For many years, mechanical turret tuners have been commonly employed in television receivers to select the VHF channels, and a second rotary or continuous tuner has been used to select the UHF channels. For most television receivers, this requires two different channel selection knobs, and the tuners themselves are relatively bulky and require a relatively large amount of space within the television receiver cabinet. Because of the nature of these tuners, it also is necessary to locate them directly behind the front panel of the receiver which imposes significant restrictions on the cabinet design and the arrangement of parts within the cabinet, reducing the flexibility of design which would be possible if such tuners could be eliminated.
Some mechanical tuners are equipped with programmable switches to permit them to be used to select either a UHF or a VHF channel at a tuner position by programming the tuner for the local area where the television receiver is to be used. The disadvantages of the cumbersome mechanical turret tuners, however, are not overcome; but the tuner is made even more complicated by such an arrangement.
It is desirable and in the United States it is becoming necessary to effect selection of the UHF and VHF channels in a comparable manner. When such tuning compatability is imposed, significant problems are encountered in providing a mechanical turret-type tuner having detented positions for all of the possible UHF channels which must be accommodated for television receivers capable of operating in any given locality in which the receiver is capable of receiving the transmitted television signals. UHF turret tuners with detent tuning selection for each of the 70 possible UHF channels are difficult and expensive to manufacture, and even the display of all the UHF channel numbers in a manner which is compatable with the display for the much smaller number of VHF channels is difficult to accomplish.
The introduction of voltage variable capacitor or varactor tuners for the VHF and UHF bands to which a television receiver can be tuned has opened the way for electronic tuning of television receivers. This replaces the cumbersome mechanical turret tuners and allows greater flexibility in the design of the channel selection panel and in the location of tuner parts within the receiver cabinet.
Even so, if the receiver is to be made capable of individual selection of any one of the 70 UHF channels, in addition to the VHF channels, a large number of individual tuning components still are required. It is desirable to reduce the number of tuning components to a number sufficient to cover all of the channels in any given area in which the television receiver is used and also to make a provision in the receiver to cause it to be non-responsive to any particular channel positions which are not used in a given locality.
It also is desirable to provide a channel selection system in which control of the channels can be effected by sequentially scanning or stopping on only those channels which can be received in a given locality and skipping over the channels which cannot be received in that locality. Such a system then also should be readily adaptable to remote control operation as well as front panel operation on the television receiver itself. Ideally, the system should be capable of sequentially stepping or scanning through the channels to which the receiver is to respond in both the "up" and "down" directions of channel sequence.