JPH0340974B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Technical Field of the Invention] The present invention relates to a channel selection method in a television receiver.

[Technical background of the invention and its problems]

In order to set the optimum reception frequency using up and down signals and station detection signals from an AFT (Automatic Fine Tuning) means, the tuning device of a television receiver has generally been constructed as shown in FIG.

The signal received from the antenna 1 is amplified by a high frequency amplifier 2, converted to an intermediate frequency by a local oscillation frequency of a VCO 4 and a frequency mixer 3, amplified by an intermediate frequency amplifier 5, and then a video signal is obtained by a video detector 6. At the same time, the signal is separated into horizontal and vertical synchronization signals by a synchronization separation circuit 7. Video detector 6
After extracting the video carrier wave and passing it through the limiter,
An AFTS curve output is obtained by the AFT detector 8.
This S curve output is supplied to the AFT signal generator 9,
The AFT signal generator 9 generates an up signal when the video center frequency is below a predetermined frequency, and a down signal when the frequency is above the predetermined frequency.
On the other hand, the output of the synchronization separation circuit 7 is supplied to the station detection signal generator 10, and when the station detection signal generator 10 detects the presence of a receiving station, it sends out a station detection signal (hereinafter referred to as an SD signal). do. These UP, DOWN signals and SD signals are sent to the control circuit 11.
The presence or absence of a station is determined based on the SD signal,
If the AFT up signal exists, the local oscillation frequency of the VCO 4 is increased, and if the AFT down signal exists, the local oscillation frequency is controlled to be decreased. FIG. 3 shows this SD signal and AFT up and down signals, and the predetermined value range is set to ±2MHz. In this figure, period a when the SD signal is H and both AFT up and down signals are L is the optimal receiving frequency range, but period b near (α±2) MHz (α: video center frequency), A similar situation exists in c, and the optimal reception frequency range may be mistaken for periods b and c. Especially, if there is an offset from the transmission frequency, it may be difficult to receive in the regular reception frequency range period a. Ta. To explain this further, for example, the frequency relationship when receiving three channels of television broadcasting in the United States is as shown in Figure 3, where the video frequency is 61.25MHz and the audio frequency is 65.75MHz, while the local oscillation frequency OSC is If it is 107MHz, the video frequency and audio frequency are respectively 5th.
As shown in the figure, 45.75MHz is converted to an intermediate frequency of 41.25MHz to obtain a 3-channel signal, and the SAW filter shown in the figure extracts only the received 3-channel signal and removes other unnecessary channel signals. is suppressed. The AFT detector 8 performs FM detection at the video center frequency of 45.75 MHz, but if the local oscillation frequency is moved above or below 107 MHz, the S-curve of its output changes as shown in FIG.
Note that FIG. 6 shows the characteristics when there is no offset in the three channels. At this time, when the local oscillation frequency is 107MHz, the S curve is centered,
In other words, adjust the video carrier wave to 45.75MHz,
When the window width shown in Fig. 5 is set relative to the center, the AFT signal generator 9 consisting of a comparator outputs an up signal when the window width is above the window width (on the left side of the S curve); (on the right side of the S curve), it can be determined whether to output a down signal to increase or decrease the local oscillation frequency. However, SAW
Due to the band characteristics of the filter, the S curve is not vertically symmetrical, and as shown in FIG. 5a, the upper frequency (the right side of the S curve according to the video frequency) becomes narrower.
Therefore, only the conventional S curve mentioned above is used.
In the case of AFT equipment, the width that can be pulled in by AFT is asymmetrical above and below the image center frequency, and there is a limit (approximately 1MHz) at the upper frequency.

Therefore, as shown in Japanese Patent Application No. 58-051503 previously filed by the applicant in order to eliminate the above-mentioned drawbacks, the down signal is detected after the SD signal is detected, and then the center frequency is adjusted by fine tuning. Then, lower the local oscillation frequency by 1 step by frequency fs, confirm that the up signal is detected, then increase the local oscillation frequency by 2 steps by frequency 2fs,
After confirming that a down signal is detected, the local oscillation frequency is finally lowered by one step to complete tuning.

However, the above method has the disadvantage that it takes a very long time to complete the channel selection.

This is because it takes several tens of milliseconds to change the local oscillation frequency, check the SD signal, etc.

Furthermore, AFT up signal, down signal,
Since channel selection is not possible unless the sensor signal is precisely detected, the AFT voltage detection circuit is required to have higher performance, which increases the number of parts, increases cost, and requires adjustment of the AFT curve.

