FM demodulator for SECAM decoder

A SECAM decoder and an FM modulator therefor are disclosed, in which a demodulated color signal is provided to indicate deviations in the frequency of modulated color information from a nominal subcarrier frequency. The demodulator comprises numerator and denominator filters operating on the modulated color information, and a divider providing a ratio result by dividing the numerator filter output by the denominator filter output. The demodulator may include two sets of numerator and denominator filters offset in phase from one another, where one of the two sets is selectively employed in order to mitigate divide-by-zero problems. Also disclosed are methods for demodulating digitized FM color signals in a SECAM decoder.

FIELD OF INVENTION

The present invention relates generally to the art of video decoders and more particularly to a SECAM decoder and an FM demodulator therefor.

BACKGROUND OF THE INVENTION

Video decoders are used in a variety of applications wherein analog video signals in a first format are digitized and decoded for use in other formats. Video consists of luma which represents a level between black and white and chroma which consists of two components containing color information. The analog video signal may comprise “S video”, wherein separate channels are used for luma and chroma, or more typically, “composite video”, where luma and chroma are included in the same signal. Typical color images are characterized in terms of red, green, and blue color components which are generated from luma and chroma. The video decoder digitizes analog video input signal information, for example, using an analog to digital converter (e.g., A/D) and separates the luma and chroma information in the digital domain.

The digitized video data may then be represented in a number of formats, including the YUV video format and the YCrCb video format. In the YUV format, the Y component represents the luma information required for a black and white system, the U component represents the difference between the value of B and the value of Y multiplied by a scale factor, and V is the difference between the value of R and the value of Y multiplied by a scale factor. The YUV format is a color space employed by the phase alternation line (e.g., PAL), national television system committee (e.g., NTSC), and systeme en couleur avec memoire (e.g., sequential color with memory, or SECAM) composite color video standards. SECAM is a color television standard developed in France, wherein 25 interlaced frames are broadcast per second (50 half frames per second) at 625 lines of resolution. SECAM is primarily found in France and Russia and many countries in Africa, Eastern Europe and the Middle East. YCrCb is another color video standard using scaled and offset versions of the YUV color space. Y generally has a nominal range of 16 to 235 with Cr and Cb ranging from 16 to 240, wherein 128 equals zero (for 8 bit output).

The PAL, NTSC, and SECAM color video standards are thus employed for transmission of composite analog video signals, which may be operated on by video decoders. A digital video decoder device requires the composite analog video signal to be first digitized using an A/D converter, and then the luma and chroma components to be separated. In the PAL and NTSC standards, the color information is amplitude (e.g., AM) modulated, whereas SECAM video signals include frequency (e.g., FM) modulated color information. SECAM utilizes FM modulation to transmit its color information, including color difference signals Db and Dr. In the SECAM format, the Db and Dr difference signals are alternatively transmitted Db, Dr, Db, Dr, . . . , and so on, wherein the Db and Dr components each have a different subcarrier frequency. For instance, SECAM employs a first nominal subcarrier frequency of 4.25 MHz for the Db component, and second nominal frequency of 4.40625 MHz for the Dr component.

Because of the FM modulation of analog color information in SECAM, the isolated digitized color information in a SECAM decoder is presented to an FM demodulator to obtain a demodulated color signal having amplitude variations representative of variations or deviations from the nominal subcarrier frequency. The demodulated color signal may then be operated on digitally in order to reformat, process, or otherwise manipulate the color information as needed. The decoder may then provide corresponding video output signals, such as in YCrCb for use in television or other video systems.

However, several problems exist in the implementation of FM demodulators for operating on the digitized video color signals. Discrete time equivalents of analog FM demodulators (e.g., like the “one-shot type”) or simple digital methods (e.g., the “frequency counter type”) require high clock frequencies (e.g., about 6 GHz). Phase locked loop (e.g., PLL) type FM demodulators have less stringent clock frequency requirements than the above. However, for a modulation index of 400 kHz at a bandwidth of 1.2 MHz and a carrier frequency of 4.286 MHz, a 13.5 MHz clock is marginal. Moreover, the PLL type FM demodulator may be sensitive to amplitude variations in the carrier signal. Although statically, the PLL type demodulator cares little about the amplitude of the FM-carrier signal, in operation the loop dynamics depend highly on well conditioned input signals such that an automatic gain control (AGC) may be required prior to FM-demodulation.

So called “product type” FM demodulators have acceptable clock frequency requirements, but suffer from higher sensitivity to carrier signal amplitude variations. In zero-crossing FM demodulators, samples occur only after a zero crossing of the FM-carrier signal. As these zero crossings occur asynchronously with respect to the clock frequency, complicated re-shuffling and sample rate conversion are needed to prevent non-equidistant sampling. Accordingly, there is a need for improved FM demodulation apparatus and techniques for video decoders, by which the effects of carrier signal amplitude variations may be mitigated without requiring high clock frequencies.

SUMMARY OF THE INVENTION

The present invention relates to SECAM decoders and FM modulators therefor, in which a demodulated color signal is provided to indicate deviations in the frequency of modulated color information from the subcarrier frequency. One aspect of the invention provides an FM demodulator having numerator and denominator filters operating on the modulated color information, and a divider providing a ratio result by dividing the numerator filter output or result by the denominator filter output. The ratio of filtered outputs provides a cancellation of carrier amplitude, thereby reducing or avoiding the susceptibility to carrier amplitude variations found in other demodulators.

According to another aspect of the invention, the demodulator may employ two pairs or sets of numerator and denominator filters offset in phase from one another, where one of the two sets is selectively employed in order to mitigate divide-by-zero problems associated with obtaining the ratio. The selective employment of one or the other filter set may advantageously provide conditional replenishment of the demodulator output, for example, where one denominator filter output crosses through zero or has a very small value. For example, in one implementation, two denominator filter result values are compared, and a numerator/denominator filter set is selected (e.g., using a comparator and multiplexer) so as to mitigate divide-by-zero conditions in producing the ratio result. In this regard, the two filter sets are offset in phase from one another, so as to ensure that where one set yields a zero or small denominator value, the other set will not. In this manner, the invention provides the advantages associated with ratio-based demodulation, while mitigating or avoiding problems associated with zero or small number denominator division through conditional replenishment of the output.

Yet another aspect of the invention involves methodologies for demodulating digitized FM color signals in a SECAM decoder. The method involves filtering a modulated color signal using a numerator filter to provide a numerator result and filtering the modulated color signal using a denominator filter to provide a denominator result. Thereafter, the numerator result is divided by the denominator result to provide a ratio result indicative of deviations in the frequency of the modulated color signal from a nominal subcarrier frequency. The filtering using the numerator filter may comprise using first and second numerator filters to provide first and second numerator results, and the filtering using the denominator filter may be performed using first and second denominator filters to provide first and second denominator results, respectively. In this case, conditional replenishment may be used, wherein one of the first and second numerator results is divided by one of the first and second denominator results to provide the ratio result, according to a comparison of the two denominator filter results.

To the accomplishment of the foregoing and related ends, the following description and annexed drawings set forth in detail certain illustrative aspects and implementations of the invention. These are indicative of but a few of the various ways in which the principles of the invention may be employed. Other aspects, advantages and novel features of the invention will become apparent from the following detailed description of the invention when considered in conjunction with the drawings.

DETAILED DESCRIPTION OF THE INVENTION

One or more implementations of the present invention will now be described with reference to the attached drawings, wherein like reference numerals are used to refer to like elements throughout. The invention relates to a SECAM decoder and an FM modulator therefor, in which a demodulated color signal is provided to indicate deviations in the frequency of modulated color information from a nominal subcarrier frequency. The demodulator comprises numerator and denominator filters operating on the modulated color information, and a divider providing a ratio result by dividing the numerator filter output by the denominator filter output. The demodulator may include two pairs of numerator and denominator filters offset in phase from one another, where one of the two sets is selectively employed in order to mitigate divide-by-zero (e.g., or divide by small number) problems. Also disclosed are methods for demodulating digitized FM color signals in a SECAM decoder.

