Signal line changeover circuit

Abstract: A signal line changeover circuit includes a first signal processing block having a first emitter follower for outputting a signal, a second signal processing block having a second emitter follower for outputting a signal, and a switch block having a first terminal connected to an output terminal of the first emitter follower through a first signal line, a second terminal connected to an output terminal of the second emitter follower through a second signal line, a third terminal for outputting a signal, a selection switch for alternatively connecting the first and second terminals to the third terminal, a first switch connected between the first terminal and a power line and turned on when the selection switch selects the second terminal and a second switch connected between the second terminal and the power line and turned on when the selection switch selects the first terminal.

Description: BACKGROUND OF THE INVENTION Field of the Invention The present invention relates to signal line changeover circuits, and more particularly, to signal line changeover circuits for use in integrated circuits. Description of the Prior Art For example, among color signal processing circuits of a video cassette recorder (VCR) consisting of the integrated circuit (IC), an automatic color control (ACC) circuit has, as shown in Fig. 1, a portion common to both a recording system and a playback system. When a signal is inputted to the common portion, a changeover of the input signal is performed by a selection switch (recording/playback changeover switch). In the Fig. 1, solid lines show the recording system, while alternate long and short dash lines show the playback system. Blocks 3, 6, and 8 are common to the recording and playback systems. In the recording system, a color signal inputted into an input terminal 1 passes through a band pass filter 2 with a center frequency of 3.58MHz, and thereafter, is provided to a gain variable amplifier 3 for ACC. An output of the gain variable amplifier 3 is decreased in frequency to 629kHz by a balanced modulation circuit 4, and thereafter, is directed to an output terminal 60 by way of a low pass filter 5. The output of the gain variable amplifier 3 is also provided to a color burst level detection circuit 8 through a selection switch 7 in the switch block 6. An output of the color burst level detection circuit 8 is supplied to the gain variable amplifier 3 so as to control a gain of the gain variable amplifier 3. In the playback system, a playback color signal inputted into an input terminal 9 passes through a band pass filter 10 with a center frequency of 629kHz, and thereafter, is provided to the gain variable amplifier 3. In this case, an output of the gain variable amplifier 3 is converted into a signal which has a carrier of 3.58MHz by the succeeding balanced modulation circuit 4. Then, after passing through the band pass filter 2 of 3.58MHz and a comb filter 11, it is amplified by an amplifier 12, and thereafter, is directed to an output terminal 13. The color signal directed to the output terminal 13 is further directed to a Y/C mixer (not shown) connected to the output terminal 13 to be combined with a luminance signal. In Fig. 1, the selection switch 7 in the switch block 6 is set to a contact R side during a recording operation so that the output of the gain variable amplifier 3 is directed to the detection circuit 8. During a playback operation, the switch 7 is set to a contact P side so that an output of the amplifier 12 is directed to the detection circuit 8. Fig. 2 shows a main portion of the circuit shown in Fig. 1, with which a problem will be described. In Fig. 2, during a recording operation when the selection switch 7 is set to the contact R, a color signal from the gain variable amplifier 3 is supplied to the color burst level detection circuit 8 through the switch 7, while the output of the amplifier 12 is blocked by the switch 7 and is not coupled to the detection circuit 8. Moreover, during a playback operation when the selection switch 7 is set to the contact R, a color signal from the amplifier 12 is supplied to the detection circuit 8 through the switch 7, while the output of the gain variable amplifier 3 is blocked by the switch 7 and is not coupled to the detection circuit 8. Since an output signal line 14 of the gain variable amplifier 3 and an output signal line 15 of the amplifier 12 run long to the switch block 6 in the IC, there arises problems that an un-used output signal line influences the other signal line and that it influences an adjoining third circuit on the way to the switch block 6. The problems are conspicuous particularly with respect to a circuit where signal lines are densely arranged such as a large scale integrated circuit. To solve the above-described problems, a method may be used where a switch is provided to each signal block (in Fig. 2, the gain variable amplifier 3 and the amplifier 12) in order that no signal is outputted to an output signal line when the signal of the block is unnecessary. In that case, however, not only the size of each block (3, 