Tv set tuning circuit

PATENT DESCRIPTION

Additional patent to patent no. Zgłoszotio: 82 08 19 (P. 237 963)

Priority: 81 08 19 United States of America

The notification was announced: 83 02 28

Patent description published: 19861128

Int. Cl *

H03J 5/09 H04B 1/29 H04N 5/50

Patent Office

Inventor: Gerald Earl Theriault

Patent holder: RCA Corporation,

New York (United States of America))

Tuning system for the television and

The subject of the invention is a tuning system for a television receiver, which is used to select one of many channels found in many frequency bands.

Known tuning systems for television sets usually use many heads, each of which has its own mixer to receive signals in many television frequency bands. For example, the first head selects channels in the VHF bands (54- ^ 88 MHz and 174-Ί2Ί6 MHz), while the second head selects channels in the UHF bands (476 (-1806 MHz). If the television is also required to receive wired television (OATV) signals, an additional third head and mixer may be required.

Tuning systems with a double transformation to eliminate the complexity and cost inherent in the use of a large number of transducers have been disclosed for VHF and UHF signals of radio (wireless) transmission, which is known, for example, from DL Ash's publication "High performance TV Receiver" quality ”), IEEE Transactions on Consumer Electronics, volume CE424, No. 1, February 1978, pp. 30i-46. However, there is still a need for simple and cheap tuning systems to receive VHF, UHF and CATV television signals.

The system according to the invention comprises a tuning signal generation control system connected to the first channel signal selection filter in the lower tuning band connected to the second channel signal selection filter in the upper tuning band. For the input of the first filter is⁵ a switch is connected and another switch is connected to the output of the first filter. However, a switch i is connected to the input of the second filter, a switch is connected to the output of the second filter.

¹⁰ The first filter and the second filter are attached to the third filter attached to the diplexer also attached to the outputs of the first filter and the second filter, and to the heterodyne via a mixer.

Suspension between the first switches¹⁵ ra input node and between the second switches the circuit contains an output node. The first switches contain two diodes connected to the two filter inputs. The second switches also contain two diodes connected to these outputs¹⁰ filters. The switches are connected by leads.

A control system is included with all diode switches.

The control system has reinforcements attached²⁵ the carrier whose input is attached to the output node and whose output is attached to the diplexer. Between the connection of the amplifier and the control system there are diodes connected to adjust the working potential. .

³⁰ Between the third filter and the diplexer is on

136 371

136 371 UHF amplifier. The first filter contains a double-tuned low-pass filter tuned by the first voltage-controlled diode. The second filter contains a double-tuned high-pass filter tuned by a second diode with voltage-controlled capacity.

The subject of the invention is illustrated in the embodiments in the drawing, in which Fig. 1 shows a block diagram of the tuning system for a television set, Figs. 2, 3, 5 and 6 - graphs ·. · - of the amplitude-frequency characteristics associated with the system of Fig. 1 and Figs. 4, 7 and 8 - diagrams of useful systems in the tuning system of Fig. 1.

In the double conversion tuning system of Fig. 1, television signals received at the UHF 10 antenna input, the VHF 30A antenna input and the CATV 30B input are fed to the diplexer 20. This system is a double conversion tuning system because two transformations are performed ( frequency shift). In the United States of America, these television signals correspond to the specific channel numbers and frequency bands shown in the following table:

Table 1

<td>TV band</td><td>Range the frequency wości &lt;MHz)</td><td>Number channel</td>

<td>Low VHF (L-VHF)</td><td> 64— 88</td><td> 2— «</td>

<td>Medium wired (MB-CATV)</td><td>00-h174</td><td>A-5 to I</td>

<td>High VHF (H-VHF)</td><td>Ϊ74-Π216</td><td> 7—13</td>

<td>Wired Superband (SB-CATV)</td><td> 216—402</td><td>J to W + 17</td>

<td>Radio UHF (UHF) '</td><td> 470-^800</td><td> 14(-33</td>

Each channel occupies a bandwidth of about 6 MHz in the frequency spectrum and has a video carrier with a frequency of 1.26 MHz greater than the lower cutoff frequency of the corresponding channel band. In the following description, specific⁵⁰frequency, it is believed that this frequency corresponds to the frequency to which the video carrier of the selected TV channel is shifted in a specific part of the described tuning system. ⁵⁵

The frequency spectrum of the channel in various television bands in the United States of America is shown in Fig. 2 (a). For the L-VHF 202, H-VHF 206 and UHF 210 bands, the amplitude of the received signals is shown as a set of⁶⁰levels indicating that the radio broadcast signals may vary widely, e.g. between 10 μν and 100 mV. On the other hand, the received CATV signals show significantly smaller changes in amplitude, usually between 1 and 6 mV,⁸⁵ as shown for the MB-CATV1204 band and the band

SB-CATV 208. ¹

Fig. 2 ('b) describes the low, high and UHF radio frequency bands associated with the filters 44, 36 and 14 of Fig. 1, which will be described later. The first intermediate frequency is selected to be 4) 15.75 MHz between SB-CATV and

UHF. It also occurs without the 420-450 MHz band and cannot interact with either¹⁰ be influenced by other sources / signals. The second intermediate frequency has a standard value of 45.75 MHz.

It is understood that although the invention is described in terms of different wireless bands ¹⁵ and wired used in the United States of America, Orf Limited for this system. For example, radio bandwidth signals can be supplied via a cable.

When the selected TV channel is in the UHF band, it is routed from the UHF antenna 10 to the diiplexer 20A input through the tuned UHF selective filter 14. The selective filter 14 receives the VT tuning potential at the 14C terminal so that it preferably transmits the frequency corresponding to the selected TV channel between input 14A and output 14B.

Fig. 3 shows the characteristic frequency 300 of selective attenuation, corresponding to the selection of TV channels at a relatively low frequency UHF ft. Selective filter 14 usually has low-pass characteristics with less attenuation and lower₃₅ 302 frequency curve 300 and relatively higher attenuation in the upper frequency 304. Selective filter 14 preferably passes signals with a frequency close to the frequency ft of the selected channel, which is indicated by the peak in the band? 306. The bandwidth 306 is determined by the intersections of the characteristic 300 with a dashed line 308 denoting attenuation 3 decibels greater than in the bandwidth.

The 300 'characteristic corresponds to the 300' characteristic when the frequency ft of the selected channel corresponds to the relatively higher frequency f't. For the selective filter 14, the 306 bandwidth is about 25 MHz, while the frequency f_t corresponds to the UHF channel 14, and the 306 'bandwidth increases slightly to about 40 MHz when the frequency Ft corresponds to the UlHF channel 83 for the system described below in connection with Fig. 4. A particular embodiment of selective filter 14 is described below in connection with Fig. 4.

The blocking circuit 12 reduces the level of all 416 MHz signals received by the UHF antenna 10 because the first 416 MHz intermediate frequency is close to the UHF band. These unwanted signals should be either used externally or removed from the first intermediate frequency response. This reduces the delivery of unwanted first intermediate frequency signals to intermediate frequency systems. Damage circuit 12 wy136 371

5 »shows the high-pass frequency characteristic 500 shown in Fig. 5, in which there is little attenuation in part 504 at frequencies higher than t_c, which corresponds to the lowest frequency in the UHF band (about 470 MHz) and greater attenuation in the part 502 at lower frequencies. Around the intermediate frequency fi (416 MHz), the blocking circuit 12 exhibits the greatest attenuation, as shown in section 506 of the 500 curve.

Amplifier; UHF 16 carries signals from the input 14B of the selective filter 14 to the 20A input of the diplexer 20 and has a gain of 14 — tl (5 dB in the UHF frequency range and has an input and output impedance of about 50Ω. The amplifier 16 works only when a channel in the band is selected UHF, because the operating voltages VB3 of band switching (about 18 V) only occur if. Have been; selected channels in the UHF frequency band, as shown by level 260 in Fig. 2 (f).

Television signals in the VHF and CATV bands are divided into lower and upper truncation bands as follows. These signals occupy the frequency range over seven to one; tuning in a range with a frequency ratio of more than three to one is impractical because of the limited range of voltage-controlled capacitive diodes. Splitting the 54-402 MHz band has MB ^ CATV and H-VHF bands (at around 174 MHz) would require a tuning range with a frequency ratio of about 3 1/4 to one for the lower band. In the described system, the division of the truncation bands takes place for the frequencies in the MB-CATV band at about 1'59 MHz, as shown in Figure 2Kb (rta). Hence, each of the tuning bands, lower and upper, contains frequencies in the aspect ratio less than three to one.

'The selection of the lower and upper tuning frequencies is also influenced by the design assumptions of filters 36 and 44. When designing filters, it is very difficult to obtain a narrow band at higher frequencies' than at lower frequencies, it is also more difficult to obtain a constant bandwidth in a tuned filter in a wide frequency range . In order to minimize distortion and interference with adjacent channels, it is necessary to use a filter with a narrower band when the signal amplitudes in the channel change in a wide range (as for VHF signals of radio transmission) than when there is a smaller range of amplitude (as for signals CATV).

Because the boundary between the low and high frequencies has been set to ISO MHz in the MB-CATV band, Both L-VHF and H-VHF radio signals varying in a wide range occur at the ends of the low-frequency tuning system bands with more controlled MB-CATV signals and SB ^ CATVs occur at the upper ends of these bands. Hence, the frequency division of the filters used in the tuning system of the invention desirably gives satisfactory work in removing the difficulties associated with design requirements for tuned filters.

Television signals in the VHF and CATV bands are diode connected to the diplexer 20 as follows. In Fig. 1, the SIA switch can be switched to the BC-A position to feed signals from the VHF 30A antenna to the input 32A of the firewall 32 or can be connected to the CA-A position to provide OATV signals from the input terminal 30B.

The blocking circuit 32 is similar to the blocking circuit 12 described above in the relationship&gt; Fig. 5, except that its maximum attenuation range 502 occurs at a second intermediate frequency fi (about 46 MHz), which jeeF close to frequency f_c (about 45 MHz) corresponds to the lowest receiving frequency (it is 2 VHF). The barrier circuit 32 provides a signal in both lower bands (54 to ISO MHz) and upper bands (up to 402 MHz) to node 32B.

If the selected channel is in the upper band, then the VB2 voltage is applied to close the switches 34 and 38 and thus enable the high band filter 36 between the nodes 32B and 40A. However, if the selected channel is in the lower band, then VB1 voltage is applied to switches 42 and 46, which are then closed to enable the low band filter 44 between nodes 32B and 4ΘΑ.

The upper band tuned filter 36 exhibits a selective characteristic of the low current mountains in the figure 600 shown in Figure 6, in which greater attenuation occurs in the portion 604 of relatively higher frequencies. As a result, the filter 36 not only selects the frequencies corresponding to the selected ft channel, but also tends to eliminate signals at lower frequencies especially in the lower tuning band. The bandwidth for the curve 600 relative to line 608 corresponding to - 3 diB is shown. When higher frequency channels are selected, the filter bandwidth 36 increases. The characteristics'WO 'corresponds to the characteristic 600, when the' Ft frequency of the selected channel is a relatively higher frequency Ft. When the filter 36 is tuned in such a way that the frequency ft corresponds to channel 'F MB-CATV, the bandwidth 606 is about 1'8 MHz; when the Ft frequency corresponds to the W + 17 SB-CATV channel, the 606 * bandwidth is about 40 MHz.

The tuned low-band filter 44 exhibits the selective low-pass bandwidth 300 shown in Fig. 3 and described above in connection with the 14 UHF selective filter, except that its bandwidth can increase substantially more when lower frequency channels are selected. The 306 bandwidth is about 8 MHz when. the frequency ft corresponds to the VHF2 channel and 306 bandwidth^/ is equal to - about 20 MHz, when Ft corresponds to channel E MB-CATV. Filter 44 doesn't just choose. so the frequencies corresponding to the selected channel, but also causes the removal of signals with higher frequencies,

136 371 especially those found in the upper tuning band and at the first intermediate frequency.

The VHF amplifier 40 of Fig. 1 feeds signals from the 40A node to input 20B of the diplexer 20 and is essentially the same as the amplifier described above. ⁵UHF 10. He differs, however, in that His⁷ the control potential VB 12 is fed through a diode element OR containing diodes D12 1 D14, so that the Amplifier 40 receives as a control potential voltage VB1 or VB2 switch ¹⁰Bandwidth is always selected when a channel in the lower lhb upper tuning range is selected, but does not receive a control signal when a UHF channel is selected. Therefore, the node 40 can be disconnected from the diplexer input 20B when selecting¹⁵wounds There is a UHF channel.

