Abstract This article describes how to transform analog TV transmission for transmitting digital television transmission equipment , and provide a transformation instance , for reference.
Keywords FM microwave ; television transmitters ; QAM modulation ; -top boxes, digital TV
With the development of digital television technology and the digital transmission of television programs commercialization, and gradually shift to digital TV analog TV . Can retrofit existing equipment used to transmit digital television , in order to improve the quality of television programs transmitted resource utilization and bandwidth , is a subject worthy of our consideration . On Fujian Province , the existing microwave station 362 television , microwave line over seven thousand kilometers ; 473 size television transmitters , TV transmitters nearly eight hundred units. If we can transform the use of these devices can not only speed up the digitization progress , but also save a lot of investment. We recently made some attempts in this regard , to achieve some practical experience , for reference.
An analog device to transmit digital television feasibility analysis :
Digital and analog television signal transmission methods are put into a frequency modulated signal , and then moved to the appropriate working band , receive, do the reverse to get the IF frequency , intermediate frequency signal by the demodulator demodulates the reduction or digital demodulators signal. Visible transmission principle of digital and analog television signals is the same , the digital modulation and the analog television signal is not the same , the transmission equipment is also slightly different. But , we just transmit analog television transmission equipment to do the necessary transformation, analog devices are capable of transmitting digital television signals.
Transmit analog television transmission system shown in Figure 1 , FM microwave mainly for long-distance transmission of television signals point to point , which represents the machine has WSF8-06 microwave transceiver , the aircraft IF is 70MHz, bandwidth of 18MHz. TV transmitter for transmission coverage point across , working in meter band or sub- meter band , frequency of 37MHz, bandwidth of 8MHz. The system is used to transmit the digital signal , a microwave transmitting section must be removed ( Fig. 1 left ) devices within the dashed line , the digital modulator is connected directly to a microwave frequency converter . Equipment within the dotted line in Figure 1 instead of the right half of the inverter with 70 MHz transfer 34.25MHz ( TV transmitter IF center frequency ) .
2 transform instance :
As the national terrestrial broadcast television transmission standard has not yet introduced, only selected transmission standard DVB-S or DVB-C. Taking into account the DVB-C receiver ( STB ) is consistent with the frequency range of TV transmitters , so we chose DVB-C standard 64QAM modulation, transmission symbol rate as 6.125Mbaud. We selected test sites over nine hundred meters above sea level in the Baima TV microwave stations, the station from downtown Ningde 20Km, launch coverage in addition to the city and the mountains , but also cover part of the marine fish culture zones . Therefore , the station has selected some representative.
2.1 Transmission and transmission equipment selection and transformation :
In order to facilitate testing , we take the province 11GHz digital microwave signal to do the program source, the digital microwave single- carrier multiplexing QPSK modulation, transmission and four sets of four sets of TV broadcasts digital transmission symbol rate 17Mbaud. See the transmission system ( Figure 2 )
2.1.1 transmission and emission device selection
Equipment installed in the left side of Figure 2 Ningde TV room , QPSK demodulator model : CHINC1 10KD01; QAM modulator model : CHINC 10K502; microwave transmitter model : WSF8-06. Figure 2 on the right equipment installed in the engine room Baimashan microwave stations , 8GHZ/34.25MHZ inverter and 34.25MHz IF AGC amplifier is custom ; television transmitter model : GSZ-3/5 in 7 channels ; STB model : DBC2100B .
2.1.2 microwave equipment modification :
(1) QAM signal through WSF8-06 microwave transmitter clipping distortion is generated in the discharge , therefore , must be removed in the discharge limiter , QAM modulator is coupled directly to the IF output to put the transmitter power of the microwave .
(2) WSF8-06 machine local oscillator using dielectric microwave oscillation , although the frequency stability and digital machines of the same order of magnitude , but the number of the local oscillator phase noise microwave , there are strict requirements, the noise is superimposed by station , especially for long distance, multi-station relay microwave circuit , this indicator can not be ignored . Therefore , the need to change the phase-locked loop oscillator ordinary media media oscillator.