Referring initially toFIG. 1, an illustration of an exemplary SECAM decoder2is provided, such as in the form of an integrated circuit, wherein the decoder2receives an analog composite video signal4, which includes luma, chroma, and synchronization information, and provides the signal4to a preamplifier (hereinafter “preamp”)6. The preamp component6receives and conditions the analog signal4, for example, by clamping the signal4to an internal reference voltage (not shown), amplifying the signal4, and offsetting the result. The conditioned analog video information or signal7is then digitized using an analog to digital (A/D) converter8, thus providing a digital composite video signal9to a digital separation filter10. The digital separation filter10separates the composite information9into a luma component12and a chroma component14.

In accordance with the SECAM color video standard, the chroma component14is frequency (e.g., FM) modulated using a carrier signal. The chroma information14is then provided to an FM demodulator16, which filters the information14and performs a division to generate a ratio result17indicative of deviations in the frequency of the chroma signal14and a subcarrier frequency. In accordance with an aspect of the invention, as illustrated and described in greater detail below, the ratio-based demodulator16mitigates problems associated with carrier signal amplitude variations. In addition, the demodulator16may employ conditional replenishment to avoid or mitigate divide-by-zero (e.g., and/or divide by small number) problems associated with performing the division to obtain the ratio result.

The demodulated chroma information17is then provided from the FM demodulator16to a logic component18, together with the digitized luma component12. The demodulated chroma signal17and luma signal18are then operated on digitally and reformatted as needed in the logic18. The decoder2then provides a corresponding video output signal20, such as in YCrCb form, for use in television or other video systems (not shown). For instance, the logic component18may form U and V signals by applying certain scale and offset factors to the luma component12and the demodulated chroma17, and adjust the information to certain levels. The logic18then outputs the digital YCrCb signal20. The digital YCrCb signal20may then be fed to a video encoder (not shown) for conversion to analog form, or may be further processed in digital form. For example, graphics may be overlayed in the image represented by the signal20, the signal20may be scaled, or other operations may be performed on the signal20using digital signal processing techniques as are known.

Referring now toFIG. 2a, the invention provides improved FM demodulators (e.g., demodulator16) for SECAM decoders such as decoder2, as well as methods for demodulating digitized color signals in such a SECAM decoder2. Although one or more demodulator hardware implementations are illustrated hereinafter, FM demodulators in accordance with the invention can be implemented in hardware, software, and/or combinations thereof, and it will be appreciated by those skilled in the art that all such implementations are contemplated as falling within the scope of the present invention. In one implementation of SECAM decoder FM demodulation of the invention, a demodulator circuit50comprises a numerator filter52receiving digitized color information54(e.g., from separation logic component10of FIG.1), for example, given by the following equation (1):
â* sin(f*t*2π+φ).  (1)
In the various filters ofFIG. 2aand other figures herein, the “z” domain transfer characteristics thereof are illustrated, as well as the time domain representations for ease of understanding. For instance, the filter52has a z domain transfer characteristic involving z0, z−1, and z−2, wherein z0is the current sample, z−1the previous sample, and z−2is the next most recent sample. As can be seen from the time domain representation of equation (1), the signal54is a function of a carrier signal amplitude â. The numerator filter52operates on the color information54and provides a numerator result56, which is given by the following equation (2):
â* sin(f*(t−T)*2π+φ)* sin2((f/fclk)*π).  (2)
The numerator result56is thus dependent upon the amplitude â of the carrier signal, a time delay T and the ratio of the frequency f to a clock frequency fclk.

The demodulator50further comprises a denominator filter58(e.g., a delay having a z-domain transfer function of z−1), which also operates on the color information signal54to provide a denominator result60given by the following equation (3):
â* sin(f*(t−T)*2π+φ).  (3)
Comparing the numerator and denominator results56and60, respectively (e.g., equations (2) and (3)), it is seen that both are functions of the carrier amplitude â, the time delay T, and a phase shift φ. A divider62receives the numerator and denominator results56and60from the numerator and denominator filters52and58, respectively, and divides the numerator result56by the denominator result60to provide a ratio result64, which is given by the following equation (4):
sin2((f/fclk)*π).  (4)

The ratio result64from the divider62is thus a function of deviations in the frequency f of the modulated color information from the clock frequency fclk. The ratio-based demodulator50thus provides the ratio result64which is independent of the carrier amplitude â, the time delay T, and a phase shift φ. The result signal value64is then input, for example, into a lookup table (LUT)66, which performs an inverse sin2operation, to provide a ratio result64′ which is simply the frequency ratio f/fclk. In the demodulator50ofFIG. 2a, it will be noted that a filtered version of the color information signal54is divided by a delay compensated version, thereby turning the frequency response of the numerator filter52into the transfer function of the FM demodulator50.