Also, when starting channel selection, set the local oscillation frequency to the frequency on the channel plan, and if an SD signal is detected, confirm that the AFT signal is the center signal, and then confirm that the signal is a regular signal. The search ends only after confirmation, so if the adjustment of the AFT curve deviates by more than ±fc/2 from the local oscillation frequency on the channel plan, or if a VTR or video disc playback device, etc. is connected as the tuner source. In this case, if the frequency of the signal differs from the frequency on the channel plan by more than ±fc/2, it cannot be received without searching, and the signal must be searched at least once before being received. It has the drawback that it takes time to receive the data, and furthermore, the image is distorted due to searching.

[Purpose of the invention]

The present invention has been made in consideration of the above, and eliminates the above drawbacks, allows reception of signals with an offset of ±f _DN /2 without searching, has fast tuning operation, and is simple and convenient for AFT signal generators. The purpose of the present invention is to provide a channel selection method that requires a low-cost circuit and can further reduce the need for AFT curve adjustment.

[Embodiments of the invention]

The present invention will be explained below with reference to Examples.

The device to which the method of the present invention is applied has the configuration shown in FIG. 2, and the AFT signal generator 9 is provided with a window of a predetermined width, and this window allows the AFT voltage to be centered for input signals without offset. It is set to output. That is, Figure 1a
As shown in the figure, the AFT signal generator 9 outputs an AFT up signal (hereinafter simply referred to as up output), an AFT down signal (hereinafter simply referred to as down output), and a center output between the up output and the down output. Output. The center output is in the frequency range fc, the up output is in the frequency range fup, and the down output is in the frequency range
It is set to correspond to f ^-6 _N.

Up output output from AFT signal generator 9,
Center output, down output, and station detection signal generator 1
The station detection signal (SD signal) output from 0 is supplied to the control circuit 11, which detects the presence or absence of these four types of signals, compares them, and sets the oscillation frequency of the local oscillator 4. are doing.

The operation of the control circuit 11 is as shown in the flowchart of FIG. 1b. When tuning begins, the local oscillation frequency is set to the local oscillation frequency on the channel plan + (f _DN + fc)/2 (step a ₀ ). Step a Detects whether there is a down output following ₀ (step a). Here, the presence of the down output is detected first because the up output may be present even when no station is present, and the down output is always present when the station is present. .

In step a, when a down output is detected, the local oscillation frequency is lowered by two steps (2fs) (step b), and it is detected whether it is within the search range (step c). Here fs is fs〓f _DN
It is set to . If the down output is not detected in step a, the local oscillation frequency is increased by one step (step d), and it is detected whether it is within the search range (step e). If it is detected in step e that it is outside the search range, the local oscillation frequency is set to the lower limit of the search range (step f),
Next, step a is executed, and when it is detected in step e that the search range is within the search range, step a is executed following step e.

Therefore, by steps a, d, e and f,
The local oscillation frequency is increased by one step (fs) from the bottom of the search range until a down signal is detected, and if no down signal is detected and the search range reaches the upper limit, the search range is increased. The search starts from the lower limit of , and is repeated until a down signal is detected.

When it is detected in step c that it is outside the search range, the local oscillation frequency is increased by one step (fs) from the local oscillation frequency at the time of the first down output detection (step g), and it is detected whether it is within the search range. (Step h). When it is detected in step h that the search range is within the search range, step a is executed following step h. Further, if it is detected at step h that the search range is outside the search range, step f is subsequently executed.
Execute.

When it is detected in step c that it is within the search range, the presence of a down output is detected (step i). When a down output is detected in step i, step b is executed. When the down output is detected in step i, it is detected that the center output is detected (step j).
Therefore, by steps a, b, g, h, and i, when a down output is detected, the local oscillation frequency is lowered by two steps (2fs) until a center output or an up output is found. If the down output is not found at the lower limit of the range, steps b, e, f, and a are repeated again from the local oscillation frequency at which the down output was detected and the local oscillation frequency started to be lowered.

When the center output is detected in step j, the presence of the SD signal is subsequently detected (step l). If the center output is not detected in step j, the local oscillation frequency is increased by one step (step k), and step l is subsequently executed.

If the SD signal is not detected in step l, step g is executed. At step l
When the SD signal is detected, following step 1, the presence of the up output is detected (step m).

Furthermore, by steps i, j, k and l,
If there is a center output or an up output (if the down output is not detected in step i and the center output is not detected in step j, then the up output is present), and the up output increases the local oscillation frequency by one step (fs) if the Then, the search ends. Next, fine tuning in a frequency width _FF which is narrower than one step is performed in steps m to s, which will be described later.

When the up signal is detected in step m, the local oscillation frequency is changed to
f Increase by _F (step n). here the frequency
f _F has a frequency width smaller than the one-step frequency of the local oscillation frequency, resulting in fine tuning. Following step n, it is detected whether it is within the hold range (step o). step o
If it is detected that it is outside the hold range at step f, execute step f, and if it is detected at step o that it is within the hold range, execute step f.
Detect the presence of an SD signal (step p).

