Classification of vehicles based on audio signals

International Journal on Soft Computing (IJSC) Vol.6, No. 1, February 2015
DOI: 10.5121/ijsc.2015.6105 53
Classification of Vehicles Based on Audio Signals
using Quadratic Discriminant Analysis and High
Energy Feature Vectors
A. D. Mayvana
, S. A. Beheshtib
, M. H. Masoomc
a
Department of Electrical Engineering, Iran University of Science and
Technology, Tehran, Iran
b
Department of Electrical Engineering, Iran University of Science and
Technology, Tehran, Iran.
c
Department of Mechanical Engineering, BabolNoshirvani University of
Technology, Babol, Iran.
ABSTRACT
The focusof this paper is on classification of different vehicles using sound emanated from the vehicles. In
this paper,quadratic discriminant analysis classifies audio signals of passing vehicles to bus, car, motor, and
truck categories based on features such as short time energy, average zero cross rate, and pitch frequency of
periodic segments of signals. Simulation results show that just by considering high energy feature vectors,
better classification accuracy can be achieved due to the correspondence of low energy regions with noises
of the background. To separate these elements, short time energy and average zero cross rate are used
simultaneously.In our method,we have used a few features which are easy to be calculated in time domain
and enable practical implementation of efficient classifier. Although, the computation complexity is low,
the classification accuracy is comparable with other classification methodsbased on long feature vectors
reported in literature for this problem.
KEYWORD
Classification accuracy; Periodic segments; Quadratic Discriminant Analysis; Separation criterion; Short
time analysis.
1. INTRODUCTION
Vehicle identification while it is in motion is a prerequisite for traffic and speed management,
classified vehicle count, traffic signal time optimization, gap/ headway measurement, and military
purposes. Moving vehicles affect the environment in different ways. Vehicle emits heats, sounds,
and magnetic field. There are many approaches investigated vehicle identification based on
different kinds of signals. Image processing techniques are used to classify vehicles under the real
time traffic management and for Intelligent Transportation Systems (ITS). Inductive loops based

54
systems are widely used for determination of vehicle counts [1]. The most promising approach
for vehicle identification is the one that is based on acoustic signals. Moving vehicles emit
characteristic sounds. These sounds are generated from moving parts, frictions, winds, emissions,
tires,etc. Assuming that similar vehicles which have the same working conditions generate the
same sounds; then these sounds can be used to classify vehicles [2]. Vehicle classification based
on sound signals have already attempted by researchers. Paulrajet al. (2013) [3] used
autoregressive modeling algorithm for the analysis to extract the features from the recorded
vehicle signals. Probabilistic neural network (PNN) models are developed to classify the vehicle
type and its distance. Nooralahiyan et al. (1997) [4] used a directional microphone connected to a
DAT (Digital Audio Tape) recorder. The digital signal was pre-processed by LPC (Linear
Predictive Coding) parameter conversion based on autocorrelation analysis. A Time Delay Neural
Network (TDNN) was chosen to classify individual travelling vehicles based ontheir speed
independent acoustic signature to four broad categories: buses or Lorries, small or large saloons,
various types of motorcycles, and light goods vehicles or vans. Ghiurcau and Rusu(2009) [5]
presented a vehicle sound classification system using time encoded signal processing and
recognition (TESPAR) method combined with the archetypes technique implemented on the
Matlab platform. The experimental results show the efficiency of the TESPAR method when
dealing with vehicle sounds. Effectiveness of features set is usually measured by how well they
represent the signal; however, features should also satisfy several conditions: their number should
be small (less than 15); also,to enable practical implementation they should be easy to calculate.
There is a number of feature extraction methods used in vehicle classification. Some of them
produce too many features for a single input vector like estimation of Power Spectrum Density
[6], [7], [8], and some are too complicated like Principal Component Analysis [9], [7].
Nevertheless, no feature extraction method fulfills all our requirements, and there is no systematic
way to select best feature according to our criterion. Harmonic line, Schur coefficients, and MEL
filters, were presented in [10] each relating to properties of vehicle audio. Methods were
compared in context of their separability and correct classification rate. Aljaafreh and Dong
(2010) [2] investigated two feature extraction methods for acoustic signals from moving vehicles.
The first one is based on spectrum distribution and the second one on wavelet packet transform.
They evaluated the performance of different classifiers such as K-nearest neighbor algorithm
(KNN) and support vector machine (SVM). It is found that for vehicle sound data, a discrete
spectrum based feature extraction method outperforms wavelet packet transform method.
Experimental results verified that support vector machine is an efficient classifier for
vehicles.Other spectrum based feature extraction can be found in [11], [12], [13], [14], and [15].
Similarly, Discrete Wavelet Transform (DWT) is used in [16] and [17] to extract features using
statistical parameters and energy content of the wavelet coefficients.
This paper describes an algorithm to classify audio signals of vehicles. Quadratic discriminant
analysis uses feature vectors of periodic segments with elements: short time energy, average zero
cross rate, and pitch frequency to distinguish between vehicles’ signals. If we separate feature
vectors with high energy from others properly and train classifier with these vectors, better
classification accuracy will be achieved due to the correspondence of low energy regions and
noises of the background, as simulation results confirm. Obviously high energy feature vectors
have less zero cross rate. Considering this point is the basic idea for separation criterion. Our
innovation in separation is assuming vectors by energy larger than ‫ܧ[ܧ‬௡], average of all
segments’ energy, and ZCR less than ‫ܼ[ܧ‬௡],average of all segments’ ZCR, are high energy
elements. We have used three features which are easy to be calculated in time domain, which
enable practical implementation of efficient classifier. The cost paid is lower, but still satisfactory,

