2011 2nd International Conference on Networking and Information Technology
IPCSIT vol.17 (2011) © (2011) IACSIT Press, Singapore
Additional Data Transmission in AM Broadcasting System Based on
Very Minimum Chirp Keying
Yajun Liu, Guoxin Zheng and Zhiyi Qiu
School of Communication and Information Engineering, Shanghai University, Shanghai,200072,China
e-mail:junerliu2003@163.com
Abstract—This paper presents an AM broadcasting system based on in-band multiplexing, of which
modulating carrier signal is VMCK(Very Minimum Chirp Keying) instead of sine wave in conventional AM
systems. Multimedia data can be transmitted during broadcasting analog audio signal without extra band. The
simulation shows that good performance of both modulations could be obtained under certain conditions.
Keywords-UNB; VMCK modulation; AM
1. Introduction
DSB-AM modulation is one of the oldest audio broadcasting modulations, which uses envelope detection
to lead to a low cost of the receiver, also plague the low spectrum efficiency and vulnerable anti-interference
performance. This kind of communication systems modulate the analog audio information with sine wave of
which the frequencies range from Mega-Hz to hundreds Mega-Hz. From the perspective of frequency
domain, it transfers the audio information in the range of 300Hz to 3400Hz to a higher frequency area. For
one thing, it avoids the low frequency information to be interfered by the low frequency surrounding noise; in
the other hand, it facilitates the signals to be sent in a small size. In the transmitting process, the sine wave
doesn’t carry any information. It only expresses the audio information in the way of varying the amplitude of
the wave.
According to the relevant national standard, the audio bandwidth of analog AM broadcasting is excepted
to be 50Hz to 5000Hz, and the voice bandwidth is from 300Hz to 3400Hz.Hence, form the point of receiving
signal view, there is a total vacuum spectrum near the carrier ±50Hz or ±300Hz. In addition, if considering the
dynamic performance of the different audio signal’s spectrum component and the breaks in the broadcasting
process, the vacuum spectrum is to be wider in statistical significance [1]. How to use this vacuum spectrum
of the DSB-AM modulation to transmit more information effectively is to be of great importance.
VMCK modulation is one kind of the UNB (Ultra Narrow Band) modulation. Compared with the
conventional modulation methods(FSK,ASK,PSK etc.), UNB modulation can transmit data in the same rate
only use a fraction of or even one of a dozen bandwidth of the former, so it can raise the band efficiency
dramatically. As one of the UNB modulation methods, VMCK has narrower spectrum, and the attenuation
degree of the side lobe is better. As a result, in order to maximally its narrow band specialty, we propose to
replace the sine wave by VMCK signal as the carrier. The additional data is transmitted on the basis of
approximately the same spectrum of the original system without changing the DSB-AM system as well as not
bring a significant impact on the analog audio transceiver performance. The mathematic analysis and
simulation are carried out. The way of multiplex modulation makes use of valuable spectrum resource
effectively and retains the broadcasting transmitted power the same.
2. AM-VMCK Modulation
173
2.1 VMCK modulation
VMCK modulation represents logic “1” and “0” with the chirp frequency increase (up-chirp) and the
chirp frequency decrease (down-chirp).The phase is continuous between any two adjacent signal waves, and it
is characterized by its extremely narrow spectrum.
Single circle VMCK signal V (t ) in a circle t ∈ [0, T ] express as:
V1 (t ) = sin[2π f s (1 − α + α f s t )t ]
(1)
(2)
V2 (t ) = sin[2π f s (1 + α − α f s t )t ]
Here, s 1(t ) and s 2(t ) represent data 1 and 0 respectively, f s is the bit transmission rate,
α( 0 < α < 1 ) is the chirp modulation factor which represents the change degree of the linear frequency. When
α = 0 , they are both sine waves and, as the carrier, don’t carry the additional information. It is the same as
the regular DSB-AM system. The more α increased, the more different between the two signals, and the
wider the spectrum is. Particularly, V1 and V2 are orthogonal when α = 0.731 , and that is the best
demodulation spot [5]. Figure1 are VMCK waveforms with different α.
Waves of VMCK
1
V1,a=0.731
V2,a=0.731
V1,a=0.2
V2,a=0.2
0.8
0.6
0.4
Amplitude
0.2
0
-0.2
-0.4
-0.6
-0.8
-1
0
0.1
0.2
0.3
0.4
0.5
0.6
Normalized Time
0.7
0.8
0.9
1
Figure 1 . VMCK waveforms with different α
2.2 AM-VMCK system
Figure2 shows the AM-VMCK compound modulation system. The analog audio, AM-DSB, modulation
adopts VMCK signal as the carrier. Firstly, DC component is added to the audio signal to ensure the
amplitudes of all signals are above zero. Here, the relative size of the DC component and the amplitude of the
signals determine the signal’s modulation depth. Meanwhile, the added digital information is converted into
binary data and then become a series of carriers after being modulated by the VMCK modulation. Secondly,
these two modulated signals are multiplied and become multi-modulation signals, that is, the audio signal with
low frequency is transferred to the signal’s center frequency f s . Lastly, after being filtered by BPF, the signal
is sent out by the antenna. The receiving signal is demodulated in two ways. In one way, the signal is passed
the BPF with the center frequency f s and then carried on envelope detection. After that the DC component
is removed, the demodulated audio information is obtained. This is the original analog audio demodulation. In
the other way, the demodulation takes the correlation operation using the up-chirp and down-chirp which have
the same frequency with the transmitting signal, and then the result is compared and decided as binary data. In
this way the additional digital information of the transmitting terminal can be recovered.
174
Figure 2 . System diaagram of the compound
c
moodulation
The trannsmitting siggnal is:
s (t ) = U c [1 + ka (t )]V (t )
(3)
Here, a (t ) is the analog audioo signal, M is the DC level, V (t ) is the digiital signal modulated
m
byy
VMCK moddulation.
Noticing that there is no much difference between VMC
CK signal annd sine wavee sin ωc t , so
o V1 (t ) can bee
separate intto an additioon of two paarts which are
a sine wav
ve and anothher wave c1 (t ) which is derived ass
follows
c1 (t ) = V1 (t ) − sin 2π f s t
= sin[2π f s (1 − α + α f s t )t ] − sin 2π f s t
(4)
= −2 cos[2π f s t − πα f s (1 − f s t )t ]sinn[πα f s (1 − f s t )t ]
That is:
V1 (t ) = c1 (t ) + sin 2π f s t
(5)
c2 (t ) V2 (t ) can be deerived in the same way:
c2 (t ) = 2cos[2π f st + πα f s (11 − f s t )t ]
• sin[πα
α f s (1 − f s t )t ]
(6)
V2 (t ) = c2 (t ) + sin 2π f s t
(7)
From (55)and (7) wee can see thaat c1 and c2 (as c (t ) ) are
a the differrence of V1 aand V2 whiich representt
‘0’and ‘1’ separately
s
.T
That means additional
a
diggital informaation is loadeed in c (t ) off which the frequency iss
the same as the carrier’ss. In the VMCK modulattion, α , chirp
p modulationn factor, reprresents the ch
hange degreee
of the lineaar frequencyy. With the increase of α , amplitud
de of c ( t ) , which is thhe diversity of V (t ) andd
T makes the
t data moddulation easy. But the spectrum and transmitting power will bee
sin(t ) , becomes lager. This
needed morre at the sam
me time and the demodullation of anaalog audio siignal will bee more interffered. So α
should be coonsidered intto all factors..
Figure33 and Figuree4 are the waveforms
w
o V1 (t ) , c1 (t ) and sin(t ) , when α =
of
=0.1,0.4 and 0.731. Thee
amplitude of
o c1 (t ) in Figure3
F
channges slowly, while
w
in Figu
ure4 it changes fast.
175
a=0.1
1
c1(t)
sin(t)
V1(t)
0.8
0.6
0.4
0.2
0
-0.2
-0.4
-0.6
-0.8
-1
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1
Figure 3 . Comparison of three waves when α = 0.1
a=0.731
1.5
c1(t)
sin(t)
V1(t)
1
0.5
0
-0.5
-1
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1
Figure 4 . Comparison of three waves when α = 0.731
After passing through a AWGN (additive white Gaussian noise) channel the receiving signal is:
r (t ) = U c [1 + ka (t )]V (t ) + n(t )
(8)
n (t ) is additive noise , U c is the amplitude of carrier , k is the AM coefficient.
As is shown in Figure2, the receiving signal is demodulated with two methods. One is to demodulate
audio signal and the other is to demodulate the added data.
The method of audio demodulation is DSB-AM demodulation by the BPF and envelope detection. Put
(4),(5) or(6),(7) into (8), we can get:
r (t ) = U c [1 + ka (t )](c(t ) + sin 2π f s t ) + n(t )
= U c c (t ) + U c ka (t )c (t ) + U c ka (t ) sin 2π f s t
(9)
+ U c sin 2π f s t + n(t )
The Figure3 shows that, V (t ) is similar with sinusoidal carrier when α is small, and the frequency of
c(t ) is the same with V (t ) ’s , which is f s . So after passing BPF and envelope detection, the result is:
d (t ) = U c ka (t )c(t ) + U c ka (t ) + n′(t )
(10)
n′(t ) is the baseband noise.
For the digital data demodulation, we design the receiver according to the criterion that is the minimum
error probability detecting signal with the noise’s interfere[6]. Suppose the two might be receiving signals are
(a (t ) + M )V1 (t ) and (a (t ) + M )V2 (t ) , and the last time is T , the noise of input n(t ) is white Gaussian
176
noise of which the mean is zero and the unilateral power spectrum density is n0 , prior probability
P (V1 ) = P (V2 ) . Comparing to the carrier, the envelope changes quite slowly, that is:
U c [1 + ka (t )] = U c [1 + ka ] = a′ (i − 1)T ≤ t < iT
(11)
So, we suppose the amplitude of carrier stays the same in a T time. In the observe time (0, T ) , the
observe wave y (t ) is expressed as:
y (t ) = {( a′V1 (t ) or a′V2 (t )}+n(t)
(12)
For n (t ) is white Gaussian noise of which the mean is zero, and the probability density fV1 ( y) and
fV2 ( y) are Gaussian distributions, so when:
P (V1 ) exp{−
1
n0

T
0
[ y (t ) − V1 (t )]2 dt}
> P (V2 ) exp{−
1
n0

T
0
(13)
[ y (t ) − V2 (t )]2 dt}
The receiving signal is decide to be V1 . For P (V1 ) = P (V2 ) , formula (14) can be simplified as:

T
0
T
y(t )V1 (t )dt >  y(t )V2 (t )dt
0
(14)
And vice versa, the receiving signal is decide to be V2 . This is the decision principle followed in this
system.
On the condition that the priori probabilities are equal, the bit error rate (BER) of any time point is:
E (1 − ρ )
1
Pe = [1 − erf [a′ b
]]
2
2n0
(15)
Here, ρ is the correlation coefficient of V1 (t ) and V2 (t ) , erf ( x ) is error function, Eb is the power of
V1 (t ) and V2 (t ) ( E1 = E2 = Eb ).
To derive the system BER, the BER of every time point needed to be integrated. If a (t ) is sine wave, and
′ = (1 + k )U c , amin
′ = (1 − k )U c .For the probabilities of angle
the modulation depth is k , then amax
appearance are equal, when k is constant, the system BER is:
Pesys = 
2π
0
=
2π
0
f (θ ) p(θ )dθ
1 1
E (1 − ρ )
]]dθ
• [1 − erf [a′(t ) b
2π 2
2n0
(16)
here,
a′(t ) = U c (1 + k cos θ )
(17)
3. Simulation and Analysis
The frequency of the carrier is at intermediate frequency in the simulation,441kHz, in order to meet for
the entire AM band. The audio material is a fraction wav file My Chinese Heart. The random series of
‘0’and’1’ are modulated by VMCK, and then VMCK signal is further modulate the analog audio and VMCK
signals with DSB-AM. The transmitting signal is shown in Figure5.
Figure6 is the input and output SNR curves of the system when α is 0.1, 0.4 and 0.731.The output SNR
means the SNR of the audio signal after the envelope detection. As is shown in the Figure, with the increase
of α , the output SNR decreases, and the situation becomes worse by the increase of the input SNR. The three
output SNR curves are alike when the input SNR is from 0dB to from 0dB to 10dB, after10 10dB the
difference behaved differently .That is because the change of α impacts the demodulation. The smaller α is,
the less VMCK wave and sine wave differ, so the audio transmission is better and the additional data
transmission is worse. When α becomes larger and larger, the result goes into opposite. The reason is that the
interference caused by the additional data accounts for more parts.
177
Figure7 is the BER curves with these different α .The Figure illustrated that BER deceases with the
increase of α .When the input SNR is above 15dB, there is no error bit in α =0.731 and α =0.4 , and the BER
is close to 10 −4 when α =0. 1.
The modulated signal
2
1.5
1
0.5
0
-0.5
-1
-1.5
-2
0
0.5
1
1.5
2
Time
2.5
-3
x 10
Figure 5 . Waveform of the transmit signals
25
20
output SNR/dB
15
10
5
0
-5
-10
0
5
10
15
20
input SNR/dB
25
30
35
Figure 6 . Curve of input and output SNR
0
10
a=0.1
a=0.4
a=0.731
-1
BER
10
-2
10
-3
10
-4
10
-20
-15
-10
-5
0
5
input SNR/dB
10
15
20
25
Figure 7 . Bit error rate(BER)caves of the additional data
4. Conclusions
The article proposes a band multiplexing method in AM broadcasting and simulates it with analysis. The
simulation demonstrate that when the input SNR is small, the change of α plays a slight effort on the output
SNR but has a serious effort on the BER performance, so a large α should be chosen at this time; When the
input SNR is fairly large, the change of BER counts less with different α input, but significantly affect output
SNR, so a small α should be adopted. A 44.1kbps additional data rate is obtained from the simulation, which
could be used to transmit words, pictures or even H.264 video.
5. References
178
[1] Chenhao qi,Lenan wu. In Band Multiplexing of AM Broadcasting system[C]. HHME2006,Hangzhou,
Nov.31-Dec.2, 2007:464-468.
[2] Zheng Guoxin, Feng Jinzhen, Jia Minghua, Very Minimum Chirp Keying as a Novel Ultra Narrow Band
Communication Scheme[C]. ICICS, Singapore, (Dec, 2007).
[3] Zheng GuoXin, Yang WeiYing, He Hui, et al. Non DC Offset Very Minimum Chirp Keying Modulation as a
Novel Ultra Narrow Band Communication Scheme[C]. CCWMSN07 Proceeding, Shanghai, Dec., 12-14,
2007:755-758.
[4] http://www.vmsk.org.
[5] Zheng Guoxin, Feng Jinzhen, Jia Minghua, Very Minimum Chirp Keyingas a Novel Ultra Narrow Band
Communication Scheme, ICICS, Singapore, (Dec., 2007).
[6] Chanxin Fan, Communication Principle, National Defence Industry Press, 2001,72-74,241-242.
[7] Lining Wang, Guangxin LE, MATLAB and Communication Simulation, The People's Posts and
Telecommunications Press, 2000,174-179.
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