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The effect of nonlinear amplification on the analog TV signals caused by the terrestrial digital TV broadcast signals
Keisuke MUTO*, Akira OGAWA**
Department of Information Sciences, Graduate school of Science and Technology, Meijo University
1-501 Shiogamaguchi Tenpaku-ku Nagoya, JAPAN Phone: +81-52-838-2388, FAX: +81-52-832-1298
E-mail: *[email protected], **[email protected]
Abstract
In this paper, the degree of the influence of the terrestrial digital TV signals to the terrestrial analog TV signals caused by the nonlinear amplification is evaluated through the computer simulation. The effectiveness due to a simple filtering to suppress the digital TV signals is also verified.
1. Introduction
The terrestrial digital TV broadcastings have been in service for three metropolitan areas in Japan since December 2003. The simulcasts of digital and analog versions will continue to be on air by about 2011 when the analog TV broadcastings are scheduled to close their services. The number of carriers within UHF-band increases greatly for this period, and it is likely for amplifiers used for the analog TV reception in many homes to be driven to the nonlinear regions, resulting in some amount of degradation in the received quality.
This paper discusses modeling of the simulcast situation with a nonlinear amplification and the way of computer simulation to estimate the performance degradation of analog TV signals received in the presence of the nonlinearity and additive white Gaussian noise (AWGN). This paper also describes the simulated results in terms of the degradations in signal-to-noise ratio. Moreover, to reduce the influence of the digital TV signals on the analog TV signals, the suppression of the digital TV signals by means of a high-pass filter is considered. This paper shows the effectiveness due to this filtering
Digital TV Carriers Analog TV Carriers
Filter
IFFT
Parallel to Serial Conversion
22QI xx +
Nonlinear Amplification
FFT
Serial to Parallel Conversion
Spectral Analysis
AWGN
Fig.1 Simulation model
Fig.2 Nonlinear characteristic of the amplifier
r
r
Input
Output
x-ax3
2. Simulation condition
2.1. Simulation model
IFFT of 1024 points is performed to all spectra of digital and analog TV signals as shown in Fig.1. The carriers for analog TV are assumed to be of no modulation with the same level. Because the digital TV signals are OFDM with about 5600 sub-carriers, in-phase (xi) and quadratic (xQ) components can be assumed to be Gaussian-distributed with zero mean and variance σ2. The output data from IFFT are converted from parallel streams to a serial stream, and its amplitude is obtained. AWGN is applied to the signal and the data are input to a nonlinear amplifier shown in Fig 2. The saturated level r is defined by the function of x-ax3. Referring to the characteristics of home-used amplifiers, the value of r is set at 76dBµV. To obtain the output spectrum, the amplified data are passed through a serial-to-parallel converter, and applied to an FFT circuit. In this paper, the allocation of TV carriers is assumed so that there are seven digital TV carriers and two analog TV carriers as shown in Fig.3, which simulates the situation in Nagoya area in Japan. 2.2. Filter
A high-pass filter with a simple structure is desirable to suppress some amount of the digital TV signals. Then, the Butterworth filters with a simple structure are assumed to be used. The filters are characterized by amplitude response with maximally flat in the pass-band and monotonic through the frequency range, at the cost of roll-off steepness.
2.3. Thermal noise
To treat the thermal noise as AWGN on the simulation, in-phase (ni) and quadratic (no) components are assumed to be Gaussian-distributed with zero mean and variance σn
2. Available power of the thermal noise is irrelevant to the
resistance that exists in an arbitrary circuit and proportional to the absolute temperature. If the frequency bandwidth of an arbitrary circuit is B Hz, the available noise power at the output is:
kTBN = [W] (1)
where k is the Boltzman constant (1.3805×10-23[J/K]), T is the absolute temperature [K].
