Signal detection function of satellite digital TV receiver when searching for stars
When we are looking for stars, the digital machine itself has some detection displays, such as signal strength, signal quality (all displayed with red bars), error correction, etc. These displays reflect what functional indicators of the receiver. After reading this article carefully , You will get the answer.
In order to facilitate the reception of satellite digital TV signals by the majority of users (receivers), the satellite digital TV receiver is equipped with a number of signal detection functions, including wiring instructions for signal strength and signal quality, automatic selection of convolution shrinkage, and signal lock instructions Wait. Based on the automatic detection of the signal, it can further realize the automatic search of the received signal, so-called "blind scan" (blindsCAN). For the majority of non-professional receivers, the "blind scan" function can greatly facilitate their viewing, and the program to be viewed can be quickly searched and stored without the tedious operation of the receiver. These signal detection functions of satellite digital TV receivers can be used for data collection in addition to their own reception needs, and the related detection data can be output through a serial data interface for other purposes, such as uplink from an earth station during rain attenuation Automatic power increase, identification of interference signals, etc.
The detection of the received signal is mainly done in the Tuner (Tuner), and is processed by the detection signal of the receiver's main control CPU. The tuner can be divided into two parts, one is the tuning circuit, and the other is the link circuit. The Link circuit is composed of an integrated circuit, such as STV0299 and STV0399 of STMicroelectronics and MT312 of Zarlink Semiconductor; the previous tuning circuit was composed of discrete components, and now there is also a single chip, STB6000 And Zarlink Semiconductor's ZL10036 and other chips. There are hundreds of registers in the Link circuit, which are used to temporarily store the detection data. The main control CPU calls these data through the I2C bus to complete the statistics and judgment. The circuit structure of the tuner, the part in the gray frame is the Link circuit. The detection point and detection principle of each signal will be analyzed in conjunction with the block diagram below.
Signal strength indication
The signal strength is the corresponding relationship through the AGC level. Take the signal from the AGC circuit and convert its true value into a logarithmic display, you get the signal strength expressed in decibels. Of course, the signal strength can also be displayed with a light-emitting diode level indicator or a bar graph, which allows users to intuitively understand the field strength of the received signal. However, with this instruction alone, the user cannot determine whether the received signal is a useful signal or an unwanted signal such as noise. Therefore, the signal strength indication is often combined with the carrier-to-noise ratio (or signal quality) indication for observation.
Signal quality (carrier-to-noise ratio) indication
The carrier-to-noise ratio indicator can reflect the quality of the received signal. Its principle is to calculate the discreteness of the received signal constellation. For the noise in the transmission channel, whether it is additive or multiplicative, it will cause the dispersion of the constellation, and the greater the noise, the greater the dispersion. Therefore, the detection of the carrier-to-noise ratio starts with the calculation of the dispersion of the constellation (Editor's Note: Discrete, this is an advanced mathematical language, which can be understood as the gap from the received signal here). For each symbol received, calculate the distance (vector) di of its constellation position from the ideal position, and then calculate the statistical average. Obviously, the larger the statistical average (dispersion), the worse the carrier-to-noise ratio.
Formula (1) is an empirical formula used by a certain type of receiver to calculate the carrier-to-noise ratio. It uses 30,000 symbols as statistical samples. For example, if the dispersion value is 7000, the carrier-to-noise ratio is calculated to be about 9.2dB.
Eb ÷ No = (13312-dispersion value) ÷ 683 (1)
In addition to noise, interfering signals will also cause the dispersion of the constellation. Therefore, the carrier-to-noise ratio calculated above actually includes the component of the carrier-to-interference ratio. The signal strength indication and the carrier-to-noise ratio indication are combined and displayed on the monitor screen in the form of a bar, which is particularly useful when searching for stars.
Detection of bit error rate after QPSK demodulation (before Viterbi convolutional decoding)
The detection of bit error rate before convolution decoding is mainly used to automatically find the convolution shrinkage rate. The left side of the dot-and-dash line is the bit error rate detection part. Re-encode the convolutionally decoded data, and then compare it with the undecoded delayed data. Count the number of inconsistent data between the two To get the bit error rate.
If the convolution shrinkage rate used in convolution decoding is wrong, the bit error rate is relatively high. Conversely, when the shrinkage rate used is correct, the bit error rate will be significantly reduced. By comparing with the pre-stored bit error rate threshold, if the bit error rate is higher than the corresponding threshold, it means that the convolution shrinkage rate used is not suitable, and the convolution shrinkage rate needs to be changed until the bit error rate is below the corresponding threshold It is considered that the correct convolution shrinkage rate is found. In this way, the automatic search for the convolution shrinkage rate is realized.
Two registers work together, one is used to count the number of error codes, and the other is used to count the time (period). Whenever the count period register overflows, an interrupt request is issued to the main control CPU, and the CPU completes the statistical calculation of the bit error rate.
Bit error rate after convolutional decoding
After Viterbi convolutional decoding, that is, the process of detecting the bit error rate before Reed-Solomon decoding. It assumes that the corresponding data bits before and after Reed-Solomon decoding are considered to be erroneous bits if there is a difference. In the 204-byte data packet before and after decoding, there are erroneous bits in the shaded bytes. See formula (2) for calculation of bit error rate:
Bit error rate = number of different bits before and after decoding ÷ (measurement time × symbol rate × convolution shrinkage rate × 2) (2)
The bit error rate of this detection point determines whether the quality of the received signal can reach "quasi error-free". As long as the bit error rate at this point is less than 10-4, the bit error rate of the signal after Reed-Solomon decoding will be less than 1 bit per hour, that is, "quasi-error-free". But the result of this detection point is not helpful for the reception of the digital satellite receiver itself, or the receiver itself does not use this result.
Bit error rate after Reed-Solomon decoding
The measurement of the bit error rate at this detection point is the same as the bit error rate detection process after convolutional decoding above, but here we compare the number of different bytes before and after Reed-Solomon decoding. Because the error correction capability of the RS (204, 188) error correction code is 8 bytes, if the number of error bytes in a 204-byte packet exceeds 8, the packet is considered uncorrected. Therefore, the bit error rate here is measured by the (error) packet error rate, and the calculation method is shown in formula (3). Whenever an uncorrected error packet occurs, the Link circuit will issue an error indication, but does not terminate the work of the subsequent MPEG-2 decoder.
Packet error rate = (number of uncorrected packets × 204 × 8) ÷ (measurement time × symbol rate × convolution shrinkage rate × 2) (3)
Blind scan of received carrier frequency
Before blind scanning, you need to determine the step interval of the scanning frequency, for example, set to 6MHz, then the corresponding capture range will be determined accordingly, which should be not less than ± 3 MHz. If the initial scan frequency is f0 (the actual frequency is generally 950MHz), and the search starts on both sides of this frequency, the second scan frequency is f0 + 6 (MHz). If a program is searched near this frequency point, the carrier frequency is f1, then the next scanning frequency point becomes (f1 + 6), and so on. For example, if the carrier f2 is searched near the frequency point (f1 + 12), the next scanning frequency point becomes (f2 + 6). In this way, the scanning frequency continues to advance to the high end until the upper limit of the receiving frequency band (generally 2150MHz), and finally the blind scanning of the receiving carrier frequency is completed.
Lock detection
Lock detection includes two aspects, one is carrier lock detection, and the other is clock lock detection.
When the local oscillator signal of the tuner is phase-locked with the input carrier, it means that the tuner can perform normal QPSK coherent demodulation. At this time, the tuner will output a CF (Carrier Recovery Flag) flag.
When the data signal demodulated by QPSK is stable, the clock can be recovered correctly, and then the received symbols are counted. As we all know, the (frame) sync byte in a 204-byte packet is 47H, but every 8 packets are flipped to B8H. A counter is used in the Link circuit to count up and down the B8H. When the B8H appears normally, it increases by 1 and loses by 1; when the positive count overflows, the TF (TIming LOCk Flag) flag is generated. The presence of this flag means that the received TS stream has been synchronized. For the "blind scan" receiver, the time interval between two adjacent B8H bytes is determined. It also means that the symbol rate of the received signal is searched, which is completed. Blind scan of symbol rate.
The logical AND of the CF flag and the TF flag produces the LK (LOCk) indication, which is the lock indication. As long as the satellite receiver issues a lock instruction, it prompts the user that the program has been received steadily. Conversely, when the receiver loses lock, the MPEG-2 decoder will stop decoding, and at the same time the machine will issue a lock loss warning. At this time, a black screen or still frame will appear on the monitor.
In summary, each signal detection function of the satellite digital TV receiver is to meet the needs of reception. These detection functions are completed by the Link circuit under the control of the main control CPU. All test results can also be output through the serial data output port. Corresponding to the serial I / O circuit (RS-232 interface) of the STI5518 chip, in satellite digital TV receivers, this interface is usually used to download and upgrade software. As long as the settings are changed, this interface can output the relevant detection results and provide them as a reference signal to devices such as rain attenuation controllers or interference recognizers. When rain attenuation occurs, both the level of the received signal and the signal-to-noise ratio will decrease; when interfered with, the signal-to-noise ratio of the received signal will decrease, and the bit error rate will increase. The post-instrument equipment can use these test results as criteria, and the satellite digital receiver at this time acts as a data collector.
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