( 3 ) television transmitter input frequency is 34.25MHz, and the output frequency microwave receiver is 70MHz, reliability and quality from the viewpoint of signal transmission equipment, we additionally customized by 8GHz becomes 34.25MHz down converter and AGC amplifier circuit receiving member WSF8-06 instead of the microwave receiver ( see Figure 3 ) .
Figure 3 is the drive gain 15dB, AGC gain circuit (30-50) dB, impedance of 50Ω, the power module BGX885 gain 17dB, the maximum output level of 60 dBmV ㏒ output level 10dB adjustable. AGC circuit to ensure the transmission of microwave output level without debilitating impact .
2.1.3 TV Transmitter transformation :
Taking into account the distribution of digital television signals across the channel average power , average power final amplifier is much lower than the peak transmitter power , in order to increase the transmission power of 50W North Canton Channel 7 TV transmitter tube solid state transformation , that is the solid gain of 20dB 300W amplifier module replacement transmitter tube amp . The board uses the Philips BLF278 power amplifier FET Class A amplifier circuit consisting of tubes , each tube is quiescent current 2A, maximum output power of 300 watts. Switching power supply output DC50V (10A) with over-current protection . Specifically, the following :
( 1 ) Open the back door tube television transmitter , the 300W power amplifier and switching power supply cooling fan module is fixed in place .
(2 ) transmitter modification, the original electrical control system can not be used , the signal lines are required grafting , therefore, must re-wiring . The transmitter signal line connection is relatively easy , as long as the transmission unit connected to 300W output power amplifier module input , output amplifier module connected to the TV transmitter directional coupler . TV transmitter only after conversion of single-phase power supply, three-phase power supply original machine can not. TV transmitter power transmission unit for DC 24V, 24V wiring just find the power supply unit CZ outlet 8,9 feet and cut them with the original machine wiring , 220V power line connected directly with 8,9 feet ( 9 feet zero line , 8-pin FireWire connection ) ; switching power supply line was installed in the amplifier heat sink 65 ℃ thermal relay ( amplifier overheating protection ) normally closed contact connected to 220V power supply ; cooling fan power line was installed in the amplifier heat 45 ℃ thermal relay on the ( power amplifier temperature higher than 45 ℃ fan work ) normally open contacts connected to 220V power , total power line should be installed and air leakage protection switch ( see Figure 4 ) .
( 3 ) the amplitude-frequency characteristics of the transmission system will affect the transmission quality of the QAM signal , and therefore , the amplitude-frequency characteristics of the system must be adjusted after the modification to ensure that the transmitter band flatness is less than 0.5dB.
3 Test Results:
In order for microwave transmission and TV transmission systems to do quantitative analysis , we have to actually do the test signal source , with Italy Leroy DL4 analog and digital spectrum analyzer testing machine for QPSK and QAM modulator demodulator as well as the transmission unit of output indicators do a number of tests , after analysis of data from a representative selection of the following :
3.1 The data source :
Table 1 QPSK and QAM modulator demodulator
|
机 器
|
符号率（Mbaud）
|
电平（dBμV）
|
载噪比（dB）
|
MER(dB)
|
BER (dB)
|
|
QPSK解调器
|
17.000
|
79.0
|
18.0
|
-----
|
<1×10-6
|
|
QAM调制器
|
6.125
|
95.3
|
37.5
|
34.1
|
<1×10-7
|
Table: Average level representatives digital multiplex power; MER for the modulation error ratio; BER bit error rate
(1) can be seen from the data in the table QPSK modulated carrier signal to noise ratio is 18dB, QPSK demodulation receiver than the threshold level 5.5dB high 12.5dB; BER <1 × 10-6 bit error rate than the critical 2 × 10-4 nearly two orders of magnitude higher, showing that the signal quality is also good sources.
(2) 64QAM receiver threshold is 28dB, critical BER of 1 × 10-4, QAM modulator carrier to noise ratio of 37.5dB, 9.5dB margin left to the transmission system.
3.2 To increase the transmission C / N, is close to the microwave power amplifiers operate in saturation, QAM are used to transmit digital signals, the output power must be used to reduce power backoff method to ensure the normal transmission of the digital signal. Table 2 is obtained through several tests in our results.