12) should be increased but also another control line should be provided for controlling the switch provided to each block, whereby the structure of the IC becomes more complicated because of the increased number of lines as well as a degree of integration decreases. JP-A-60192 discloses an analog switch circuit comprising a resistance and a diode, which is used instead of a load circuit of an PNP-transistor as a load of a differential circuit. The diode is connected to compensate the potential difference between the base of an emitter of an output terminal. Since the high-response speed and stable resistance load R is used the responsiveness for current switching is good. Further, since the diode is connected commonly to two side circuits the collector current of the terminal is always followed. Therefore, the speed is made high because the internal impedance of the diode can be always made low. Since the output impedance is lowered, the circuit is not affected by the variance of a load connected to a terminal. SUMMARY OF THE INVENTION An object of the present invention is to realize a signal line changeover circuit with a simple arrangement where a signal line which is not selected by a selection switch does not influence circuits adjacent thereto. This object is achieved according to the present invention by a signal line change over circuit according to claim 1. The dependent claims are related to further advantageous aspects of the present invention. According to the above-described feature, since no signal is provided to a signal line which has not been selected by the selection switch between a signal block thereof and a switch block, the other signal line and other circuits are not influenced. Furthermore, although an emitter follower which is disabled in order that no signal is outputted to the unselected signal line exists in the signal block, there is no need to increase the number of lines since the unselected signal line is used for the disabling control. BRIEF DESCRIPTION OF THE DRAWINGS This and other objects and features of this invention will become clear from the following description taken in conjunction with the preferred embodiments with reference to the accompanied drawings in which: Fig. 1 is a block diagram of a color signal processing circuit for VCRs employing a conventional signal changeover circuit; Fig. 2 is a block diagram plainly showing a main portion of Fig. 1; Fig. 3 is a block diagram of a main portion of a color signal processing circuit for VCRs employing a signal changeover circuit of the present invention; and Fig. 4 is a circuit diagram showing a part of Fig. 3. DESCRIPTION OF THE PREFERRED EMBODIMENT In Fig. 3 showing an embodiment of the present invention, portions the same as those of the prior art shown in Fig. 2 are represented by the same reference designations. In this embodiment, emitter followers 20 and 21 are provided on the output side of a signal block 3a including a gain variable amplifier 3. Similarly, emitter followers 22 and 23 are provided on the output side of a signal block 12a including an amplifier 12 as shown in the figure. The emitter follower 20 of the block 3a provides an output of the gain variable amplifier 3 to a balanced modulation circuit 4. The emitter follower 21 provides an output of the gain variable amplifier 3 to a contact R of a selection switch 7 in a switch block 6. On the other hand, the emitter follower 22 in the block 12a provides a color signal amplified by the amplifier 12 to an output terminal shown in Fig. 1 and a Y/C mixer, etc. (in Fig. 3, arranged in a block 16 on the next stage) connected thereto. Moreover, the emitter follower 23 provides the color signal amplified by the amplifier 12 to a contact P of the selection switch 7 in the switch block 6. Signal lines 14 and 15 connected between the emitter followers 21 and 23, and the contacts R and P of the selection switch 7, respectively, are coupled to a power line 26 at the switch block 6 through switches 24 and 25. The switches 24 and 25 as well as the selection switch 7 are controlled led by a switch control circuit (not shown) provided in the switch block 6. The control is performed in synchronism with the above-described control of the selection switch 7. During a recording operation, the selection switch 7 is set to the contact R so that the output of the gain variable amplifier 3 is directed to the detection circuit 8. When that happens, although the switch 24 is OFF, since the switch 25 is ON, a voltage Vcc of the power line 26 is applied to an emitter of the emitter follower 23 in the block 12a through the switch 25 and the signal line 15. Thereby, the emitter follower 23 is disabled, so that the amplifier 12 outputs no signal to the signal line 15. When that happens, the emitter follower 22 can be activated without being influenced by an operation of the switch 25. During a playback operation, the switch 25 is OFF and the selection switch 7 is set to the contact P, so that the output of the amplifier 12 is supplied via the signal line 15 to the detection circuit 8 through the switch 7. When that happens, the switch 24 is ON so that