55
classification accuracy than other classification applications reported in literature for this
problem. Sections of paper are as follow: After introduction first,we determine important
parameters and features.Then we study quadratic discriminant analysis and high energy element
separation criterion in section III. Simulation results show improvement in classification accuracy
for separation criterion. Finally, the paper ends by conclusion.
2. FEATURE EXTRACTION
Changing the characteristics of an audio signal over time seems normal. For example, Changes in
the signal’s peak value or rate of sign changes of amplitude are such a time varying
features.Although these changes exist in signals, short time analysis is a method to illustrate them
asobvious way. Basic assumption of this method is based on these changes occur slowly; then
wedivide signal into smaller parts and features are extracted.
2.1. Short Time Energy
This feature indicates how the signal amplitude changes over the time which is defined as
follows:
‫ܧ‬௡ = ∑ [‫ݔ‬ሺ݉ሻ‫ݓ‬ሺ݊ − ݉ሻ]ଶ∞
௠ୀି∞ (1)
Where ‫ݔ‬ሺ݊ሻis the audio signal and ‫ݓ‬ሺ݊ሻis the window that slides along the audio sequence
selecting the interval to be involved in the computation.
2.2. Short Time Average Zero Cross Rate
This feature indicates how the signal sign changes over the time which is defined as follows:
ܼ௡ = ∑ |‫ݔ[݊݃ݏ‬ሺ݉ሻ] − ‫ݔ[݊݃ݏ‬ሺ݉ − 1ሻ]|∞
௠ୀି∞ ‫ݓ‬ሺ݊ − ݉ሻ (2)
Where
‫ݓ‬ሺ݊ሻ = 1 2⁄ 0 ≤ ݊ ≤ ܰ − 1
= 0 ‫ݐ݋‬ℎ݁‫݁ݏ݅ݓݎ‬ (3)
The rate at which signal sign changes occur is a simple measure of the frequency content of a
signal. This is particularly true for narrowband signals. For example, a sinusoidal signal of
frequency ‫ܨ‬଴, sampled at a rate‫ܨ‬௦, has 2 ‫ܨ‬଴ ‫ܨ‬௦⁄ average rate of zero-crossings.Thus, the average
ZCR gives a resonable way to estimate the frequency of a sine wave. Audio signals are
broadband signals and the interpretation of average ZCR is therefore much less precise. However,
rough estimates of spectral properties can be obtained using a represantation based onthe short
time energy and zero cross rate together.This representation was proposed by Reddy, and studied
by Vicens [18] as the basis for a large-scale speech recognition system.

56
2.3. Pitch Frequency
Pitch period estimation (or fundamental frequency) is one of the most important problems in
speech processing. Pitch detectors are used in vocoders [19], speaker identification and
verification systems [20, 21]. Because of its importance, many solutions to this problem have
been proposed [22]. In this paper we use the fact thatthe autocorrelation function for periodic
segments attains a maximum at samples 0, ±ܶ,±2ܶ,… where T is pitch period. Since, there may
be exist other autocorrelation peaks except those due to the periodicity, the simple procedure of
picking the largest peak at sampleܶin the autocorrelation will failto estimate Pitch period. To
avoid this problem it is useful to process the signal so as to make periodicity more prominent
while suppressing other distracting features of the signal. Spectrum flattener with the objective to
bring each harmonic to the same amplitude level as in the case of a periodic impulse trainhas
been used. Specifically, center clipped signalhas been used in computing autocorrelation function.
In the scheme proposed,similar to [23], the center clipped signal is obtained by following
nonlinear transformation
For samples above ‫ܥ‬௟, the output is equal to the input minus the clipping level. For samples below
the clipping level the output is zero.We set clipping level, ‫ܥ‬௟, equal to 68% of the minimum of
two maximum amplitudes found in both the first third and last third of the audiosegment;denoted
respectively by݉ܽ‫ݔ‬ଵ and ݉ܽ‫ݔ‬ଶ. However, for the un-periodic segment, there are no strong
autocorrelation periodicity peaks and it seems to be a high frequency noise-like waveform [22].
We assume segments with weak peaks below 30 % clipped signal’s energy, ‫ܧ‬௖௟௜௣௡
=
∑ [‫ݔ‬௖௟௜௣ሺ݉ሻ‫ݓ‬ሺ݊ − ݉ሻ]ଶ∞
௠ୀି∞ ,are un-periodic. Finally pitch frequencies of frames smoothed
through median operation. Following figures present features.
−‫ܥ‬௟‫ܥ‬௟
Fig.1: center clipping function