Table 2 corresponds to 34.25MHz transmit power microwave downconverter output
|
发信功率（mW）
|
接收电平（dBμV）
|
载噪比（dB）
|
MER(dB)
|
BER(dB)
|
|
96
|
94.2
|
30.8
|
27
|
<1×10-3
|
|
60
|
93.7
|
32
|
30
|
<3×10-4
|
|
40
|
93.5
|
37
|
32
|
<1×10-5
|
|
20
|
93.1
|
37.6
|
34
|
<3×10-6
|
As can be seen from Table 2: the microwave transceiver system WSF8-06, power backoff factor may be between (14-20) dB is selected. The test selected microwave transmit power is 17dB that 20 mW.
3.3 For the transmitter power amplifier, there is also the power back-off factor selected issues. Table 3 is different from the corresponding input power television transmitter output data, it can be seen from the table when the TV transmitter output power is less than 50 W, indicators better. Power back-off coefficient between (3-10) dB selected. The test selected 10dB namely television transmitter power 30W.
Table 3 TV transmitter output data: (detection level of directional coupling the output level)
|
输入功率（W）
|
输出功率（W）
|
检测电平（dBμV）
|
载噪比（dB）
|
MER(dB)
|
BER(dB)
|
|
1.1
|
100
|
92
|
26.4
|
26.7
|
<2×10-3
|
|
0.81
|
75
|
90.8
|
28.4
|
28.8
|
<6×10-4
|
|
0.56
|
50
|
89
|
32.6
|
32.6
|
<7×10-5
|
|
0.34
|
30
|
86.8
|
34.6
|
34
|
<1×10-5
|
|
0.22
|
20
|
85
|
36.7
|
34
|
<8×10-6
|
|
0.12
|
10
|
82
|
36.5
|
34
|
<2×10-6
|
3.4 Baimashan room, we set a good receiver (STB) parameters as required, with a single wire antenna will be able to do normal open receiver. Ningde urban areas must be equivalent to a half-wave dipole antenna in order to stabilize the receiver.
Open to receive data in Table 4: (three units receiving antenna Yagi antenna)
|
接收地点
|
接收电平（dBμV）
|
载噪比（dB）
|
MER(dB)
|
BER(dB)
|
|
白马山机房
|
87
|
35.4
|
30
|
<8×10-5
|
|
宁德电视台机房
|
57.4
|
34.5
|
23.7
|
<1×10-4
|
The above test data can be seen, when the analog microwave transmission 64QAM signal , as long as the transmit power of less than 60mW indicators that can meet the requirements , which we can also be verified by the system indicators , WSF8-06 microwave transmitter chain -weighted S / N ≥ 75dB, 18dB FM improvement factor considered , the microwave transmission signal C / N ≥ 57dB. The same link , if 64QAM transmission quadrature amplitude modulated digital signal , as long as the C / N ≥ 28dB to obtain good image quality can be seen , compared with the analog FM 64QAM digital microwave microwave threshold is away from 29dB, to transmit power of 1W (30dBm) of WSF8-06 microwave machine, even if the transmitter power is reduced 50mW (17dBm), as well as receiving threshold 12dB margin. Most of the work on the set-top box and 64QAM mode, the actual threshold is only 24dB, therefore , to change the analog microwave digital signal transmission , the actual margin will be greater than the threshold 16dB. When we actually use , transmit power is only 20mW (13dBm). TV transmitter maximum power 300W, when the digital signal transmission , the transmission power can not be greater than 30W.
Except as the fixing test , we also have to be placed in the receiving apparatus on board the ship and , at sea and road tests made mobile reception , the receiver can see the location Baimashan long , the reception level of about 50 dBμV, the normal reception. However, from the open receiving data, due to the presence of multipath interference received and various high level and the received signal STB C / N value required . The system began from February 2003 put into operation , has been stable.
Conclusions :
Analog TV transmission through the transformation of the transmitting device can transmit compressed digital television programs, with digital compression and reduce the cost of set-top boxes can be used in the following areas:
( 1 ) The transformation of analog microwave backbone transmission system or backup digital television transmission ;
( 2 ) the transformation of analog TV transmitters for transmitting digital signals, digital cable TV as a complement and extension ;
( 3 ) Open TV reception program to solve the problem of low copy number ;
( 4 ) to solve sea fish culture zones do not see the problem of television programs .
Our other product:
