Abstract: The digital TV test signals, instruments and methods are introduced. Focusing on the analysis and research of the field test technology, the coverage test is the most important, emphasizing the importance of selecting test points, and then detailing the various points of the test, the test plan and the process that must be followed.
Keywords: digital TV; field test; coverage test
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Some conventional field measurement schemes and field measurement purposes of digital television systems are introduced. Its scope of work includes receiving, demodulating, and restoring transmitted data. Digital TV testing tends to focus on a few specific aspects, and the testing program focuses on achieving the following goals.
(1) Determine the environmental variables and determine their minimum variable range.
(2) Measure the coverage of the actual service.
(3) Collect data useful for improving the performance of digital television systems.
1 Classification of field measurements The Ad Hok Broadcasting Team defined four receiving modes: fixed reception, portable, walking, and mobile reception.
(1) Fixed reception refers to reception by a fixed antenna and receiver.
(2) Portable reception is defined as the fact that the receiver can be moved from one location to another. It uses a set of fully equipped antennas for reception, but is already fixed during reception operations.
(3) Walk reception is defined as receiving at low speeds with a speed of ≤ 5 kph (3.1 mph).
(4) Mobile reception refers to reception under high-speed motion, and the speed is generally >5 kph (3.1 mph).
According to the four different receiving modes, the field measurement also has four aspects, as shown in Table 1.
The article mainly describes the outdoor reception, and the external field test is mainly divided into the following aspects:
(1) Coverage test.
The meaning of coverage is to determine that the actual electromagnetic strength reaches a specified indicator. The coverage measurement usually has two purposes: the first is to detect the function of the transmitting antenna; the second is to provide auxiliary data for the transmission algorithm of different terrains for the planning of the frequency band and to estimate the potential interference.
These test schemes firstly mount the antenna about 10 m off the ground, then correct the antenna to a standard dipole to verify its coverage, transmit mode of the transmit antenna, and provide measured data for distribution. Transmission algorithms based on these factors have been developed on the frequency band.
(2) Service testing.
Service testing, also known as admissibility testing, is defined as the minimum condition under which test digital television signals can be correctly received and decoded in a variety of practical environments.
Service testing typically involves connecting the receiver to a data logging device and using the latter to obtain signal levels, edge thresholds, bit error rates, adaptive equalization parameters, and other information. These results are obtained by testing various times using various antennas and, in general, are not repeatable in formal testing.
(3) Obtain channel characteristics.
Capturing channel characteristics has a special meaning. It refers to obtaining channel parameters by measuring with known signals at a specific time and place using a specific fixed or mobile antenna. The effects of the channel on the known signal include changes in signal levels, impulse noise, boundary interference, and multipath effects.
2 field measurement signals and instruments
2.1 Field measurement signal (1) In-service measurement. In-service measurement is the use of an unmodified digital television signal itself, by repeating a video sequence with audio and evaluating the picture quality.
(2) Measurement outside the service. The so-called out-of-service means that the TV program cannot be received normally. For the measurement outside the service, the special cropped signal will be used for transmission and reception. These signals have an average power equal to the normal number and occupy the same frequency band, but some clipping is performed because of the specific channel characteristics outside the service area. A common test signal is a series of repeated video sequences with accompanying sounds. For evaluating the error of the stream. These test streams are constructed of undisturbed, uninterrupted loop sequences. The bit rate of the signal is close to the channel capacity to maximize the accuracy of detecting video error probabilities. Optimize fast error detection.
2.2 Field measuring instrument The field measuring instrument is placed on the vehicle. A power pole is placed on the vehicle. A receiving antenna is placed 10 m away from the ground, and some receivers, monitors, measuring instruments and other equipment are attached. The signal received by the antenna is sent into the car, as shown in Figure 1.
The design of the field test tool is divided into two types, one is mandatory, the device must be used in the field test process, and the other is optional to help the tester to judge.
3 Field measurement method Coverage test is performed on the circumference according to a specific radius and angle. The selection of these test points includes a total of 18 different angles (12, 27, 42, 58, 72, 87, 102, 117, 132, 147, 162, 177, 182, 207, 222, 287, 300, 357), corresponding to 3, 6, 9, 12, 15, 20, 25, 30 and 40 km centered on the transmitter Several radii. The selection point is based on the consideration of the angle, exhibiting a field strength estimate of at least 45 dBμV/m, and obtaining useful information through the algorithm based on the terrain information. Figure 2 shows the terrain allocation of the field selection points. The specific parameters of the test points are shown in Table 2.
In practice, the coverage test must take into account the time factor, and the measurement of the channel parameters is associated with different seasons and climate changes.
There are two methods for recording the field test results. One is to record the percentage of the field strength of each test point relative to the field strength threshold of the area. For different points, there are two cases. If the edge region is received, the field strength threshold (Elim) is defined as: Elim=Erec-M, se M>0 dB, and if not in the edge region, Elim>Erec, se M=0 dB. Another method of recording is to record the percentage of the field strength of each test point relative to the receive threshold SNR, as shown in Figure 3.
4 Conclusion This paper introduces the various classifications of field test and the specific scheme of coverage test, which can give readers a clear conceptual understanding of digital TV field test technology.
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