57
Fig.2: Energy for Bus in ﬁrst row for three differentsounds, for Car in the second row, for Motor in the
third row, and for Truck in the fourth row.
Fig.3:ZCR for Bus in ﬁrst row for three different sounds, for Car in the second row, for Motor in the third
row, and for Truck in the fourth row.

58
Fig.4: Pitch Frequency for Bus in ﬁrst row for three different sounds, for Car in the second row, for Motor
in the third row, and for Truck in the fourth row.
Features are extracted from rectangular windowed segment of length 15 Milliseconds and
overlapping of 5 Milliseconds between consecutive windows. Feature vectors of periodic
segments are used for training and classification. After determining the important features and
parameters for audio signal modeling, we will review quadratic discriminant analysis and high
energy separation criterion will be described.
3. CLASSIFIER AND SEPARATION CRITERION
Now the discriminant boundaries between classes should be determined. Quadratic
discriminantanalysis is used to discriminate the classes. The classifier that separate K classes has
K quadratic functions as follow [24]:
‫ݕ‬௞ሺܺሻ = ்ܺ
ܳܺ + ்ܸ
ܺ + ‫ݒ‬଴ (4)
The coefficient‫ݒ‬଴is constant coefficient,ܸis vector of linear coefficients, and ܳ is matrix
containing quadratic coefficients. Feature vectorX will belong to class k, if‫ݕ‬௞ሺܺሻ > ‫ݕ‬௝ሺܺሻ; ∀݆ ≠
݇.In classification step, after feature vectors were extracted, we are able to define class of each
vector separately. Finally, signal will allocate to a classwhichachieved great supply in feature
vector classification phase. In other words, probability of belonging to each class is proportional
to abundance of that class in allocated labels to vectors by quadratic discriminant analysis.

59
3.1. High Energy Vector Separation Criterion
Up to now all periodic vectors are considered in training and classification. However, low energy
regions corresponding with noises of the background consist of high frequency variations may
not be suited for vehicle classification. Hence by ignoring mentioned vectors, an improvement in
classification accuracy can be achieved.On the other hand, it can be observed that when
amplitude is high zero cross rate decreases. So, high energy elements have low zero cross rate,
and low energy vectors have fast sign alternation. A criterion for separation elements with high
energy and low zero cross rate can be defined as follow:
‫ܧ‬௡ > ߙ. ‫ܧ[ܧ‬௡]Andܼ௡ < ߞ. ‫ܼ[ܧ‬௡] (5)
Where ‫ܧ[ܧ‬௡]and ‫ܼ[ܧ‬௡]denote average of all segments’ energy, ZCR. The coefficientα is
more than 1, andߞis less than 1. The more α and the less ߞ than unit, the less data
coincides in condition. In this paper α = ߞ = 1are chosen. Figure 5 presents a block
diagram of our method.
Fig.5: Block diagram of the proposed scheme for vehicle classification