3. Simulation
3.1. Simulation parameters
The simulation parameters are shown in Table 1 and the amplitude characteristic of the filter is shown in Figure 4. The simulation is repeated 5000 times. Because the bandwidth of one channel is 6MHz in the terrestrial TV
Table 1 Simulation parameter Parameters Value
FFT-IFFT points Simulation iteration Thermal noise
Bandwidth of the amplifier Room temperature
Type of high-pass filter Order Cutoff frequency
1024 5000
AWGN 6MHz 300K
Butterworth 2
29ch
0 10 20 30 40 50-20
-18
-16
-14
-12
-10
-8
-6
-4
-2
0
TV channe l (ch )
Mag
nitude
(dB
)
Fig.4 The amplitude characteristic of filter
ch.13 18 19 20 21 22 23 25 35 473 MHz 503 509 515 521 527 533 545 605
Fig.3 Example for frequency allocation in the simulation
Digital TV Analog TV
Fig.5 Power reduction (Ch.25)
Fig.6 Power reduction (Ch.35)
Fig.7 CNIR(Ch.25)
Fig.8 CNIR(Ch.35)
Fig.9 Power reduction when the digital TV carriers are filtered (Ch.25)
Fig.10 Power reduction when the digital TV carriers are filtered (Ch.35)
Fig.11 CNIR when the digital TV carriers are filtered (Ch.25)
Fig.12 CNIR when the digital TV carriers are filtered (Ch.35)
-6
-4
-2
0
2
4
6
8
10
35 40 45 50 55 60 65 70level of one digital TV carrier[dBμV]
pow
er
redu
ction [
dB]
ATV:40[dBμV]
ATV:45[dBμV]
ATV:50[dBμV]
ATV:55[dBμV]
ATV:60[dBμV]
ATV:65[dBμV]
ATV:70[dBμV]
-6
-5
-4
-3
-2
-1
0
35 40 45 50 55 60 65 70level of one digital TV carrier[dBμV]
pow
er
redu
ction [
dB]
ATV:40[dBμV]
ATV:45[dBμV]
ATV:50[dBμV]
ATV:55[dBμV]
ATV:60[dBμV]
ATV:65[dBμV]
ATV:70[dBμV]
0
10
20
30
40
50
60
70
80
35 40 45 50 55 60 65 70level of one digital TV carrier [dBμV]
CN
IR[d
B]
ATV:40[dBμV]
ATV:45[dBμV]
ATV:50[dBμV]
ATV:55[dBμV]
ATV:60[dBμV]
ATV:65[dBμV]
ATV:70[dBμV]
-6
-5
-4
-3
-2
-1
0
35 40 45 50 55 60 65 70level of one digital TV carrier[dBμV]
pow
er
redu
ction [
dB]
ATV:40[dBμV]
ATV:45[dBμV]
ATV:50[dBμV]
ATV:55[dBμV]
ATV:60[dBμV]
ATV:65[dBμV]
ATV:70[dBμV]
-6
-5
-4
-3
-2
-1
0
35 40 45 50 55 60 65 70level of one digital TV carrier[dBμV]
pow
er
redu
ction [
dB]
ATV:40[dBμV]
ATV:45[dBμV]
ATV:50[dBμV]
ATV:55[dBμV]
ATV:60[dBμV]
ATV:65[dBμV]
ATV:70[dBμV]
0
10
20
30
40
50
60
70
80
35 40 45 50 55 60 65 70level of one digital TV carrier[dBμV]
CN
IR[d
B]
ATV:40[dBμV] ATV:45[dBμV]
ATV:50[dBμV] ATV:55[dBμV]
ATV:60[dBμV] ATV:65[dBμV]
ATV:70[dBμV]
0
10
20
30
40
50
60
70
80
35 40 45 50 55 60 65 70level of one digital TV carrier[dBμV]
CN
IR[d
B]
ATV:40[dBμV] ATV:45[dBμV]
ATV:50[dBμV] ATV:55[dBμV]
ATV:60[dBμV] ATV:65[dBμV]
ATV:70[dBμV]0
10
20
30
40
50
60
70
80
35 40 45 50 55 60 65 70level of one digital TV carrier[dBμV]
CN
IR[d
B]
ATV:40[dBμV]
ATV:45[dBμV]
ATV:50[dBμV]
ATV:55[dBμV]
ATV:60[dBμV]
ATV:65[dBμV]
ATV:70[dBμV]
broadcasting, the frequency bandwidth of the amplifier is set to 6MHz. The room temperature at the input to the amplifier is assumed 300 K(27℃). The adopted filter is 2nd-order high-pass Butterworth type.
3.2. Simulated results without filtering
The simulated results are obtained in terms of power reduction and CNIR (Carrier to Noise + Intermodulation Ratio) of the analog TV carriers. As simulated results, the output power reductions for 25ch and 35ch are shown in Fig.5 and Fig.6, respectively, the CNIRs for 25ch and 35ch are shown in Fig.7 and Fig.8, respectively.
In Fig.5, the power reduction begins at about 55dBµV of the level of each digital TV carrier. When the level of each digital TV carrier is 60dBµV or more, on the contrary, some increases in the level of analog TV carrier are observed. This is because the amount of intermodulation products may become significant.
In Fig.7, in the case of analog TV of 25ch, the CNIR starts decreasing at about 45dBµV of the level of digital TV carrier. The lower limit of the quality for analog TV signals is generally 30dB in CNR. When the level of digital TV carrier is about 60dBµV, the CNIR is found to be below the above-mentioned quality.
In case of the analog TV carrier of 35ch, which is relatively far from the digital TV group, the amount of power reduction is not so large. 3.3. Simulated results with filtering
In order to reduce the level of digital TV carriers and improve the performance, a high-pass filter is inserted at the input to the amplifier. The effects of this filtering are shown in Figures 9-12.
Power reductions of analog TV carrier of 25ch are about 5dB, which is almost constant over the range below 65dBµV of the level of the digital TV, while the power reductions for the analog TV of 35ch are about 2dB. This is because the analog TV carrier of 25ch is attenuated by virtue of the high-pass Butterworth filter.
On the other hand, CNIR improved as a whole about 7-8dB compared with the time that had not been filtered. Analog TV signals 25ch can achieve 30dB and over in CNIR at all levels of analog TV where the level of one digital TV carrier is 60dBµV.
4. Conclusion
This paper discussed the influence on the analog TV signals when the amplifiers placed at the user premises for the purpose of amplifying analog TV signals are driven to the nonlinear zone. The received quality of analog TV signal of 25ch became poorer by the influence of nonlinear amplification when the level of each digital TV carrier is 60dBµV or more. Then, the high-pass filter is inserted at the input to the amplifier to suppress the digital TV signals. As a result, for 60dBµV of the level of digital TV carriers, CNIR can be of about 30dB, which is regarded as the lower limit in quality serviceable for analog TV. It was found that the filtering is effective for keeping the quality of analog television signals.
Acknowledgment
This work has been partly supported by the Ministry of Education, Science, Sports and Culture, Government of Japan, under Grant-in Aid for Scientific Research.
Reference
[1] Fumio Ikegami, "Electricity and electronic engineering basic course communication engineering", Rikogakusha
The effect of nonlinear amplification The effect of nonlinear amplification
on the analog TV signals caused by on the analog TV signals caused by
the terrestrial digital TV broadcast signalsthe terrestrial digital TV broadcast signals
Keisuke MUTO and Akira OGAWA
Meijo University
JAPAN
BackgroundBackground
In Japan, the terrestrial digital TV
broadcastings have been in service for three
metropolitan areas since December, 2003.
The provision of simulcastsimulcast has been made
for smooth transition from analog TV to
digital TV.
The period for the simulcastsimulcast will continue by
around 2011.
BackgroundBackground
Analog TV Analog TV carrierscarriers
Digital TV Digital TV carrierscarriers
UHFBooster
TV
The number of carriers
within UHF-band increases
greatly during the simulcast
There is a possibility that the
boosters (home amplifiers)
may be driven to nonlinear region.
Some amount of degradation
in the quality of analog TV signals
is anticipated.
ObjectivesObjectives
To evaluate the influence of the digital TV
signals on the analog TV signals caused by
the nonlinear amplification through
the computer simulation.
To estimate the effectiveness of filtering,
which is placed for reducing the effect of
the digital TV signals.
Object for the simulation Object for the simulation UHFUHF--band allocation in Nagoya areaband allocation in Nagoya area
Analog TV : 25ch,35ch
Digital TV : 13ch,18ch 22ch Same levelSame level
Digital TV : 23ch 1/31/3
Digital TV Digital TV carrierscarriers
Analog TV Analog TV carrierscarriers
13ch 18 19 20 21 22 23 25 35
Evaluated items Evaluated items
Items to be evaluated for the quality are :
Power reductionPower reduction
CNIRCNIR(Carrier-to-Noise plus Intermodulation Ratio)
in analog TV carriers.
