated by simply taking the difference between sucessive pairs of samples,

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We see that the uncertainty in the position that the signal crosses the zero amplitude axis is a function of the signal amplitude and noise level. (See area inside red circle.)

If we increase the noise level while holding the peak signal amplitude constant, the uncertainty in zero-crossing position increases.

If we hold the noise level constant and reduce the peak signal amplitude, the uncertainty in zero-crossing point also increases.

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peak signalamplitude

peak signalamplitude

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noiselevel

Development of Applications in Signal Processing

Creating effective solutions to signal processing problems requires a combination of theoretical knowledge and expertise in recognizing the essential elements of a problem. The good news is, these are skills that can be acquired with practice. The bad news is, you have to practice. Use the following exercises to develop your problem solving techniques.

1. A person with normal hearning can determine the direction of a sound source to within a few degrees. This is possible because the brain can detect defferences in the time of arrival of sounds to the left and right ears. Expand on this idea as you answer the following:

a. A sound course called a clicker is placed 10 meters in front of a blindfolded test subject. It is discovered that the person can detect a change in the position of the clicker when it is moved left or right by at least 45 centimeters. Use the speed of sound to determine the temporal resolution (in seconds) of this subject.

b. Imagine that a sound source (e.g. the clicker) is placed approximately 10 meters in front of you with the distance to each ear the same. Now imagine that the source is moved in an arc over your head so that the distance from the source to either ear remains constant. Assuming that you could tell that the source was being moved, develop a theory to explain how the ears/brain detect the change in position of the sound source.

2. Using the concept of aliasing, explain why the spokes on a wagon wheel appear to move backwards in a movie.

3. Given that a sine wave of frequency f0 is being sampled at a rate of m samples per cycle, give two other frequencies f1 and f2 (in terms of f0 and m) that when sampled at the same rate would appear identical to the original sine wave (i.e. f1 and f2 are aliased frequencies of f0 at the sampling rate m).

Challenge Problem: Use the expression for the Gaussian function and its derivative to analytically derive the relationship for the magnitude of the peak-detection error for a Gaussian pulse as a function of SNR. (Assume the zero-crossing method described above.)

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Spatial Domains

In the time domain, the independent variable is time which is plotted against a dependent variable we call signal amplitude S(t). This signal can be in units of voltage, decibels, kilograms or any measurable quantity. The function S(t) is an indication of how the dependent variable changes with time. In some applications the signal strength is a function of a spatial parameter x that is not, itself, a function of time. Signals obtained from an entity's position, or shape in a two-dimensional (or higher) spatial domain are called spatial signals.

Parametric Forms - An example of a spatial function based on an objects shape is a parametric representation of characters as shown below.

The parametric function shows the slope of the line of the character 5 as a function of x, the fraction of the length along the character line (starting at the bottom of the character).

Barcodes as Spatial Functions

A barcode scan is an example of a spatial function that is measured directly. The 3-of-9 Code is one of a number of barcodes commonly used in product labeling.

Symbol codes are separated by a narrow white bar. The code string *12345* is shown above. A laser scanner repeatedly illuminates the bar with a Helium-Neon (red) laser while a photocell detects the reflected light level across the bar image. When the laser beam scans across the bar a signal

similar to the one shown above is produced. This signal is converted to the decoding vector for each character 0-9 or *.

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threshold 2

An Example Problem in the Time Domain

Consider a time-varying signal composed of a large amplitude, low frequency sine wave, low amplitude random noise and a series of relatively narrow width, low amplitude Gaussian pulses. In this example the peaks of the Gaussian pulses are much less than the peak-to-peak amplitude of the sine wave. Our task is to detect (i.e. count and locate the positions of) the Gaussian pulses. We cannot use a fixed amplitude

threshold due to the large variations in signal imposed by the sine wave. Either the threshold will miss the pulses in the valleys or it will incorrectly detect the peaks of the sine wave as signal pulses.

Since sample-to-sample changes of the signals of interest are more rapid than those of the sine wave, we can compute a running average of the last 10 to 15 samples and use it as an adaptive threshold.

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composite signal

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where the kth sample Sk of the signal is compared with the value Tk derived from the samples in the range k-NW to k+NW. We accept any sample Sk that exceeds the value of the adaptive threshold Tk.

The adaptive threshold Tk can be scaled to detect lower amplitude pulses by setting to a lower value. However, Tk must be kept above the noise level to prevent excessive false alarms (i.e. detection of background noise as signal). When we study the frequency domain, we will learn how to selectively remove the sine wave so that a fixed threshold can be used.

Systems Effects on Signals

We have looked at time and spatial domain signals in terms of their information and noise content. We have discussed the existence of distortion or changes in the shape of a signal due to imperfect recording, amplification or transmission.

The first homwork problem introduced the effects of an optical system on an image. We learned that a point of light entering the front aperture of an optical system is transformed into a blurspot on the focal plane. We used this effect to help us detect and remove noise spikes in images.

The effect of a physical system (amplifier, telescope, tape recorder) on a signal can be determined by inputing a known signal and measuring the resulting signal. The simplest possible input signal we can use is a single sample of a known amplitude called an impulse. The effect on this input spike by a system is called its impulse response.

System

impulse impulse response

The Dirac Delta Function

The impulse function is expressed mathematically as a rectangular function of infinitesimal width and unit area (weird). In this form the impulse function is known as the Dirac delta function (t).

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The effects of a system on a signal can be estimated by replacing each individual sample of the input signal with a collection of samples corresponding to the systems impulse response.

The computation of the response is expressed as the superposition integral also called the convolution,

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Output

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where h is the impulse response function and x() is the input signal.

Convolution for Discrete-Time Systems

In computers and digital signal processors we are limited to discrete samples in time, so our superposition integral becomes a summation,

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Let's see how a discrete convolution is computed. Assume we have determined the impulse response of our system given by h(t) and we want to see how the system will affect the input signal x(t) (in this case a simple square pulse).

To compute the convolution, the response function h(t) is flipped horizontally and then slid over the signal function x(t). At each sample position ti, the product of the two functions x(ti)*h(ti-) is summed and the total is plotted as r(ti).

Before the two functions overlap the output r(t) is zero.

The first non-zero r(t) is just the product of h(t-t) and x(t) at the indicated sample.

When there is more the one sample overlap r(t) is the sum of the sample products.

Once h has passed beyond x(t) the convolution is complete.

Homework:

4. A system has an impulse response represented by the sampled function h(t) shown below. Derive the total output function for the system presented with the signal function x(t).

5. Be prepared to discuss in class each of the following phenomena in terms of an impulse and response (Web section may omit this question).

a. the sound of a gong (or bell).b. feedback in an audio amplifierc. an earthquaked. out of focus photograph

6. Challenge Problem: Given any two constants a and b and any two functions f(x) and g(x), we can define a linear system L by the relationship,

L(a f(x) + b g(x)) = a L(f(x)) + b L(g(x))

Given f(x)=sin x and g(x) = cos 3x determine if the system L(s)=2s+1 is linear.