the voltage Vcc of the power line 26 is applied to an emitter of the emitter follower 21 in the block 3a. Thereby, the emitter follower 21 is disabled, so that no signal is outputted to the signal line 14 by the gain variable amplifier 3. The emitter follower 20 remains ON since it is not influenced by the switch 24, and provides the output of the gain variable amplifier 3 to the balanced modulation circuit 4. As described above, according to this embodiment, since when a signal is selectively supplied to the detection circuit 8 common to the two signal blocks 3a and 12a, no signal is provided from the signal blocks to a signal line which has not been selected by the selection switch 7, the unselected signal line does not influence the other signal line and other circuits. In the above-discussed embodiment, NPN-type transistors are used for the emitter followers. When PNP-type transistors are used, the circuit may be designed so that electric potential of a contact is provided to an emitter of an emitter follower by the switches 24 and 25. Fig. 4 shows a case where a main portion of the circuit shown in Fig. 3 is constituted by IC. In the figure, a constant current source 28 is provided between the selection switch 7 and the ground. The selection switch 7 connects the constant current source 28 to the contact P or R. Practically, the selection switch 7 and switches 24 and 25 are constituted by devices such as transistors. According to the present invention, as described above, since no signal is provided to a signal line which has not been selected by a selection switch from a signal block including the line, the other signal line and other circuits are not influenced. Furthermore, although an emitter follower which is disabled in order that no signal is outputted to the unselected signal line exists in the signal block, there is no need to increase the number of lines since the unselected signal line is used for the disabling control. Thus, an effect is obtained that an entire circuit does not become complicated in structure and large in size. While a case where two signal lines are changed-over has been described in the above embodiment, a signal changeover circuit of similar arrangement as that of the above embodiment may be employed for a case where three signal lines are changed-over. Obviously, many modifications and variations of the present invention are possible in light of the above teachings. It is therefore to be understood that within the scope of the appended claims, the invention may be practiced other than as specifically described.

Claims: A signal line changeover circuit comprising: a first signal processing block (3a) for outputting a signal; a second signal processing block (12a) for outputting a signal; and a switch circuit (6) including a first terminal (R) connected to an output terminal of said first signal processing block (3a) through a first signal line (14), a second terminal (P) connected to an output terminal of said second signal processing block (12a) through a second signal line (15), a third terminal for outputting a signal, and a selection switch (7) for alternatively connecting said first and second terminals (R, P) to said third terminal, characterized in that said first and second signal processing blocks (3a, 12a) include a first emitter follower (21) and a second emitter follower (23) respectively, the first and second terminals (R, P) are connected to the output terminals of said first and second emitter followers (21, 23) through said respective signal line (14, 15) respectively, and a first switch (24) is connected between said first terminal (R) and a power line (26) and turned on when said selection switch (7) selects said second terminal (P), and a second switch (25) is connected between said second terminal (P) and said power line (26) and turned on when said selection switch (7) select said first terminal (R). A signal line changeover circuit according to claim 1, wherein a common circuit (8) which operates similarly in response to an output signal of said first signal processing block (3a) and in response to an output signal of said second signal processing block (12a) is connected to said third terminal. A signal line changeover circuit according to claim 2, wherein said common circuit (8) is a color burst level detection circuit for automatic color control which is common to a recording system and a playback system in a video cassette recorder, wherein said first signal processing block (3a) is a circuit for providing a color burst for a recording operation and wherein said second signal processing block (12a) is a circuit for providing a color burst for a playback operation. A signal line changeover circuit according to claim 1, wherein said first and second signal blocks (3a, 12a) and said switch circuit (6) are formed with an integrated circuit.

Inventors: KAMEI SHINJI, KAWAMURA YASUNORI, KAMEI, SHINJI, KAWAMURA, YASUNORI, KAMEI, SHINJI, C/O ROHM CO., LTD., KAWAMURA, YASUNORI, C/O ROHM CO., LTD.
Applicants: ROHM CO LTD, ROHM CO., LTD.
Application year: 1992
Main classification: H03K17
IPC codes: G11B15, H03K17, H04N9