Simulation modelSimulation model
Digital TV Analog TV
Filter
IFFT
S/P
FFT
Spectral Analysis
Nonlinear
Amplifier
P/S
22
QI xx +
AWGN AWGN
Ix Qx
Conditions of TV carriersConditions of TV carriers
Analog TV carriers Analog TV carriers
No modulation
Same level between two-carriers
Digital TV carriers Digital TV carriers
OFDM with about 5600 subOFDM with about 5600 sub--carrierscarriers
xI : In-phase component
xQ : Quadratic component
GaussianGaussian--distributiondistribution(Zero mean ,Variance )
Setting of the filterSetting of the filter
To suppress the effect of the digital TV signals,
a filter should be placed at the input to the amplifier.
The filter should be as simple as possibleThe filter should be as simple as possible
2nd2nd--order order
highhigh--passpass
ButterworthButterworth
filterfilter0 50 100 150 200
-20
-15
-10
-5
0
Frequency (Hz)
Magnitude (dB)
The amplitude characteristic
The cut-off frequency
29ch
Setting of AWGNSetting of AWGN
Gaussian-distribution : zero mean
variance n2.
Available power of the AWGN
[W] kBTN =
k : Boltzman constant (1.3805 10-23 [J/K])
B : Bandwidth of one channel [Hz]
T : Absolute temperature [K]
Setting of nonlinear amplifierSetting of nonlinear amplifier
The saturated level r is defined by the function
of x-ax3.
Referring to the characteristics of home-used
amplifiers, the value of r is set at 76dB V.
x
x ax3
Input
Output
r
rx ax3 r
Simulation parametersSimulation parameters
1024
2.48 10-14 W
6MHz
300K
Butterworth
2
29ch
The number of FFT-IFFT points
AWGN
Bandwidth of the amplifier
Room temperature
Type of high-pass filter
Order
Cut-off frequency
ValueValueParametersParameters
Power reduction (35ch)Power reduction (35ch)
The case without filteringThe case without filtering The case with filteringThe case with filtering
-6
-5
-4
-3
-2
-1
0
35 40 45 50 55 60 65 70
level of one digital TV carrier[dBµV]
Received power[dB]
40[dBµV]
50[dBµV]
60[dBµV]
70[dBµV]
-6
-5
-4
-3
-2
-1
0
35 40 45 50 55 60 65 70
level of one digital TV carrier[dBµV]
Received power[dB]
40[dBµV]
50[dBµV]
60[dBµV]
70[dBµV]
-6
-4
-2
0
2
4
6
8
10
35 40 45 50 55 60 65 70
level of one digital TV carrier[dBµV]
Received power[dB]
40[dBµV]
50[dBµV]
60[dBµV]
70[dBµV]
Power reduction (25ch)Power reduction (25ch)
IntermodulationIntermodulation productsproducts
-6
-4
-2
0
2
4
6
8
10
35 40 45 50 55 60 65 70
level of one digital TV carrier[dBµV]
Received power[dB] 40[dBµV]
50[dBµV]
60[dBµV]
70[dBµV]
The case without filteringThe case without filtering The case with filteringThe case with filtering
0
10
20
30
40
50
60
70
80
35 40 45 50 55 60 65 70
level of one digital TV carrier[dBµV]
CNIR[dB]
40[dBµV]
50[dBµV]
60[dBµV]
70[dBµV]
CNIR (35ch)CNIR (35ch)
Improvement by Improvement by
7 to 8dB7 to 8dB0
10
20
30
40
50
60
70
80
35 40 45 50 55 60 65 70
level of one digital TV carrier[dBµV]
CNIR[dB]
40[dBµV]
50[dBµV]
60[dBµV]
70[dBµV]
The case without filteringThe case without filtering The case with filteringThe case with filtering
0
10
20
30
40
50
60
70
80
35 40 45 50 55 60 65 70
level of one digital TV carrier[dBµV]CNIR[dB]
40[dBµV]
50[dBµV]
60[dBµV]
70[dBµV]
0
10
20
30
40
50
60
70
80
35 40 45 50 55 60 65 70
level of one digital TV carrier[dBµV]
CNIR[dB]
40[dBµV]
50[dBµV]
60[dBµV]
70[dBµV]
CNIR (25ch)CNIR (25ch)
The case without filteringThe case without filtering The case with filteringThe case with filtering
Conclusion Conclusion
The influence of digital TV signals on the analog
TV signals is caused by the nonlinear
amplification, and is evaluated through the
simulation.
The simulated results show the amount of the
degradation may not be negligible for 60dB V
or more of the level of a digital TV carrier.
It is found that the filtering is effective for keeping
the quality of the analog TV signals.