The development of electronic tuners, based upon the use of varactor diodes for producing capacitance changes as a function of an applied DC potential, has vastly simplified tuning systems in general, and television receiver tuning systems in particular. In these tuners, the varactor diode exhibits capacitance variations with changes in bias voltage and serves as the variable reactance in an -otherwise- conventional tuned circuit. Electronic tuning systems are gaining increasing acceptance because of their ease of tuning, freedom from RF signal carrying contacts and the versatility afforded the designer in styling the receiver. Their most serious drawbacks are the limited range of diode capacitance change and the non-linear relationship between frequency and bias voltage. The limited range of the diode is circumvented by band switching arrangements in VHF (and separate diodes in UHF) for changing the inductively reactive elements in the tuner stages to enable a limited change in capacitance to produce resonance over a large frequency range. The non-linearity of the varactor diode bias voltage-versus-frequency characteristic poses more difficult problems, especially in systems using conventional tuner addressing approaches.
Further complications in tuning system design were introduced a few years ago when the Federal Communications Commission decreed "equal tuning" (or tuning parity) between VHF television channels and UHF television channels. The ultimate goal is for the viewer to experience the same ease (or degree of difficulty) in tuning any television station, whether UHF or VHF. There is thus a need for readily "equalizing" the tuning characteristic of varactor diodes whereby equal tuning information increments will be translated into equal frequency changes irrespective of the particular portion of the tuning characteristic at which the increment occurs.
At the outset, it should be understood that the terms "equalizing" and "equalized" have been greatly abused in the art. Generally speaking, any attempt at making the tuning curve seem more linear has been called equalizing. Thus a single resistor may be switched between the VHF and UHF bands and the system may be referred to as equalized. In the discussion of the invention, true equalization is meant in the sense that equal tuning information increments give rise to equal frequency changes.
Some recent, all-electronic tuning systems inherently produce "equal ease" tuning. Specific reference is made to U.S. Pat. No. 3,961,266, issued to Akio Tanaka and assigned to Zenith Radio Corporation, which is directed to an electronic tuning system having a "tuning window". Its disclosure describes a tuner with the local oscillator frequency being digitally sampled and converted into the two digit channel number and intrachannel frequency increment corresponding to the FCC frequency allocation. The derived channel number (and intrachannel increment) is digitally compared with the desired channel number and a preselected intrachannel increment to determine when proper tuning exists. The system uses modulo 6 counting and consequently a one count change in the modular residue corresponds to a 1MHz change in oscillator frequency, independent of the tuning curve. Equal tuning is always provided in that system because the tuning voltage, per se, is not involved except as the driving means for the oscillator. The system doesn't provide for fine tuning.
Less sophisticated systems do not, in general, produce equal tuning because conventional tuners do not have linear tuning curves and changing the bias or tuning voltage by a given increment will not result in equal frequency changes. As a practical matter, for abrupt junction varactor diodes used in television receiver tuning systems, a tuning voltae change of one volt in the vicinity of VHF channel 2 may produce a 6MHz change in frequency whereas a similar tuning voltage change in the vicinity of VHF channel 6 may produce a 1/2MHz change.
Another desirable characteristic for a viewer is "equal tuning feel". By this is meant that, if for example, the range of fine tuning is one-half turn of the control knob fo VHF channel 2, it should be one-half turn for VHF channel 6. In many receivers, fine tuning of UHF channels is more difficult than fine tuning of VHF channels. As will be seen, the invention enables a viewer to perceive "equal tuning feel" even for UHF channels. Also tuning confusion may be materially reduced since the effect of the fine tuning control may be limited to less than .+-. one channel and preclude tuning to a "wrong" channel. In this context, a wrong channel is one for which the receiver tuning readout or channel indicator displays a channel number other than that assigned to the actual received signal. Similarly in an environment in which counters produce the tuning voltage, each count of the counter may be readily made to correspond to an equal frequency change in the oscillator.
While the invention will be described in the environment of a television system and indeed is contemplated to provide very attractive benefits therein, it is not to be so restricted. As will be apparent to those skilled in the art, the inventive method and apparatus may be applied to tuning systems generally - where there is a non-linear voltage to frequency characteristic.
One ready example is that of providing Automatic Frequency Control (AFC) for the tuner when the receiver has been tuned to a desired channel. The subject of AFC's is quite complex in itself. Suffice it to say that the "pull-in" and "lock-in" of the AFC system is of great importance and that invariably the designer must make a number of compromises in an effort to produce an optimum arrangement. Since AFC is in reality a form of "continuous" fine tuning, it should be apparent that the requirements of the AFC circuit may be relaxed considerably by providing equalization over the television spectrum. There are a number of approximate AFC equalization schemes in the art, most of which involve interposing dividing resistors, tailored to selected channels or channel groups, to proportion the AFC voltage in accordance with the tuning voltage magnitude. While the correction obtained is in the right direction it is not related to the slope of the tuning curve. Thus it is well-known to apportion the AFC voltage, especially with respect to signals in the VHF and UHF bands, and the same circuitry can be readily adapted for approtioning the fine tuning voltage.
U.S. Pat. No. 4,005,256 discloses an AFC circuit for a television receiver. The circuit includes bias means for providing one of a plurality of bias voltages each corresponding to a particular channel to be received, an AFC means and a signal combining circuit for application to a voltage controlled tuner. The signal combining circuit may include either; a combination of first, second and third resistance means to produce a variation in the amount of error voltage coupled to the tuning means in accordance with the bias voltage; or a first resistance means and a second non-linear resistance means for doing substantially the same thing. The objective of the patented circuit is to provide a substantially constant automatic frequency control pull-in range throughout the frequency range of interest despite the non-linear frequency versus voltage tuning characteristic of the tuner. As can be readily seen, the AFC voltage is truly equalized, if at all, at only a very few points.
U.S. Pat. No. 3,962,643 shows an "equalized" AFC system for VHF channels. Here again the AFC voltage is apportioned based upon the tuning voltage magnitude and represents a coarse approximation rather than true equalizing.
The art has also pursued digital memory type tuning systems. U.S. Pat. No. 3,990,027 dicloses a memory block for storing, in digital form, the tuning voltages for each receivable television channel. The memory block is addressed by counters in accordance with the channel number of the desired signal. The channel tuning voltages are adjustable for precise tuning and on readout are converted by a digital to analog (D/A) converter into a tuning voltage for the tuner. Suitable band decoding is also performed to compensate for the particular frequency band in which the channel lies. Such systems are common but do not provide equalized fine tuning or AFC.
The basic distinction between the so-called equalized systems in the art and the method and apparatus of the invention is that the art by-and-large uses the magnitude of the tuning voltage for determining the "equalization" value, whereas the invention uses the slope of the tuning voltage-versus-frequency characteristic to determine the weight to be given to the tuning voltage increment.
In this respect prior art U.S. Pat. No. 3,878,466 issued to Melvin Hendrickson and assigned to Zenith Radio Corporation, discloses a complex electro-mechanical system with a sequentially connected plurality of resistors having contact junctions presenting the end tuning voltages for each channel. A potentiometer is sequentially switchable thereover for channel selection and produces true equalized fine tuning to the extent that the resistors are selected to match the tuning characteristic. However the slope factor information is not available and is of no value for equalizing AFC and the fine tuning range is arbitrarily fixed at .+-. one channel.
On the contrary the inventive method overcomes the problem in the prior art and paves the way for economical all-electronic, all-channel equalized tuning systems.