If the up output is not detected in step m, it is detected whether the down output is detected (step q). When a down output is detected in step q, following step q, the local oscillation frequency is decreased by the frequency f _F (step r),
Subsequently, it is detected whether it is within the hold range (step s). If it is detected in step s that it is outside the hold range, step f is executed following step s. When it is detected in step s that it is within the hold range, and when no down output is detected in step q, step p is executed. step p
When the SD signal is detected in step m
is executed, and if no SD signal is detected, step f is executed.

In other words, in finching, for example, when a down output is detected, the local oscillation frequency is lowered in steps of frequency f _F until the center output is detected, and when an up output is detected, the local oscillation frequency is lowered in steps of frequency f _F . The local oscillation frequency is increased until the center output is detected in each step, and while the desired station is being received, step m → step n → step o → step p → step m or step m → step q → (→ step The process loops from step r to step s) to step p to step m. At this time, if the local oscillation frequency exceeds the hold range or if the SD signal is no longer detected, the search will be started again from the lower limit of the search range.

Therefore, in one embodiment of the present invention, when selecting a channel, the local oscillation frequency + (f _DN + fc/
Since the center output or up output is detected by lowering the local oscillation frequency by two steps, the capture range that can be selected without searching is ±f _DN /2 from FIG. 1a.

By the way, the capture range in the conventional case is ±
fc/2. Furthermore, since fc is actually about 150 KHz and _fDN is about 800 KHz, the capture range is improved by more than five times in this embodiment.

Further, the adjustment range of the AFT curve can be ±f _DN /2 instead of ±fc/2 if performance is not taken into consideration, so that no adjustment or adjustment steps can be performed.

Incidentally, in the above embodiment of the present invention, a television receiver is described; however,
It can also be applied to PLL synthesizers in general.

Furthermore, although the present invention has been described in the case of the upper local oscillation frequency, the direction of frequency change in the above embodiment may be reversed also in the case of the lower local oscillation frequency.

Furthermore, it is not restricted by the system of the television receiver.

Also, when selecting a station, set the local oscillation frequency to (f _DN + fc)/
Although an offset of 2 is used, if the offset is in the range of fc/2<offset amount<fc/2+ _fDN , the same effect can be obtained although the capture range changes.

〔Effect of the invention〕

As explained above, according to the present invention, when selecting a channel, the local oscillation frequency is set to the frequency on the channel plan + offset frequency, so the capture range is widened, and then only the down output is detected, then the center output or up output is detected, and then the capture range is widened. Since the search is completed by confirming the presence of the SD signal, the channel selection operation can be performed quickly.
Further, after detecting the down output, it is sufficient to detect either the center output or the up output, so there is no requirement for strictness in the up output, the center output, and the down output. Therefore, the AFT signal generator is simple and
It will be cheaper. Furthermore, since the up output, center output, and down output are not required to be precise, there is no need to adjust the AFT curve.

[Brief explanation of the drawing]

FIG. 1a is a diagram showing an AFT curve of an AFT signal generator in an embodiment of the present invention, FIG. 1b is a flowchart showing an embodiment of a channel selection method according to the present invention, and FIG. A block diagram showing the configuration of a channel selection device capable of performing a channel selection operation according to the present invention,
Figure 3 is a diagram showing the station detection signal, up output, and down output to explain the conventional channel selection method, Figure 4 is a diagram showing an example of video and audio frequencies of broadcasting stations, and Figure 5 is a diagram showing the station detection signal, up output, and down output. FIG. 6 is a diagram showing the detected frequencies of the video and audio signals in FIGS. 3 and 4. FIG. 6...Video detector, 7...Synchronization separation circuit, 8...
...AFT detector, 9...AFT signal generator, 10...
...Station detection signal generator, 11...Control circuit.

Claims (1)

[Claims] 1 The AFT voltage is set so that it falls within a window for a received signal without offset, outputs a center signal for signals within this window, outputs a down signal for signals exceeding the center signal, and In a channel selection method for a television receiver equipped with an AFT signal generator that outputs an up signal for signals below the center signal and a station detection signal generator that outputs a station detection signal that detects the presence of broadcasting, The local oscillation frequency at the start of tuning is added to the local oscillation frequency on the channel plan by 1/2 of the frequency range fc where the center signal is output and 1/2 of the frequency range f _DN where the down signal is output. It scans sequentially from this frequency in steps of a predetermined frequency width during the period in which the down signal is detected, and when the down signal is no longer detected, the presence or absence of the center signal and up signal is detected, and the center signal or up signal is detected. A channel selection method for a television receiver, characterized by detecting the presence or absence of a station detection signal when either one of the signals is detected, and entering fine tune tuning when the station detection signal is detected.