MXPA98008172A - Tuning system to achieve rapid acquisition times for a satellite digital receiver - Google Patents

Abstract

The present invention relates to the RF signals received from the low noise block converter and the corresponding IF signal produced by the tuner can be shifted in frequency due to reasons other than a frequency shift of the block converter oscillator. low noise, such as the frequency settings of the satellite repeater made by the satellite transmission system. A tuner (9) includes a local oscillator (911) controlled by a controller. The controller (a) controls the frequency of the local oscillator (911), (b) stores words representative of the digital nominal frequency for the respective signals of the received RF signals, (c) stores words representative of the digital displacement for the respective signals of the RF signals; (d) determines a frequency shift of said carrier signal after having tuned an RF signal; (e) updates all the words representative of the digital displacement according to the frequency shift of the carrier; the representative word of tuning is derived for a selected RF signal to be tuned to the word representative of the previously updated offset for the selected RF signal to be tuned, and (g) updates the word representative of the respective digital shift for a signal of RF individual that is tune-tuning if the correct tuning is not achieved with the p representative scroll of previously updated

Description

TUNING SYSTEM TO ACHIEVE QUICK ACQUISITION TIMES FOR A DIGITAL SATELLITE RECEIVER.

Title of the invention The invention relates to a tuning system for a satellite receiver, especially one capable of receiving and processing television signals transmitted in digital form.

BACKGROUND OF THE INVENTION Satellite television reception systems usually comprise an "outdoor unit" including a dish-type receiving antenna and a "block" converter, as well as an "indoor unit" including a tuner and a processing section signs The block converter converts the range range ("block") of relatively high frequency RF signals transmitted by a satellite to a more manageable and lower frequency range. In a conventional satellite television transmission system, the information is transmitted in analog form and the RF signals transmitted by the satellite are in the C (for example, 3.7 to 4.2 GHz) and Ku (for example, 1 1 7) bands. at 14 2 GHz) The block converter converts the RF signal received from the satellite by the antenna of the reception system to the L-band (for example, 900 to 2000 MHz). A RF filter section of the indoor unit's tuner selects one of the RF signals received from the block converter corresponding to the selected channel, a section of the tuner's local mixer / oscillator oscillator converts the selected RF signal to a range lower intermediate frequencies (IF) for filtering and demodulation. In recent satellite television systems, such as the DirecTv ™ operated by the Hughes Corporation of California, television information is transmitted digitally. The RF signals are transmitted by the satellite in the Ku band and the block converter converts them to the L-band. The frequency range of the RF signals transmitted by the satellite is a bit smaller (for example, between 12.2 and 12.7 GHz) that for the analog satellite television system, and the frequency range of the RF signals produced by the block converter is consequently slightly lower (for example, between 950 and 1450 MHz). As with analogue satellite television reception systems, the RF signal corresponding to the selected channel must be reduced in frequency to a frequency range of Fl for filtering and demodulation. In a digital satellite receiver, in addition to the normal filtering of Fl to select the desired RF signal and reject unwanted RF signals, it is desirable that the Fl filter perform what is known as a "symbol conformation" to reduce decoding errors due to "inter-symbol interference" caused by bandwidth limitations.

The conversion stage of the block converter of the outdoor unit usually includes a local oscillator which is not stabilized against variations in temperature and age. The result is that the frequency of the signal of the local oscillator of the block converter changes, causing a corresponding change or displacement of the frequencies of the carrier signals of the RF signals received by the tuner of the internal unit. As a consequence, the frequency of the Fl signal produced by the tuner also changes or shifts from its nominal value. If the frequency of the Fl signal changes too much from its nominal value, the digital signals modulated in the Fl signal can not be demodulated adequately and the information they represent can not be reconstructed properly. To solve this problem, the displacement frequency is monitored and a shift to the nominal frequency command is added to change the local oscillator of the tuner to the central signal in the Fl filter. BRIEF DESCRIPTION OF THE INVENTION Part of the invention resides in the recognition that the RF signals received from the low noise block converter and the corresponding Fl signal produced by the tuner can be shifted in frequency due to reasons other than an offset of oscillator frequency of the low noise block converter. More specifically, the frequency settings of the satellite repeater can be made by the operator of the satellite transmission system to reduce the possibility of interference between carrier signals. By way of example, a repeater frequency can be changed up to +/- 2 MHz. The frequency settings of the repeater cause the RF signals received from the low noise block converter and the corresponding Fl signal produced by the tuner to have an frequency offset. The present invention relates to provisions for tuning frequency shifts due to the adjustment of individual frequencies of the repeater performed by the operator of the satellite transmission system. These provisions allow the operator of the satellite transmission system to adjust the transmit frequencies of the repeaters without unduly increasing the time of the indoor unit to acquire the digital signal when a new channel is selected. Briefly, the tuning system measures and stores individual frequency shifts originating in the repeater. Any displacement due to the frequency derivation of the low noise block converter is added to all frequency shifts of the repeater as a "global" shift. An individual displacement of the repeater is updated if it is not possible to tune a frequency of the repeater or its successful acquisition required a frequency shift greater than a predetermined value or a broad frequency search was required to acquire the signal. These and other aspects of the repeater frequency offset provisions are described below. These and other aspects of the invention will be described in detail with reference to the drawings that Ig accompanies. BRIEF DESCRIPTION OF THE DRAWINGS In the drawings: Figure 1 is a block diagram of a digital satellite television receiver that includes a tuning system that can be used by the invention; Figure 2 is a block diagram of a digital data demodulation for use in the satellite receiver shown in Figure 1 and useful for understanding the digital data recovery of the tuning system shown in Figure 1; and Figure 3 is a flow chart of the acquisition routine that is used to control the tuning system shown in Figure 1, in accordance with an aspect of the present invention. In the different Figures, the same or similar reference designations are used to identify the same or similar elements. Description of the preferred embodiment The invention will be described with reference to a digital satellite television system in which television information is transmitted in encoded and compressed form according to a predetermined digital compression standard, such as MPEG. MPEG is an international standard for encoded film representation and associated audio information developed by the Group of Film Experts. The DirecTv ™ satellite television transmission system operated by the Hughes Corporation of California is said digital satellite television transmission system. In the transmitter, the television information is digitized, compressed and organized into a series or stream of data packets corresponding to respective portions of video, audio and data of television information. The digital data is modulated in an RF carrier signal in what is known as QPSK modulation (Quaternary Phase Change Transmission) and the RF signal is transmitted to a satellite in Earth orbit from where it is retransmitted back to Earth. In the QPSK modulation, the phases of the two quadrature phase signals, I and Q, are controlled in response to the bits of the respective digital data streams. For example, the phase is set to 0 degrees (°) in response to a low logic level ("0") and the phase is set to 180 ° in response to a high logic level ("1"). The modulated I and Q signals of phase change are combined and the result is transmitted in a QPSK carrier signal modulated by RF. Consequently, each symbol of the modulated Q PSK carrier indicates one of four logical states, that is, 00, 01, 10 and 1 1. Commonly, a satellite includes a number of repeaters to receive and retransmit respective modulated carriers of R F. In a conventional terrestrial television system, each RF carrier or "channel" contains information for only one television program at a time. Therefore, to see a program, it is only necessary to select the corresponding RF signal. In a digital satellite television system, each modulated RF carrier carries information for several programs simultaneously. Each program corresponds to groups of audio and video packages that are identified by a single header attached to the packages, which identifies the program. Therefore, to view a program, it is necessary to select the corresponding RF signal and the corresponding packets. In the digital satellite television receiver shown in Figure 1, RF signals modulated with digital signals representing video and audio information which have been transmitted by a satellite (not shown) are received by a type 1 antenna plate. The received RF signals of relatively high frequency (for example, in the Ku frequency range between 12.2 and 12.7 GHz) are converted by a block 3 converter, including a 3-1 RF amplifier, a 3-3 mixer and a oscillator 3-5, to RF signals of relatively lower frequencies (for example, in the L band between 950 and 1450 MHz). The amplifier 3-1 is a "low noise" amplifier and therefore the block 3 converter is commonly referred to by the initials "LN B" for "low noise block converter". The antenna 1 and the low noise block converter 3 are included in a so-called "external unit" 5 of the receiving system. The remaining portion of the receiver is included in a so-called "internal unit" 7.

The internal unit 7 includes the tuner 9 for selecting the RF signal containing the packets for the desired program from the plurality of RF signals received from an outdoor unit 5 and for converting the selected RF signal to a corresponding intermediate frequency signal (Fl) minor. The present invention relates to controlling the tuner 9 and will be described in detail below. The remaining portion of the indoor unit 7 demodulates, decodes and decompresses the digital information carried in the form of QPSK modulation by the Fl signal to produce currents of digital audio and video samples corresponding to the desired program, and subsequently converts the currents of the samples digital to respective analog audio and video signals suitable for playback or recording. More specifically, a demodulation QPSK 11 demodulates the signal of Fl to produce two IP and QP pulse signals containing respective data bit streams corresponding to the data represented by the phase change modulated I and Q signals generated in the transmitter. A decoder 13 organizes the bits of the IP and QP signals into blocks of data, corrects the transmission errors in the data blocks based on error codes that have been included in the data transmitted in the transmitter and reproduces the audio packets and video transmitted from M PEG. The audio and video packets are routed by a transport unit 15 to respective audio and video sections of a data processing unit 17, where they are decompressed and converted to respective analog signals. A microprocessor 19 controls the operation of several sections of the indoor unit 7. However, only the control signals generated and received by the microprocessor 19 with which the invention is directly related are indicated in Figure 1. The digital receiver of Satellite television described so far is similar to the RCA ™ DSS ™ digital satellite system television receiver commercially available from Thomson Consumer Electronics, Inc. of Indianapolis, Indiana. As mentioned above, the present invention relates to control of tuner 9 and demodulation. The tuner 9 receives the RF signal provided by the low noise block converter 3 at an input 901. The RF input signals are filtered by the wideband filter 903, amplified by the RF amplifier 905, and filtered by the tunable bandpass filter 907. The tunable band pass filter 907 selects the desired RF signal and rejects the RF signals unwished. The resulting RF signal is coupled to a first input of the mixer 909. A local oscillator signal produced by the local oscillator 91 1 is coupled to a second input of the mixer 909. The output of the mixer 909 is amplified by the amplifier 913 and coupled at the entrance of the Fl filter 915 comprising a SAW device. The filter output of Fl 915 is coupled to output 917 of tuner 9.

The frequency of the local oscillator 911 is controlled by a synchronization circuit arrangement 919 comprising the synchronization circuit integrated circuit 921, the external frequency reference crystal 923 and the external filter network 925. The frequency of the local oscillator is controlled by the synchronization circuit 919 according to the instructions generated by the microprocessor 19. The carriers of the RF signals transmitted by the satellite and received by the antenna 1, have very stable frequencies that remain in "nominal" values. Therefore, as long as the oscillator frequency 3-5 of the low noise block converter 3 is stable and remains at its nominal value, the carrier frequencies of the RF signals received by the tuner 9 of the indoor unit 7 will be in its nominal values. Unfortunately, the frequency of oscillator 3-5 can change with time and temperature. The frequency shift of the oscillator 3-5 with respect to its nominal frequency produces corresponding displacements of carrier frequencies of the RF signals received by the tuner 9. To compensate for these frequency shifts, the frequency of the local oscillator 91 1 of the tuner 9 is changed under the control of the microprocessor 19, in response to the received frequency status information of the QPSK demodulation 1 1. As shown in Figure 2, the signal of F l produced by the SAW filter of Fl 915 is coupled to respective first inputs of the mixers 1 1011 and 1 101 Q. The letters "I" and "Q" mean "in phase" and "square". The output signal of the relatively stable frequency oscillator 1103 is directly coupled to the mixer 1 1011 and indirectly coupled to the mixer 1101 Q, via the phase change network 1105 of 90 degrees (90 °). The mixer 1 1011 produces a "near" baseband version "in phase" (much lower frequency) (IA) of the Fl signal, while the 1 101 Q mixer produces a baseband version close to the "quadrature" (QA) of the Fl signal, which is changed 90 degrees with respect to the signal "in phase". The letter "A" means "analog." The signals IA and QA are coupled to respective analog to digital converters 1 1071 and 1 107Q. Analog-to-digital converters 1 1071 and 1 107Q also receive a clock signal from the "symbol timing recovery circuit" 1 109 and produce respective series of digital samples I D and QD. The "D" means "digital." The symbol timing recovery circuit 1 109 includes a controlled oscillator (not shown) from which the clock signal for the analog-to-digital converters 1 1071 and 1 107Q is derived. The controlled oscillator is controlled by a hybrid synchronization circuit (digital part and analog part) (not shown) so that the digital samples are synchronized with the incoming symbol phase and value. The analog signals can be viewed as a pulse current. The function of the symbol timing recovery circuit 1 109 is to phase-lock the clock for the analog-to-digital converter. Sample the analog signal at the peak of the pulses. In other words, the symbol timing recovery circuit 1009 synchronizes the sampling operation of analog-to-digital converters 1 1071 and 1 107Q with the arrival of each received symbol. The ID and QD signals are also processed by a "carrier tracking circuit" 1 1 1 1. The carrier tracking circuit 1 1 1 1 demodulates the digital sample signals ID and QD to form respective IP and QP pulse signals. The letter "P" means "pulse." Although the signals have been demodulated (separated into IA and QA components) they were demodulated with a non-synchronous carrier. Since the demodulating carrier was not synchronized with the transmitted carrier, the constellation will still be rotating. It is commonly called a Near Baseband Signal at this point. Once it has been unpinned, it is called "Baseband Signal". Hence the nomenclature I BB and QBB at the output of the 1 1 1 1 -4 Derotator. The baseband signals can be plotted on a graph I vs. Q, which creates the "constellation" diagram. The baseband signal is input to a 1 1 1 -2 amplifier that estimates which of the four points of the constellation was transmitted. Each of the pulse signals I P and Q P contains a series of pulses corresponding to data bits. The data bits have either a low logic level ("0") or a high logical level ("1") corresponding to the phase changes of 0 ° and 180 °, respectively of the I and Q signals of the RF transmitted carrier of QPSK. The IP and QP signal components are coupled to the decoder 13, where the packet data bits are formatted and the direct error correction (FEC) is performed. Carrier tracking circuit 1111 includes the complex 1111-4 spoiler, the amplifier 1111-2, the numerically controlled oscillator (NCO) 1111-1, the phase detector 1111-3 and the circuit filter 1111-5. The complex 1111-4 decomposer is a complex multiplier that shifts the rotating constellation to produce a stable constellation. Derrotation is carried out by multiplying the digital input signals ID and QD by the estimated sine and cosine of the estimated phase and frequency offset. The estimated frequency offset is the value at which the signal near the baseband is rotating. The manner in which this estimated displacement is generated is described below. The amplifier 1111-2 takes the constellation unrooted and produces decisions based on the input quadrant. Each pair I, Q produced by the amplifier 1111-2 is the estimate of the symbol that was transmitted. The phase detector 1111-3 takes the input and output of the amplifier 1111-2 and generates a phase error signal for each symbol. This phase error signal is applied to the 1111-5 circuit filter. The 1111-5 circuit filter controls the numerically controlled oscillator 1111-1 and provides an estimate of the displacement frequency. This estimate is available for the microprocessor 19. A frequency error, for example, due to a frequency offset derived from the low noise block converter of the selected RF signal, causes a "rotation" or "rotation" of the position of the two-bit demodulated data of the QPSK signal over time. The direction of rotation depends on whether the frequency shift is positive or negative. As shown in Figure 2, the data constellation for the QPSK modulation has four points corresponding to the four possible logical combinations (00, 01, 10 and 11) of the two respective possible logical levels represented by the two change values of possible phase of the I and Q signals. The phase detector 1111-3 measures the position of the demodulated data relative to the ideal position in the data constellation. To correct the inclination and rotation of data, the frequency and therefore, the phase of the numerically controlled oscillator 1111-1 is changed by the circuit filter 1111-5 in response to the output signal of the phase detector 1111-3 until that the rotation stops and the tilt is eliminated. With this rotation stopped, the constellation is stabilized and the carrier tracking circuit 1111 is considered "closed". Under this steady state condition, the 1111-5 circuit filter has correctly estimated the phase and frequency changes required to unravel the data for the constellation to stabilize successfully. The 1111-5 circuit filter has integral and proportional paths that add up to form the control of the numerically controlled oscillator 1111-1. The value of the integral path (which integrates the phase error) represents the frequency shift that causes the "rotation". This value is available for the microprocessor 19 as the FREQUENCY signal shown in Figures 1 and 2. The microprocessor 19 compares successive samples of the FREQUENCY signal to determine if the constellation has been stabilized. If the difference in the successive samples is small, the demodulation is recognized as "CLOSED". Under this steady state condition, the demodulated data of IP and QP are reliable and are passed to the decoder of direct error correction 13. During the acquisition of a channel, if the current frequency of the local oscillator 91 1 of the tuner does not allow a closure After the successful completion of the carrier tracking circuit 1 1 1 1, then the microprocessor 19 will adjust the frequency until a CLOSED condition is found or a suitable frequency range has been covered. The entire process of signal acquisition will be described in greater detail in the description of the flow diagram in Figure 3. Within limits, the carrier tracking circuit 1 1 1 1 can demodulate the QPSK data even when the frequency of the signal of Fl and therefore the frequency of the signals IA and QA, is incorrect or is displaced. However, if the frequency shift is very large, a portion of the frequency spectrum of the Fl signal will fall outside the passband of the SAW 915 filter, due to the change of the Fl signal relative to the center frequency of the filter. SAW filter 91 5. This will cause a degradation of the signal-to-noise ratio of the receiver. Accordingly, as mentioned above, the microprocessor 19 monitors a FREQUENCY signal generated by the carrier tracking circuit 1111 to indicate the frequency shift of the signal of Fl. As the frequency shift caused by the change of the frequency converter changes, low noise block, the carrier tracking circuit 1111 tracks the changes and updates the FREQUENCY signal monitored by the microprocessor 19. In the next channel acquisition, the microprocessor 19 will use the last recorded frequency offset to provide a positioning more accurate local oscillator 91 1. This should allow you to acquire the signal quickly without having to search by moving the local oscillator 91 1 again. If the frequency offset is so great as to cause degradation in the reliability of the demodulated data, eventually the direct error correction decoder 13 will be unable to correct the errors and break the closure. The microprocessor 19 will request a reacquisition of the same channel and again the last frequency offset will be used to place precisely the local oscillator 91 1 for a quick reacquisition. As mentioned above, the derailed data streams IP and QP are processed by the direct error correction decoder 13 shown in Figure 1. The function of the direct error correction decoder 1 3 is to correct errors in the transmission of the data. In order for the decoder to be able to correct errors, the demodulated signal must be stabilized. Additionally, to correct the data, the direct error correction decoder 13 must be set to the same code value as the transmission code value and synchronized with the packet limits. The ERROR ERROR CORRECTION LOCK signal D generated by the direct error correction decoder 13 and monitored by the microprocessor 19, indicates whether all of the aforementioned conditions are met and the direct error correction decoder 13 is passing error-free data. . For example, the ERROR IC RECEIVED ER RECORD signal has a low logic level when the direct error correction decoder 13 can not correct the data, and the CI signal ERROR CORRECTED STRAIGHT DI ERROR has a high logic level when the direct error correction decoder 13 can correct the data. The signal CI ERRE DE CO RRECCONIO R ECT OF ERRO R is used as the final determination of whether the tuner 9, the demodulation QPSK 11, and the decoder CORRECTION DI STRAIGHT OF ERROR 13 are properly closed because the carrier tracking circuit 1 1 1 1 can be falsely stabilized at a "false closing point". At a "false closing point", the constellation does not appear to be spinning. But in reality the constellation is rotating 90 degrees (or a multiple of 90 degrees) per symbol. Since there is another constellation at 90 degrees, it seems to be stable. The "false closing points" occur at multiples of the value of the symbol divided by four. When the carrier tracking circuit 1111 is stabilized to a false closing point, the direct error correction decoder will not be able to decode the data. Hence, the DIRECT ERROR CORRECTION CLOSE signal will remain at a low logic level (not closed). The acquisition of signals that has been described so far with respect to frequency shifts due to the frequency shift of the low noise block converter. As mentioned earlier, frequency shifts can also be due to other reasons. More specifically, the frequency settings of the satellite repeater can be made by the operator of the satellite transmission system to reduce the possibility of interference between carrier signals. As an example, a repeating frequency can be changed up to +/- 2 M Hz. The frequency settings of the repeater make the RF signals received from the low noise block converter and the corresponding Fl signal produced by the tuner have a frequency shift. The following aspects of the present tuning system refer to provisions for tuning frequency shifts due to the adjustment of individual repeater frequencies made by the operator of the satellite transmission system. These provisions allow the operator of the satellite transmission system to adjust the transmit frequencies of the repeaters without unduly increasing the time for the indoor unit to acquire the digital signal when a new channel is selected. Without the provisions for tuning frequency offsets due to the adjustment of individual repeater frequencies by the operator of the satellite transmission system, the tuning system operates in the following manner when a new frequency of the repeater is selected: Usually, the frequencies of the signals that are being transmitted are known in advance and stored in a table (known as the "baseline frequency" plan). Then, during the operation when a repeater is selected for tuning, the baseline frequency is retrieved from the table and a frequency offset is added. This displacement as described above, is determined from the displacement required to close in the previous repeater. This displacement is called "global displacement" because it is applied globally to all repeaters. The cause of the global displacement is due to any change of frequency in the oscillators common to the communication path. For example, if the low converter oscillator in the low noise block converter is shifted by 3 MHz because being a cold night, all repeaters will be changed 3 MHz below their baseline frequencies. This global change is initially found by a search algorithm that passes the tuner through a specified frequency range while trying to acquire the signal (referred to as the "find displacement" algorithm). Once the algorithm of finding the offset finds a signal, the exact displacement of the signal can be used to initialize the global shift for future tuning. Once the global offset is initialized, the value is tracked by monitoring the FREQUENCY signal in the carrier tracking circuit 1 1 11. Each time a new repeater is requested, the microprocessor updates the global displacement by adding the last value of the FREQUENCY sign. With the normal system described above, if a repeater of its baseline frequency plan was moved, it would result in slow channel change times when tuning that repeater and any repeater tuned later. This is due to the fact that the system mentioned above assumes that the displacement is global for all repeaters. For example, for a system with 10 repeaters, evenly spaced 30 MHz between them starting at 1000 MHz, the baseline frequency plan for the repeaters would be as shown in the following TABLE 1. If the displacement of the low noise block converter produces a change of 2 MHz in the frequencies, the repeaters are located at the frequencies shown in the column "with displacement of low noise block converter". If the operator of the satellite transmission system moves repeater 3 from the other 1.5 MHz, then the last column in TABLE 1 shows where each repeater is located.

Frequency Number Frequently with # 3 moved Repeater Baseline displacement of LNB LNB 1 1000 MHz 1002 MHz 1002 MHz 2 1030 MHz 1032 MHz 1032 MHz 3 1060 MHz 1062 MHz 1060.5 MHz 4 1090 MHz 1092 MHz 1092 MHz 5 1120 MHz 1122 MHz 1122 MHz 6 1150 MHz 1152 MHz 1152 MHz 7 1180 MHz 1182 MHz 1182 MHz 1182 MHz 8 1210 MHz 1212 MHz 1212 MHz 9 1240 MHz 1242 MHz 1242 MHz 10 1270 MHz 1272 MHz 1272 MHz TABLE 1 (Frequency Plan) With respect to In the exemplary situation shown in TABLE 1 above, the global offset would be initialized to 2 MHz if repeater 1 is selected. As all repeaters except for repeater 3 are tuned correctly, the tuner would be tuned to the desired signal. However, if the repeater is selected.3, the tuner would be tuned to a frequency of 1.5 MHz above the required one and, therefore, the signal is not acquired until the search algorithm begins to broaden its search by going to the local oscillator 911. This would cause the signal to be found, but at a new displacement of 0.5 MHz. It would be assumed that this new displacement would be the new global displacement and would cause the next repeater to be selected while being badly tuned as well. As a result, the tuner has to perform the extended search. Therefore, whenever repeater 3 is selected, a slow change of undesirable channel occurs. The present invention deals with provisions for independent tuning frequency shifts due to the adjustment of independent repeater frequencies performed by the operator of the satellite transmission system. The following description is made with respect to Figure 3. The flow chart of Figure 3 has five main scenarios that need to be described: (1) the maintenance mode (viewing a channel); (2) a normal channel change; (3) the repeater has moved slightly and does not require a broad search; (4) the repeater has moved or is not at the expected displacement or value and requires a broad search; (5) the tuning starts with a repeater at the beginning of the box; and (6) an unsuccessful channel change. (1) Maintenance mode. Stable operation is when a user is watching a channel and is not suffering or experiencing any type of rain fading. Under this scenario, the following path would be taken: The question "New Channel Requested?" It would be answered No. This would lead to the question "direct correction of closed error?" (direct correction of error - closed means that the decoder is successfully decoding the bit stream without errors) that would be answered Yes since everything is properly closed. In the box with the number 3, the FREQUENCY signal and the carrier tracking circuit is read. This value is stored in the variable "Last_displacement" and represents the frequency offset that has occurred since the last tuning (assuming that the last tuning put the tuner one step away from the correct frequency tuner). Since it is in a stable state, the Notification indicator will not be established (it is deleted after the notification of a successful closure) and the routine is repeated to verify if a channel change request has occurred and the cycle repeats. (2) Normal channel change. Under a normal channel change scenario, the new repeater to be acquired is one step away from the tuner of the expected frequency. The expected frequency is the base frequency plus the displacement stored in the displacement table. The displacement table contains the individual displacement frequency for each repeated r. The code follows the following path: the question "New Channel Request" is answered Yes and it executes the box with the number 2. Here the variable "Last_displacing" (last updated in the maintenance mod mentioned above) is added to each element of the displacement table. This makes the assumption that the displacement has occurred in the previous repeater because the last tuning is applicable to all repeaters and is usually due to the aging displacement and temperature of the local oscillator of the low noise block converter (similar to the normal systems that track a global displacement). Then the tuner is sent to the new repeater frequency which is the sum of the base frequency plus the recently updated offset frequency of the shift table. After tuning, the status indicator is removed, acquisition indicators are established including the notification indicator. After a short delay, the direct error correction for closure is consulted. The delay allows sufficient time for the direct error correction to close if the tuner is properly positioned and the correct code value is selected. Under a normal channel change, the direct error correction will close at this point and the path will follow the "S" "branch. The frequency offset is read again (and should be with the frequency increase step of the tuner's local oscillator under this scenario) and stored as Last_displacement. Now the Notification indicator is checked and the trajectory will follow Yes because it has just been established. Then the "First_indicator_of tuning" is checked and it should not be established because it has already been closed in this scenario. The value of the Last_displacement is revisited against a minimum value that is approximately one step higher than the tuner. Again, under this scenario, it is assumed that the displacement is within the minimum value and follows the trajectory No.

At this point, the link is successfully closed and the routine notifies the task of the computation programs that requested the channel change that the link is ready. The Notification indicator is removed. Then the trajectory returns to the maintenance path and will follow the maintenance cycle until another channel change is requested or a disturbance causes the direct error correction to break the closure. It is important to mention that in this trajectory the acquisition indicators are never used, because the acquisition was successful without having to try or readjust anything. (3) Channel change with minor adjustment to the frequency of the repeater. Under this scenario, the repeater being acquired is close but not exactly where the displacement table predicts (in terms of frequency). The frequency is so close that the demodulation and the direct error correction can still close, but it is considered far enough that the displacement of the individual repeater will be corrected in the shift table. The trajectory that follows is identical to the previous one (case 2) except that the Last_displacement is outside the minimum value. Therefore, the routine executes the box with the number 5. Here, the value of the last_displacement is added to the new entry of the repeaters in the displacement table. Then, this new displacement is used to place the tuner exactly in the signal-centering it in the SAW Fl. To reach this point in the routine, the direct error correction must have been closed and therefore the code value must have been and therefore the try_value flag is set to zero. As the tuner is moving, the demodulation could have problems and the try_desmod flag is set to give it an extra opportunity if necessary. The trajectory returns to the top and will fall through it to verify the closure of the direct error correction. Under this scenario, the direct error correction should close and this time follow the path of a normal channel change with the Last_displacement within the minimum level. (4) Channel change with broad frequency search required. In this scenario, the repeater being acquired is so far from the predicted value that the algorithm must search for the signal by staggering the tuner. However, before the frequency search begins, the algorithm check for closing the symbol timing recovery (STR), resets the carrier tracking circuit in case it is in a false lock, and verifies each code value for direct error correction, also verifies the stability of the AGC to determine if there is a signal there to acquire it. If these corrective actions do not allow a closure of the direct error correction, then the frequency search is performed. This is a last resort because it takes a little time to do it. This is also the reason for tracking individual displacements for repeaters, to avoid this time-consuming search under conditions of normal channel change. The scenario begins as a normal channel change, the shift table is updated in box 2, the tuner is tuned to the predicted frequency, the indicators are reinitialized but after the delay, the direct error correction is not yet closed. At this point the corrective actions begin. Following the path of No after the decision of "direct correction of closed error", the status indicator is NOT CLOSED, then the routine follows the path No. However, the indicator "try_desmod" is displayed, therefore it is not equal to zero and the routine removes the indicator lntentar_desmod, verifies the recovery of symbol timing (STR) for closure. The closure of STR is evaluated by comparing consecutive readings of the STR filter circuit and comparing it to a permissible delta. When the STR is opened, the filter will be jumping and will be easily detected. If the STR is closed, then the carrier tracking circuit is reinitialized to provide another opportunity in a clean close. If the STR is not closed, then it is checked periodically until you have been given enough time to jump through all the possible values. If it closes at that time, then as previously mentioned, it restarts the carrier tracking circuit. If the STR does not close in the time period, then the indicator of trying value is eliminated (it is pointless to treat the other code values if the symbol timing can not be closed). The path returns to verify the new request of the new channel and if there is none, then check to see if the corrective action was successful resulting in a direct error correction closure. If the direct error correction is not yet closed, then the No path is followed again, but this time the indicator "lntentar_desmod" is free, then it falls to check the indicator "Intent_value". If the STR was closed, then this indicator will still be displayed and will not be zero. Therefore, the path No is followed and the indicator "lntentar_valor" is reduced and the code value of the direct error correction is changed to the next value. In the example, the indicator "try_value" is initialized to 3, then three values will be tried before falling to the Automatic Gain Control check. After each value, the routine returns to check if there is a new channel request or to see if the direct error correction closed. Assuming nothing happens, the Automatic Gain Control is checked to see if there is closure. Again, the closure is determined by comparing consecutive samples of the Automatic Gain Control circuit filter. The Automatic Gain Control is reviewed to see if there is closure in order to accelerate the installation of the clients. If there is no signal present, then the Automatic Gain Control will not close and there is no point in wasting time searching for the frequency. For this scenario, the Automatic Gain Control must be closed and the variable "try_displacement" revised. While the variable of If the movement is still positive, the tuner will be passed through a series of positions to cover a certain pattern. In each step, the variable intent_displacement will be reduced and the algorithm will look for the closure of the carrier tracking and STR circuit. ("Signal found?"). First, the STR is reviewed in a manner similar to the one described above in the lntentar_desmod portion. Once the STR is closed, the carrier tracking circuit is reinitialized and checked for closure. Again, the closure of the carrier tracking circuit is determined by comparing the differentiation of the frequency indication of the circuit filter to a fixed minimum value. Unless both the STR and the carrier tracking circuit are declared closed within a certain period of time, the No path will be followed and the next tuner location will be attempted until a signal is found or a stop = displacement = 0. If the STR and the carrier tracking circuit are declared closed within the allowed time, then the signal is considered "found" and the trajectory is followed Yes. In the box with the number 4, the frequency of the carrier tracking circuit 1 1 1 1 is added to the position of the tuner step and that is stored in the displacement table for that repeater. The tuner is tuned to this new offset and the acquisition indicators are displayed to repeat the portions "try_desmodD e" try_value. "After box 4, the routine returns to the top to check again if there is a new request of channel and to see if the direct error correction is closed.After the correct value and frequency offset are discovered, the direct error correction should close and the rest of the normal channel change path continues. "idp_displacement" is initialized to 10 because there are 10 positions of the tuner (bands) that are searched in. The frequencies sought allow to locate a signal that is displaced by the aging and temperature specifications of the low noise block converter for the maximum displacement individual of the repeater allowed from the top link provider, as an example, it is specified that the If the low noise block rter is at +/- 5 MHz of the desired frequency and the upper link provider was allowed to move individual repeater frequencies up to +/- 2 MHz, then the algorithm searched for +/- 7 MHz. ) In the initial tuning of a repeater. The scenario is similar to that of the number 4 in which the repetition frequency of the repeater is not known or is incorrect. The only difference is that once the direct error correction is Closed, this time the "first_indicator_indicator" will be displayed and the box with the number 1 will be executed. In this stage, all entries in the displacement table are initialized to the offset found for the first repeater. This includes the Last_displacement read in box 3 and the current_shift, which is the value determined in box 4. Then, the "first_handle_indicator" is eliminated so that this initialization is not done again. Then, the trajectory continues as a normal channel change. (6) Acquisition not successful. During an unsuccessful acquisition, all portions of tryout, try_value, and try_displacement are eventually set to zero due to having tried that portion or to be eliminated due to another prerequisite. An example was mentioned in (4) above, in lntentar_desmod, if the STR does not close and then try_value becomes zero automatically. Then, once the routine has all the variables "try" at zero and if the Notification indicator is displayed, then the tuner returns to zero offset for that repeater, the Notification indicator is deleted and the task of the computer programs requested by the repeater is notified of the unsuccessful acquisition. The routine will continue in the cycle looking for a new channel request and closing of direct error correction. The present invention relates specifically to the way in which frequency shifts are handled for individual repeaters. In a normal system, only one frequency shift is tracked or monitored and that displacement applies to all repeaters in a similar way.

The present invention similarly tracks frequency offset during visualization and applies that offset to all repeaters, but maintains separate values for each repeater, so that each repeater can be recorded separately if required. The scenarios 3 and 4 mentioned above are examples of when a repeater displacement is adjusted individually. The main factor is when a repeater is acquired in a position other than the mentioned displacement, then only the displacement of that repeater is updated. It should also be mentioned that the present invention will only require the longest tuning time for a repeater shifted from the base plan in the first acquisition of that repeater since it has been changed. After that, the shift should have been recorded and rapid channel changes will occur. Although the present invention has been described in terms of a specific embodiment, it will be apparent that modifications may be made that are within the scope of the invention.