60
4. RESULTS AND DISCUSSION
Simultaneous arrival of different types vehicles, overtaking at the study location, and random
back ground noises were the different challenges encountered in the data collection. Sound
emitted by vehicles was captured for a divided road carrying moderate traffic. Roadside recording
was performed on clear sunny working days. The recordings’ sites were selected to capture the
sound of moving vehicles without much acceleration change.Environmental back ground noise is
at moderate level. The conditions under which data set is recorded are realistic.Our algorithm is
implemented on the Matlab7.10.0 (R2010a) platform.
Preprocessing: Each signal is normalized such that −1 ≤ ‫ݔ‬ሺ݉ሻ ≤ 1. Windowed signal segments
are filtered by 4௧௛
degree Butterworth low pass filter.
The data set includes four classes; bus, car, motor, and truck. All signals sampled at 11025 rate.
Each signal is segmented by rectangular window of length 15 milliseconds and overlapping of 5
milliseconds between consecutive windows. Vectors containing short time energy, average zero
cross rate, and pitch frequency are calculated for each segment and those with 0 pitch frequencies
are removed. We used k-Fold cross validation with k=10 in our applications. In this method data
is randomly divided into a test and training setk different times [25] and the average error across
allk trials is computed. Table 1 provides the values of some important parameters employed in
this work. Classification accuracies for following classifier have been provided in table 2.
• least square classifier,
• k-nearestneighbor classifier,
• quadratic and linear discriminant analysiswhen training and testing have been done just
based on vectors which be able to satisfy the (5) conditions;denoted respectively by
ܳ‫ܣܦ‬∗∗
and ‫ܣܦܮ‬∗∗
,
• SVM classifier with harmonic line based feature vectors [10],
• SVM classifier with Schur filter coefficients based feature vectors [10],
• SVM classifier with Mel filter coefficients based feature vectors [10].
We see improvement in classification accuracy has been achieved forquadratic discriminant
analysis and separation criterion, ܳ‫ܣܦ‬∗∗
. The correct classification rate depends not only on the
feature extraction method, but also on the type of the classifier. This is the reason that why not we
see a considerable improvement for ‫ܣܦܮ‬∗∗
.
Table 3 shows computation complexity in terms of feature space dimensionality for our proposed
algorithm and last 3 methods in table 2. In comparison, we should note that the accuracy of
ܳ‫ܣܦ‬∗∗
with 3 short time parameters based feature vectors is comparable with SVM with 16 Schur
coefficients. SVM with 12 Mel filter coefficients accuracy is 80% and accuracy of QDA∗∗
is
68.75%. Although the computation complexity of QDA∗∗
is low, but the classification accuracy is
lower thanSVM with Mel filter coefficients.

61
Signal sample rate 11025 sample/sec
Length of window for classification 165 sample
Overlap between windows for classification 55 sample
Type of window Rectangular
Preprocessor 4௧௛
degree ButterworthLPF
LPF’s cut off frequency 4 KHz
Clipping level 0.68 ∗ min {݉ܽ‫ݔ‬ଵ, ݉ܽ‫ݔ‬ଶ}
Maximum pitch frequency f<= 1000 Hz
Periodic/Un-periodic threshold 0.3 ∗ ‫ܧ‬௖௟௜௣௡
Number of frames over which median operates 3
α and ζ 1
Number of cross fold validation 10
examples per class: [bus, car, motor, truck] [50, 70, 80, 40]
Table 1: Values of some important parameters
Classifier Accuracy (%)
Least Square 30
kNN, k=25, Cosine 52.09 ± 2.2
kNN, k=25, Euclidian 53.33 ± 3.83
LDA 46.66
‫ܣܦܮ‬∗∗ 49.58
QDA 56.25
ܳ‫ܣܦ‬∗∗ 68.75
SVM with harmonic line 83
SVM with Schur coefficients 68
SVM with Mel ﬁlter coefficients 80
Table 2: Classification accuracy

62
Feature extraction method Number of features
Short time analysis 3
harmonic lines 5
Schur coefficients 16
Mel filter coefficients 12
Table 3: Number of features
Additionally confusion matrix, contain information about actual and predicted classifications
done by ܳ‫ܣܦ‬∗∗
classification system, shown in table 4.
Actual Predicted
Bus vehicles Car vehicles Motor vehicles Truck vehicles
Bus vehicles 27 20 3 0
Car vehicles 0 70 0 0
Motor vehicles 16 12 48 4
Truck vehicles 9 11 0 20
Table 4: Representation of confusion matrix
Although recording conditions were realistic, but we should examine proposed algorithm when
environmental noises increase like rainy days.Since we use high energy feature vectors, ܳ‫ܣܦ‬∗∗
can save its effectiveness.
CONCLUSION
In the research reported in this paper, QDA with a criterion to separate high energy feature
vectors was applied on the task of classification of vehicles based on audio signals and the
simplest method with satisfactory accuracy was evaluated.
Short time energy, average zero cross rate, and pitch frequency made feature vectors; then,signals
were classified using quadratic discriminant analysis. We found that classification accuracy
improved from 56.25% to 68.75%just by considering components which have (5) conditionsdue
to the correspondence of low energy regions and noises of the background.At last, the obtained
accuracy was 68.75% by using three features while the 80% correct classification rate was
achievedby SVM with 12Mel coefficients [10].The results strongly suggest that proposed method
can aid the practical implementation.

63
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