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Position:IcFull.com » IC Electronic information » Category: T

IC Electronic information

Troubleshooting case of digital television

In Electronic Infomation Category: T | on April 12,2011

1 digital TV signal source Ningguo About

Ningguo digital TV signal by the provincial center of mass under a unified platform of digital television, the province will DS3 signal SDH backbone network spread all over the city to the front-end digital TV QAM points, around the city and MAX706PCSA datasheet and then digital TV QAM modulated output RF signal mixing to 1 550 nm optical transmitter, by 1 550 nm optical transmission system will transmit digital TV signals to the county the front room (Figure 1), currently used in digital television channel frequency band from 18 until the addition of 42 additional channels, a total download 113 sets of digital television programs.


Figure 1, digital TV Anhui province, city and MAX706PCSA price and county network system diagram

2 Symptom reproduction According to the local technical staff

description of a digital TV users reflect the 6 sets of Anhui Province (center frequency 411MHz) and MAX706PCSA suppliers and the central 4,9 channels (center frequency of 419MHz) stop frame the phenomenon occurs, accompanied by mosaic and crackling, and the fault appears uncertainty, no law could be found, these two respective sets of frequency shows a relatively high user ratings, leading to the views of users is very large, and technical personnel to the scene detected technical data related to digital television (DTV level, MER, etc. ) were normal.

3 troubleshooting process and analysis

3. 1 source investigation and

Ningguo digital TV according to signal routing, the source of the signal from the engine room from the investigation, that is, through digital Strength Meter on the 1 550 nm optical receiver test port, local optical node optical transmitter test port for testing, the test showed that the MER value of the frequency points are stable at around 38 dB, BER values are stable at 1E - 9 below, these two indicators are very good, and then stream analyzer with 411 MH z frequency for testing, shown in Figure 2 .


Figure 2 411MHz Stream analyze the situation

Stream Analyzer displays the section of the secondary stream exist PCR accuracy error, and a small number of three errors. According stream analysis, the second error can cause the phenomenon of mosaic and static frame, three errors had no effect on the image. However, the provinces other cities by the stream analysis, there are also the same situation, but did not reflect a mosaic and static user frame, that means the set-top box decoder error correction mechanism is part of two errors can be corrected , by asking, room monitors are the normal reaction of these two frequencies. So far ruled out front-end signal source under the influence of the problem lies in determining the initial allocation of net users.

3. 2 investigation process

Whole turn based on previous digital TV experience in troubleshooting, most of these problems appear in the user home, decorating the house because the previous user in the process of construction is not a professional cable TV technicians, and inferior materials are used, sometimes even found in the distribution of television signals the user when the signal to be allocated directly hinged together, instead of using the splitter, the user F is the card the first ring, very easy to loose. In the analog age, the user is not too bad as long as the TV screen can, and to the digital TV era is not the same, the digital television signal line shielding requirements of high, pre-digital TV switch forward very fast the whole, some of the network is simply too late rectification, if the connection did not do a good job, or the joints of aging water lines are likely to experience mosaic static frame, the user will not be able to watch.

The investigation is reflected in the most serious from the beginning of a user. We first optical node the user is at the mouth with a digital optical receiver test stream Strength Meter and Analyzer, various indicators of normal, indicating that the optical transmission is no problem. The user of the optical node where the plot is straight with the user stands to reason that this signal should not be too bad. Then we went to the user at home, its home of the F head replaced cold type, and measured with digital Strength Meter digital home of the user level and MER values were 60dB and 35 dB in about stream analyzer and no big mistake. By observing the 411MHz frequency point where the two-hour program, indeed, as reflected in the user like a mosaic and still frames appeared, the phenomenon of unequal and irregular intervals to be found, sometimes once in 5 minutes, sometimes 20 minutes once. Error at the same time we carefully observed the stream analyzer and Strength Meter, found that there is a transition moment values, and in the meantime we have contacted the office monitors the front room, the results of all the normal front room, which is more proved that under the provincial centers with digital TV signal transmission is not associated.

3. 3 fault location

Preliminary investigation by the user at home, this phenomenon is probably due to interference, in order to verify that we connected to the spectrum analyzer and spectrum analyzer to maintain the signal to the maximum observed frequency spectrum as shown in Figure 3 and Figure 4 ( The circle marked interference signal).


Figure 3, digital television signals received 411MH z spectrum


Figure 4 Lou Dong disconnect connector distributor directly test interference signal spectrum

By spectral analysis, the digital television signal was disrupted, and the interference signal level is high, which most likely co-channel interference, than the peak level of the users home, resulting in digital television error. Where is the source of interference from the safe side, we had a long-term users Lou Dong lira directly to the user at home, and home users to turn off all active devices, through observation of the TV screen, or a case of interference, indicating that interference was not from the user home , should come from the air.

Reflected by re-combing users found no users in urban areas reflects the external ratings failure, and some users did not reflect the Area, the user at night than during the day to reflect more, if it is interference, then, this phenomenon is probably directional transmitting antenna due to interference. Technical staff through multiple visits, and finally found a local taxi paging using a center frequency band for the 415MH z, which landed just add 31 digital TV channels (center frequency 411MH z) and the addition of 32 channels (center frequency 419MH z) 8 MB within the bandwidth, wireless paging towers facing the users of digital television is indeed often reflect a failure, and a taxi at night than during the day and be more active, as long as a paging signal has been given a taxi, then cause interference to digital television, finally found that digital television signals caused by Ningguo main source of interference.

3. 4 solution

Divided according to domestic channels, add cable TV channels are not part of a dedicated channel, other communications can also be used. Had to be taken to avoid the situation for Ningguo approach, these two frequency # a copy of the program and then into the other frequency transmission problems on the users way to manually search for the original frequency interference replacement program out. At this point digital TV signal interference Ningguo fault has finally been resolved.

4 Conclusion

The above analysis shows, cable digital TV signals after processing through the carrier modulation, is not a pure digital signal, and its anti-interference ability has decreased, especially in the face of the same frequency pulse or signal interference, caused by not a single symbol error, and often a string of code element exists in a large number of errors, the error between before and after the performance symbol to have some correlation, resulting in set-top boxes can not be decoded, we do a good job shielding the network, while but also to avoid interference with the signal source, to ensure stable transmission of digital television signals.

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