Claims (8)

1. CLAIMS 1. A method for controlling a tuner (9) that receives a plurality of RF signals and produces a carrier signal containing information corresponding to a tuned signal of said RF signals, said tuner (9) includes a local oscillator ( 91 1) having a frequency controlled in accordance with representative digital tuning words to tune the respective signals of such RF signals; said method comprises the steps of: storing representative words of digital nominal frequency for the respective signals of said R F signals; store representative words of digital displacement for the respective signals of said RF signals; determining a frequency offset of said carrier signal after having tuned an RF signal; updating all said representative words of digital displacement in accordance with the frequency offset of said carrier; deriving the representative word of tuning for a selected RF signal to be tuned by combining said nominal frequency representative word for said selected RF signal to be tuned to said previously-displaced representative word for such selected RF signal to be tuned; and updating the respective word representative of the digital displacement for an RF signal that is being tuned if the correct tuning is not achieved with the word representative of the previously updated offset. The method of claim 1, wherein: said word representative of the digital displacement for a single RF signal being tuned is updated if such a frequency offset of that carrier exceeds a predetermined value. The method of claim 1, wherein: all said representative words of the digital offset are updated in response to the selection of a new RF signal for tuning. 4. The method of claim 1, wherein: such RF signals are associated with respective repeaters of a satellite, are provided with a frequency conversion unit and have frequencies with nominal values corresponding to the respective representative words of nominal frequency, but are susceptible to being displaced from the nominal values due to displacements associated with one of the conversion unit and the tuner and displacements of a transmition frequency of a repeater. 5. The method of claim 4, wherein: said carrier contains digitally coded information that is processed by a digital processing section that includes an error correction unit; and the respective word representative of the digital shift pa to an RF signal being tuned is updated if any frequency shift of such a carrier exceeds a predetermined value or the correct error correction is not possible. 6. A method for controlling a tuner that receives a plurality of RF signals associated with respective repeaters of a satellite and provided with a frequency conversion unit and that produces a carrier signal containing information corresponding to a tuned signal of said frequency signals. RF, such RF signals have frequencies with nominal values, but are susceptible to being shifted from the nominal values, said tuner includes a local oscillator (911) having a frequency controlled in accordance with digital words representative of digital tuning to tune one of the aforementioned RF signals; said method comprises the steps of: storing representative words of nominal digital frequency corresponding to the nominal frequency values of the respective signals of said RF signals; store representative words of digital displacement corresponding to the respective signals of said RF signals; when selecting an RF for tuning, combine the word representative of the nominal frequency and the word representative of the shift for such selected RF signal to derive the said tuning word for said selected RF signal; determining a frequency offset of such a carrier signal; and updating said displacer words for all said RF signals in a first mode of operation and updating said frequency representative word for only such selected RF signal in a second mode of operation. The method of claim 6, wherein: the frequencies of such RF signals are susceptible to being shifted from the nominal values due to (1) shifts associated with one of the conversion unit and the hoist sinker and (2) a displacement of a transmit frequency of a repeater; such first mode of operation pertains to displacements due to displacements associated with one of the conversion unit; and said second mode of operation pertains to displacements due to a displacement of a transmit frequency of a repeater. 8. Apparatus comprising: a tuner (9) that receives a plurality of RF signals and produces a carrier signal containing information corresponding to a signal tuned to said RF signals, said tuner (9) includes a local oscillator ( 91 1) having a frequency controlled in accordance with representative words in accordance with representative digital tuning words to tune the respective signals of such RF signals; and a controller (921, 923, 11, 19, 13) for controlling the frequency of such local oscillator (91 1); such controller stores representative words of digital nominal frequency for the respective signals of such RF signals, stores representative words of digital displacement for the respective signals of such RF signals, determines a frequency shift of said representative words of digital displacement in accordance with said frequency offset of said carrier, derives the word representative of the tuning for a selected RF signal to be tuned by combining a representative word of nominal frequency for said selected RF signal to be tuned to said displacement representative word previously updated for the RF signal selected to tune it, and updates the respective representative word of digital displacement for an individual RF signal that is being updated if the correct tuning is not achieved with the representative word of the d previously updated displacement. The apparatus of claim 8, wherein: said controller (921, 923, 11, 19, 13) updates the word representative of the offset for an RF signal that is being updated if said displacement of displacement exceeds a predetermined value. . The apparatus of claim 8, wherein: said controller (921, 923, 11, 19, 13) updates all the words representative of the displacement in response to the selection of a new RF signal for tuning. . The apparatus of claim 8, wherein: such RF signals are associated with respective repeaters of a satellite, are provided with a frequency conversion frequency and have frequencies with nominal values corresponding to the respective words representative of nominal frequency, but they are liable to be displaced from the nominal values due to shifts associated with one of the conversion unit and the tuner and shifts of a transmit frequency of a repeater. . The apparatus of claim 1, wherein: said carrier contains digitally encoded information that is processed by a digital processing section that includes an error correction unit; and the respective word representative of the digital shift for an RF signal being tuned is updated if any frequency shift of such a carrier exceeds a predetermined value or the correct error correction is not possible. . The apparatus of claim 8, wherein: said local oscillator (91 1) is included in a synchronization circuit (919) that includes a programmable separator having a division factor established in accordance with such a digital word representative of the tuning for controls the frequency of said local oscillator. SUMMARY The RF signals received from the low noise block converter and the corresponding Fl signal produced by the tuner can be shifted in frequency due to reasons other than a shift in the low noise block converter oscillator frequency, such as the frequency settings of the satellite repeater made by the satellite transmission system. A tuner (9) includes a local oscillator (91 1) controlled by a controller. The controller (a) controls the frequency of the local oscillator (911); (b) stores words representative of the nominal digital frequency for the respective signals of the received RF signals; (c) stores words representative of the digital shift for the respective signals of the RF signals; (d) determining a frequency shift of said carrier signal after having tuned an RF signal; (e) updates all the representative words of the digital displacement according to the frequency offset of the carrier; (f) the representative word of tuning is derived for a selected RF signal to be tuned by combining such word representative of the nominal frequency for the selected RF signal to be tuned to the word representative of the previously updated offset for the RF signal selected for be tuned; and (g) updates the representative word of the respective digital offset for an individual RF signal that is being tuned if the correct tuning is not achieved with the displacement representative word previously updated.