Another implementation of one or more aspects of the invention is illustrated inFIG. 2b, where another FM demodulator70is illustrated having numerator and denominator filters72and74, respectively, operating on an incoming digitized FM modulated color information signal76. The filters72and74provide numerator and denominator results78and80, given by the following equations (5) and (6), respectively:
â* sin(f*(t−T)*2π+φ)* cos2((f/fclk)*π  (5)
â* sin(f*(t−T)*2π+φ)* sin2((f/fclk)*π).  (6)
The filter results78and80are provided to a divider82, which divides the numerator result value78by the denominator result value80to provide a first ratio result84, given by the following equation (7):
cot2((f/fclk)*π).  (7)
A lookup table LUT86, for example, then provides the inverse cotangent squared of the ratio result84to generate the final ratio result88of f/fclk.

A further refinement of the ratio-based demodulation is illustrated inFIG. 2c, wherein the filter characteristics are altered to result in a transfer characteristic for another exemplary FM demodulator100that approximates a linear function, eliminating the need for a lookup table. The modulator100comprises numerator and denominator filters102and104, respectively, receiving a digitized FM modulated color information signal106. The filters102and104provide numerator and denominator results108and110, given by the following equations (8) and (9), respectively:
â* sin(f*(t−T)*2π+φ)*(32* sin2((f/fclk)*π)+8).  (8)
â* sin(f*(t−T)*2π+φ)*(16* cos2((f/fclk)*π)+40).  (9)
The filter results108and110are provided to a divider112, which divides the numerator result value108by the denominator result value110to provide a ratio result114, given by the following equation which is a linear approximation of the actual output (10):
f/fclk*2.696−0.1735.  (10)

As this linear ratio result114does not involve complex trigonometric formulas, no lookup table is required to obtain the result114, which is indicative of deviations in the signal frequency from that of the nominal subcarrier. Referring also toFIGS. 5 and 6, graphs130and140thereof illustrate output vs. frequency and linearity error vs. frequency, respectively, for the linear approximation provided in the ratio result114ofFIG. 2c. InFIG. 5, the linear approximation132for the ratio result114in demodulator100is illustrated in dashed line along with the actual output134versus frequency. The graph140ofFIG. 6illustrates a curve142of linearity error for the approximation versus frequency. From the curves132,134, and142ofFIGS. 5 and 6, it is seen that the linear approximation advantageously eliminates the need for a lookup table within the frequency band of interest (e.g., from about 3 MHz to about 5 MHz) without significant error.

It is noted at this point that the ratio-based FM demodulators illustrated and described above with respect toFIGS. 2a-2cprovide for dividing out the carrier amplitude dependency in computing the ratio result. This provides advantages over prior digital FM demodulation techniques, such as digital equivalents of analog demodulators (e.g., one-shot or frequency counter types), PLL, product-type, and Philips-type demodulation. This is accomplished in part by the provision of a divider component in the demodulator to take the ratio of two filter result values. However, as can be appreciated, where a division is performed on digital data, there is a possibility that the denominator will be very small or zero from time to time, causing result overflow and other undesirable conditions.

Another aspect of the invention provides refinement for such ratio-based demodulators, by which such divide-by-zero (e.g., or divide by small number) problems may be reduced or avoided through conditional replenishment as illustrated and described below with respect toFIGS. 3-4b. The conditional replenishment techniques of this aspect of the invention may be implemented separately or in combination with the filtering techniques ofFIG. 2c, wherein no lookup table is needed for the final ratio result f/fclk. In this regard, it will be appreciated that in the denominator filters58,74, and104of the above demodulators50,70, and100, respectively, individual samples (e.g., filtered and/or delayed) may, from time to time, be coincident with or come close to a zero crossing, whereby the denominator results associated therewith may be small or zero.

The invention provides for conditional replenishment of the denominator and/or the numerator to remedy this situation. Referring now toFIG. 3, another exemplary FM decoder150is illustrated, which may be employed to demodulate digital FM modulated color information signals in a SECAM video decoder. The demodulator150comprises a first numerator filter152(e.g., similar to the numerator filter102of demodulator100,FIG. 2c), which receives a color information signal154and provides a first numerator result156given by the equation (11) below:
â* sin(f*(t−2T)*2π+φ)*(32* sin2((f/fclk)*π)+8).  (11)
A first denominator filter158(e.g., similar to denominator filter104ofFIG. 2c) also receives the color information signal154and operates to provide a first denominator filter result160given by the following equation (12);
a* sin(f*(t−2T)* 2π+φ)*(16* cos2((f/fclk)*π)+40).  (12)

The demodulator150further comprises a divider162generating a ratio of numerator and denominator filter results164and166obtained via numerator and denominator multiplexers168and170, respectively, to provide a ratio result172. As with the demodulator100ofFIG. 2c, the ratio172of the numerator and denominator filter results164and166provides a linear approximation indicative of deviations in the frequency of the color information with respect to the clock frequency, without a lookup table, due to the transfer characteristics of the numerator and denominator filters (e.g., filters152and158). However, other implementations of the invention may employ filters having transfer characteristics different from those illustrated and described herein, which are contemplated as falling within the scope of the present invention. For example, other numerator and denominator filter transfer characteristics may be provided, wherein a lookup table is employed to obtain a ratio result indicative of such frequency deviations from the result of the division.

Another aspect of the invention is implemented in the demodulator150, wherein conditional replenishment is employed to avoid or mitigate divide-by-zero or divide by small number conditions in the divider162. This is accomplished by introducing second numerator and denominator filters180and182, respectively, having the same ratio as the first filters152and158and the same group delay, but which have a 90 degree phase relationship with respect to the first filters152and158. The second numerator filter180receives the modulated color signal154and provides a second numerator result184given by equation (13) below, and the second denominator filter182operates to provide a second denominator filter result186given by equation (14):
â* sin(f*(t−2T)*2π+φ)*(32* sin2((f/fclk)*π)+8)*2* sin((2f/fclk)*π).  (13)
â* sin(f*(t−2T)*2π+φ)*(16* cos2((f/fclk)*π)+40)*2* sin((2f/fclk)*π).  (14)

Both the numerator and the denominator results164and166, respectively, are then derived from that set of filters (e.g.,152and158, or180and182) providing the better conditioned value for the denominator, as selected by the multiplexers168and170. Since the phase of both sets of filters is 90° apart, if a zero crossing (e.g., or small value) is obtained in one denominator filter result, the other denominator result will have a larger value, and the other set of filters is selectively employed to provide the ratio result172. Accordingly, the demodulator150employs a comparator190to compare the first and second denominator filter results160and186to provide a selection signal192to the multiplexers168and170to select the appropriate set of filter results for provision to the divider162.

In accordance with one exemplary aspect of the present invention, in the demodulator150, the comparator190determines whether the absolute value of the first denominator filter result160is greater than twice the absolute value of the second denominator filter result186. If so, the signal192has a first state, wherein the multiplexers168and170provide the first numerator and denominator filter results156and160, respectively, to the divider162. Otherwise, the selection signal192has a second state, wherein the second numerator and denominator filter results184and186, are provided to the divider162via the multiplexers168and170, respectively. In this manner, the set of filters which avoid zero divide (e.g., or small number divide) situations is selectively employed.

It will be appreciated that while the comparator190of the exemplary demodulator150selects the filter set according to a determination of whether the filter result160is greater than twice the filter result186, that other comparisons may be made to select the filter set in accordance with the invention. For instance, the comparator190could alternatively select the filter set corresponding to the denominator filter having the largest absolute value result. Furthermore, although the filter sets in the exemplary demodulator150have a 90 degree phase relationship to one another, other implementations having different phase relationships are contemplated as falling within the scope of the present invention.

A more detailed illustration of the exemplary FM demodulator150is provided inFIGS. 4aand4b. The digitized FM input signal154is fed into a series of four delay stages200, indicated by the delay transfer characteristics z−1in the z domain. The delay stages provide the delayed samples (e.g., z−1, z−2, z−3, and z−4) employed in the filters152,180,158, and182, wherein the z0factor is obtained from the current sample of the signal154. Each set of filters (e.g., first filters152and158, or second filters180and182) also share most of the coefficients, implemented as binary shifts and adds inFIG. 4a. In the drawingFIGS. 4aand4b, binary left shifts are illustrated as a less than (“<<”) symbol followed by an integer indicating the number of bits shifted, summation operations are indicated by blocks having a plus (“+”) symbol, and multiplications are illustrated as blocks having an “x” (seeFIG. 4b). The color information signal154is thus processed by various delays, shifts, and adds as illustrated inFIG. 4ato implement the filters152,180,158, and182in the demodulator150.

In this manner, the filters152,180,158, and182provide filter results156,184,160, and186, respectively, and the absolute values of these results are then provided to the multiplexers168and170. The second denominator filter result186is further shifted left by one bit at a shift210, in effect, multiplying the result186by two. The shifted result is then provided to the comparator190along with the first denominator result160. The comparator190provides a selection signal192to the multiplexers168and170, which corresponds to the comparison. The multiplexers168and170thus provide the first numerator and denominator filter results156and160, or the second numerator and denominator results184and186to the divider162(e.g., see alsoFIG. 4b) in accordance with the selection signal192.

Since both the respective numerator and the denominator values inFIG. 4ahave identical signs, sign extension in the divider162is avoided by turning all four filter output result values156,184,160, and186into absolute values. The two denominator results160and186are then compared and the first set of filters (e.g., filters152and158) is selected for input to the divider162if its absolute denominator value160exceeds twice the absolute denominator value186of the second set of filters (e.g., filters180and182). InFIG. 4b, the divider162is implemented in accordance with one exemplary aspect of the invention using a lookup table220after normalization, shifting, and rounding, to generate the reciprocal of the selected denominator result value166, which is then multiplied with the normalized numerator result164in a multiplier block230. The ratio is then provided via the multiplexers240and250as the output ratio result172unless the numerator result164is zero or if the denominator result is less than 0.01, in which case data values corresponding to a NO COLOR signal is provided as the output172via multiplexer250.

For example, both the numerator and denominator results164and166are normalized to fall in a range from 1.0 to 2.0 by such shifting. The shift constants from both normalization blocks may be subtracted to generate a final shift constant that is applied to the product output. Entries in the lookup table220fall in a range from 0.5 to 1.0, by which a range at the output of the multiplier230is 0.5 to 2.0. The multiplexer240selects the final shifted output based on the sign of the final shift constant. If the numerator result164is zero or if the denominator result166is small, the multiplexer250selects NO COLOR for the output172.

In the exemplary FM demodulator150, the filter coefficients are chosen according to various factors, and it will be appreciated that filters having other coefficients and transfer characteristics are contemplated as falling within the scope of the present invention. For example, the individual coefficients may be realizable by a minimum number of simple shifts and adds in order to economize hardware implementation. Alternatively or in combination, the coefficients may be chosen so as to result in an acceptable approximation of a linear ratio of output value versus FM input frequency for the entire FM demodulator150. In this regard, this second condition advantageously avoids the need for a look-up table for linearization purposes, as illustrated and described above with respect toFIG. 2c.

Referring again toFIGS. 5 and 6, the illustrated coefficients in the filters of the demodulator150are one implementation which satisfies both goals. First, the coefficients are relatively simple to implement in hardware (e.g.,FIGS. 4aand4b), requiring mostly delays, shifts, and summations (binary). Secondly, as demonstrated inFIGS. 5 and 6, the FM demodulator output172approximates very closely a linear function (dashed) over the frequency range of interest (3.5 to 5.0 MHz). This linear function satisfies the following equation (15):

FIG. 5draws the deviation of the FM demodulator output172from this linear function. For the range of frequencies from 3.5 MHz to 5.0 MHz, the excursions from linearity remain less than 0.25%, satisfying 8 bit resolution, differentially and integrally.