Diplexer 20 of FIG. 1 receives radio frequency signals from the UHF paten signal at input 20A and radio frequency signals from the signal path at input 20B ²⁰VHF and CATV bands and connects these signal paths to carry these signals to the 20C input. In dipleloser 20, the system path includes a high-pass filter 22 connecting the 20A input to the node 24, to which the filter is attached²⁵Lowpass 26 connecting this node to the 20C output. Input 20B is connected to node 24 via multiple cascaded low-pass filters 26.

J Mixer 50 receives frequency signals ³⁰radio from the 20C output of the diplexer and signals with heterodyne frequencies from the amplifier 52 through the node 52A. In response, mixer 50 shifts the signal at the frequency of the selected channel to the signal at the first intermediate frequency.³⁵about 416 MHz (i.e. the first frequency transition).

The amplifier 52 provides a signal with a relatively higher frequency in the range of 10- / 16 dBm to the mixer 50 and maintains the impedance on the values ⁴⁰about 50 Ω at node 52A. When the amplifier 50 is controlled by talks with a higher frequency signal, the radio frequency signal from the diplexer 20 may also have relatively higher power without introducing additional distortion⁴⁵value.

It is preferred to choose a relatively high intermediate frequency of about 416 MHz and a high level of mixer control 50 for the reasons described above. It is also beneficial⁵⁰minimizing mixer 50 distortion by selecting the gain of circuits between antennas 10 and 30A with mixer 50 to obtain an acceptable noise figure at the receiver. In this case, the relevant⁵⁵the bands of the filter bands 14, 36 and 44 may be relatively larger for a double conversion tuning system compared to the smaller band widths that would be required for a single conversion tuning system to achieve ⁸⁰equivalent distortion and noise ratio. This advantage makes it possible to increase the filter bandwidth along with the frequency of the selected channel, as described above.

The first signal with intermediate frequencies ⁶⁵ at the output 50A of the mixer is therefore amplified by the amplifier 60. The amplifier 60 may include a two-section input filter, tuned to an intermediate frequency of 416 MHz with a bandwidth of about 1 * 2 MHz and a three-section output filter also tuned to an intermediate frequency of 416 MHz with a bandwidth of about 10 MHz . The amplified intermediate frequency signal at the output of amplifier 62A is then mixed with the 370 MHz signal obtained from heterodyne 04 on input 62B, through the mixer 62B through the mixer 62 in the second frequency conversion to produce a conventional 46 MHz signal. This second intermediate frequency signal is then fed to output 66 through filter 66.

The tuning control system 70 reacts to channel selection to generate the tuning potential VT and the potentials VB1, VB2 and VB3 band switching. The tuning potential VT, shown in Fig. 2 (c), usually varies between a low level of about 1/5 V; marked with dashed line 220 and high level of about 214 V, marked with dashed line 222. When the selected channel is in the lower tuning lane, the VT tuning potential goes towards the lower value at point 224 when channel 2 VHF is selected and goes towards the high value at point 226 when channel E MB-CATV is selected. When the selected channel is in the upper tuning band, the VT tuning potential also goes towards the lower value at point 228 when the F MB-CATV channel is selected and goes towards the high value at point 230 when the W + channel is selected (17 SB- Similarly, the VT tuning potential tends toward a low value at point 232 when UHF 14 channel is selected and towards a high value at point 234 when UHF channel 83 is selected. The band switching signals VB1, VB2 and VB3 occur at a high level of about 18 V, as shown by the characteristics 240, 250 and 260 in Figs. 2 (d), 2 (e) and 2 (f), only when selected the channel in the band to which they correspond and appear at 0 V when a channel outside this particular band is selected.

Three tunable voltage-controlled heterodines 54, 56 and 58 are used to generate heterodyne frequency signals in three tuning bands. This is done so that the frequency of each heterodyne 54, 56, 56 can be conveniently changed to track the frequency tuning of the associated filter 14, 36, 44, responding to the same VT tuning potential. The frequency range at which signals are to be fed to the amplifier 52 at node 52B is shown in the following table.

Heterodines 54, 56 and 58 receiving the VB1, VB2 and VB3 signals respectively as band switching potentials, so that they are controlled only when the selected channel falls in the frequency band in which the particular heterodyne is bound.

'The respective embodiments of the filters 14, 36 and 44 shown in Fig. 4 17 will now be described.

136 371

18

Table 2

<td>Band</td><td>Channel number</td><td>Heterodyne frequency in MHz</td>

<td>Band</td><td>2 (L-VHF)</td><td> 471</td>

<td>lower</td><td>0 (L-VRF)</td><td>4i99</td>

<td>heterodyne</td><td>A-5 (MB-CATV)</td><td> 507</td>

<td> 58</td><td>E (iMB-CATV)</td><td> 501</td>

<td>Band</td><td>F (MB-CATV)</td><td> 567</td>

<td>upper</td><td>I (MB-CATV)</td><td> 585</td>

<td>heterodyne</td><td>7 (VHF-H)</td><td> 591</td>

<td> 56</td><td>19 (VHF-L)</td><td> 027</td>

<td></td><td>J (SB-CATV)</td><td> 033</td>

<td></td><td>W + 17 (SB-CATV)</td><td> 013</td>

<td>Band</td><td>{14 UHF)</td><td>AARZ</td>

<td>UHF heterodyne 54</td><td>83l (UHF)</td><td> 1301</td>

The UHF band 14 filter shown in Fig. 4 is a double-tuned low-pass filter with inductive coupling on the upper side between the 14A input and the 14B output by connecting the coils in series Ι&gt; 4Ο2ζ L406, L418 and L414. The C408 capacitor serves as a DC blocking capacitor with negligible impedance variable waveforms at UIHF frequencies. Coils L404 and L406 serve as a system of coils with catches to maintain impedance at the 14A input of about 50 Ω.

Similarly, the coils L410 and L412 serve as a coil system with taps to maintain impedance at the 14B output of about 50 Ω. The input and output coils L402 and L414 "help to maintain a substantially constant bandwidth over a wide filter tuning range 14. The € 5404 capacitor is connected in parallel with the L408 coil for resonance at around 1000 MHz.

Variable frequency tuning is given by C5D42 and C5D44 variable capacity diodes, connected respectively from the ends of the tuned circuit L408— € 5404 to ground by the coupling capacitors C402 and (5406, which show very low impedance at the frequencies of the TV signal passed through filter 14. Potential VT tuning at the 14C terminal is supplied to change the capacitance of the CIM2 and C1M4 diodes, respectively, through the isolating resistors R402 and R404. The VT potential can vary from 1.6 to 24 V for UHF 14- ^ 93 channels.

Fig. 7 is a detailed schematic of switches 34, 38, 42 and 44 of tuned filters 38 and 44. Switch 34 conducts television signals from node 32B to input 36A through blocking capacitors € 702 and C7O4 when pin SD72 is put into conduction by feeding VB2 voltage switching the band through the L702 choke. Resistor R702 controls the forward current flowing through the SD72 diode when the channel in the upper band is selected. At this time, the SD74 pin diode is reverse-polarized by the potential across the R702 resistor. When the selected channel is in a band other than the upper band, the SD74 diode is polarized in the forward direction either by the voltage VBl adapted to the diode D84 or by the voltage VB8 supplied⁵ for diode D78. The forward current flowing through the SD74 diode is determined by the RT02 resistor and the potential on it provides the reverse polarization of the SD72 diode. The R7O4 resistor creates a return path when the SD74 diode is reverse-polarized *¹⁰ Capacitor C706 is a blocking capacitor, giving a low impedance connection at TV frequencies between the SD72 cathode and ground when the SD74 diode conducts.

Switch 38 is of the same type as for¹⁵ connector 34 and they are put together in a conductive or non-conductive state. Switches 42 and 46 are also similar to switch 34, however they are put into conduction only when a channel in the band is selected²⁰ lower. Suitable terminals and components that perform similar functions in switches 34, 38, 42 and 46 are listed in the following table.

High-band tunable filter 36 exhibits * high-pass characteristics due to a series connection of resistors Ć708, CJ710 and diodes CD72 and CD74 between input 36A and output

Table 3

<td>Switch 34</td><td>Switch 38</td><td>Switch 42</td><td>Switch ik 46</td>

<td> 1/702</td><td>L714</td><td>LT20</td><td> 1/732</td>

<td>C7O2</td><td> 07114</td><td>CT0O</td><td>C784</td>

<td>C7O4</td><td> 0712</td><td> 0702</td><td>CT3 &amp;</td>

<td> 0706</td><td> 0716</td><td> 0704</td><td> 0708</td>

<td>RT! "</td><td>r712</td><td>RTtóO</td><td>RTO8</td>

<td>RT04</td><td></td><td>RT702</td><td>RTO0</td>

<td>SD7E</td><td>SDTO</td><td>SD92</td><td>SD88</td>

<td>SD74</td><td>SO76.</td><td>SDB4</td><td>SD86</td>

<td>D78</td><td>D8G</td><td>D76</td><td>DM</td>

<td>D84</td><td>D86</td><td>DT2</td><td>D74</td>

<td>34A</td><td>39A</td><td>40A</td><td>46A</td>

<td>3βΑ</td><td>35Β</td><td>44A</td><td> 44®</td>

36B and the "bottom side * of the induction coupling ce60 v L708, L708 and L710. The section containing the capacitors € 5708, C710 and L708 has high-pass characteristics tuned to eliminate signals in the lower band. Change of tuning is carried out by means of a change of responses⁵⁵ appropriate capacities of the CD72 and CD74 diodes with variable capacities reacting to the barrier polarization provided by the resistors R708 and R708 respectively from the VT tuning potential at the 80C input. The capacity of the C5D72 capacitor causes resonanceⁱⁿ with L704 and L7G6 coils, while the capacitance of the C5D74 capacitor causes resonance with the coils

L710 and L712.

The low band tunable filter 44 is inductively coupled on the upper side * and is similar * to the UHF 14 filter described above * Coil L726

13 «371 and capacitor C728 are tuned for resonance at about MHz, which blocks signals in the upper band. This frequency is then tuned to a lower value by supplying the VT tuning potential from the 4AC input to the CD76 and CD78 voltage tuned diodes. Capacitors C726 and C730 give paths with relatively low impedance at television frequencies between the corresponding cathodes of CD76 and CSD78 diodes and ground. Coils L722 and L724 serve as a coil system with catches just like coils L730 and L728.

Modification of the invention may be considered. For example, the SIA switch has been shown as a mechanical switch, because it is assumed that the TV set is normally used with the VHF 30A antenna or the CATV 30B connection, but not both at the same time, because programming of the VHF channel is usually also carried out by the CATV source. However, if fully automatic reception is required, the SIA switch can be replaced by a relay or switch on a pin diode, controlled by the band switching voltage or by a diplexer.

Then the UHF 16 amplifier and the VHF 46 amplifier can be removed and replaced with switches ^ on the pin diodes, similar to switch 34, if a single amplifier is placed on the 20C output between the diplexer 26 and the mixer 50.

The particular modification required when the first intermediate frequency is around '416 MHz is shown in Figure 8. Television signals for the 12 VHF channel have a video carrier at about 205 MHz and a sound carrier at about 210 MHz. Because the second harmonics of these carrier frequencies, as well as the sum signals of these frequencies are close to frequencies¹Intermediate, it is desirable to damp these carriers. To this end, the coil L706 described above in connection with Fig. 7 is selectively coupled to form a barrier circuit except in those cases where channel 12 or adjacent channels 11 and 13 are selected.

The L806 coil is positioned so that its inductance is coupled with two turns of the L706 coil closest to the G mass. As a result, the C801 capacitor is mapped by the transformer connected between the L806 and L706 coils as a Serial with the part of the L766 farthest in relation to the G mass. This connection serves as a series barrier circuit and is tuned in on IB, giving attenuation of approximately 20 dB at 210 MHz and approximately 10 dB at 2Q5 MHz. Part of the + V control voltage generated by the voltage divider containing resistors K861 and R862 is fed to the switching anode of the SD801 diode. The C802 capacitor connects the upper connection of the 0801 capacitor and L806 coil to ground at TV frequencies at which it has negligible low impedance, similarly, the 0863 capacitor connects the SD861 diode cathode to ground.

To receive the radio broadcast, the SOS switch, which is the second pole of the SIA switch described above in connection with Fig. 1, is in the BO-B position to supply + V voltage to the cathode of the SD801 diode through the switch S2 and the resistor R803 for reverse polarization SD861 LEDs. For the reverse-polarized SD801 diode, the C861 capacitor is connected as described above. When one of the channels 11, 12 or 13 is selected, the switch S2 moves from the IN position to the IN position to polarize the SD861 diode in the direction of conduction by connecting its cathode to ground. As a result, the C801 capacitor is connected to ground via negligibly small impedances of the SD801 diode, CJ803 and C802 capacitors, basically shorting the 0801 capacitor, and therefore disconnects the just described barrier circuit C601-L706.

In order to receive CATV signals, the S1B switch is moved to the OA-B position for similarly disconnecting the C801-L706 blocking circuit.