The above is one implementation of a ratio-based FM demodulator150employing conditional replenishment in order to avoid or mitigate divide-by-zero problems. The ratio output172obtained has an amplitude indicative of the deviations in frequency between the color signal and the nominal subcarrier, and is less susceptible to carrier amplitude variations than are other non-ratio based FM demodulators. While the invention finds particular utility when employed in association with SECAM and other video decoder systems (e.g., integrated circuits), it will be appreciated that other implementations of the various aspects of the present invention fall within the scope of the appended claims, and that the invention is not limited to those applications or implementations illustrated and described herein. For example, the invention can be implemented in non-video type FM demodulation applications.

Another aspect of the present invention relates to methods and techniques for FM demodulation, by which various shortcomings associated with the prior art may be avoided or mitigated. This aspect provides for filtering a modulated color signal using a numerator filter to provide a numerator result, and filtering the modulated color signal using a denominator filter to provide a denominator result. The numerator result is then divided by the denominator result to provide a ratio result indicative of deviations in the frequency of the modulated color signal from a nominal subcarrier frequency. The filtering may comprise using first and second numerator filters to provide first and second numerator results, and first and second denominator filters to provide first and second denominator results. In this case, conditional replenishment may be employed, wherein one of the first and second numerator results is divided by one of the first and second denominator results to provide the ratio result, according to a comparison of the two denominator filter results.

One exemplary method300is illustrated inFIG. 7in accordance with the invention. While the exemplary method300is illustrated and described herein as a series of acts or events, it will be appreciated that the present invention is not limited by the illustrated ordering of such acts or events, as some acts may occur in different orders and/or concurrently with respect to other acts or events apart from those illustrated and/or described herein, in accordance with the invention. In addition, not all illustrated steps may be required to implement a methodology in accordance with the present invention. Moreover, it will be appreciated that the method300may be implemented in association with the apparatus and systems illustrated and described herein as well as in association with other systems not illustrated.

Beginning at302, a color signal is filtered at304using first and second numerator filters, such as the filters152and180, respectively, of the demodulator150ofFIGS. 3-4b. At306, the color signal is filtered using first and second denominator filters (e.g., filters158and182, respectively). A determination is made at308as to whether the first denominator filter result is greater than twice the second denominator filter result. This determination is made in order to selectively use one or the other of the two sets of filters, so as to avoid divide-by-zero situations discussed above. If the first denominator result value exceeds twice the second denominator filter result (e.g., YES at308), the method300proceeds to310. At310, the first numerator filter result is divided by the first denominator filter result to provide a ratio result representative of deviations in the frequency of the color signal from that of a nominal subcarrier. Thereafter, the method300ends at312.

However, if the first denominator result value does not exceed twice the second denominator filter result (e.g., NO at308), the second numerator filter result is divided by the second denominator result at314, after which the method300ends at312. Although the exemplary method300involves a comparison at308of the first denominator result value with twice the second denominator filter result value, other comparison criteria are possible in mitigating divide-by-zero (e.g., and/or divide by small number) conditions, and that all such comparison criteria are contemplated as falling within the scope of the present invention. For example, the values of the first and second denominator filter result values could be employed in accordance with the present invention.

Although the invention has been illustrated and described with respect to one or more implementations, equivalent alterations and modifications will occur to others skilled in the art upon the reading and understanding of this specification and the annexed drawings. In particular regard to the various functions performed by the above described components (assemblies, devices, circuits, etc.), the terms (including a reference to a “means”) used to describe such components are intended to correspond, unless otherwise indicated, to any component which performs the specified function of the described component (i.e., that is functionally equivalent), even though not structurally equivalent to the disclosed structure which performs the function in the herein illustrated exemplary implementations of the invention.

In addition, while a particular feature of the invention may have been disclosed with respect to only one of several implementations, such feature may be combined with one or more other features of the other implementations as may be desired and advantageous for any given or particular application. Furthermore, to the extent that the terms “includes”, “including”, “has”, “having”, “with”, or variants thereof are used in either the detailed description or the claims, such terms are intended to be inclusive in a manner similar to the term “comprising.”