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BACKGROUND OF THE INVENTION
This invention concerns a widescreen television system that is compatible
with conventional television receivers having a relatively smaller display
aspect ratio.
A conventional television receiver, such as a receiver in accordance with
NTSC broadcast standards adopted in the United States and elsewhere, has a
4:3 aspect ration (the ratio of the width to the height of a displayed
image). Recently, there has been interest in using higher aspect ratios
for television receiver systems, such as 2:1, 16:9 or 5:3, since such
higher aspect ratios more nearly approximate or equal the aspect ration of
the human eye than does the 4:3 aspect ratio of a conventional television
receiver. Video information signals with a 5:3 aspect ratio have received
particular attention since this ratio approximates that of motion picture
film, and thus such signals can be transmitted and received without
cropping the image information. However, widescreen television systems
which simply transmit signals having an increased aspect ratio as compared
to conventional systems are incompatible with conventional aspect ratio
receivers. This makes widespread adoption of widescreen systems difficult.
It is therefore desirable to have a widescreen television system that is
compatible with conventional television receivers. In accordance with the
principles of the present invention, there are disclosed herein method and
apparatus for encoding and decoding a compatible widescreen video signal
representing a picture having an aspect ratio greater than the standard
4:3 aspect ratio.
SUMMARY OF THE INVENTION
In a disclosed preferred embodiment of a compatible widescreen television
system employing apparatus in accordance with the principles of the
present invention, a widescreen signal having left side, right side and
center video information panels is converted into a signal compatible with
a standard system, such as NTSC for example, by compressing side panel low
frequency information of the widescreen signal into left and right
overscan regions which are present in the standard system but not seen by
a viewer, and by simultaneously time expanding the center panel
information to occupy the standard display region seen by a viewer. High
frequency side panel information is encoded by quadrature modulating such
high frequency information on an alternate subcarrier signal other than
the chrominance subcarrier.
In accordance with the principles of the present invention, video signals,
illustratively time-expanded/compressed video signals, are peaked before
being interpolated. In accordance with a feature of the invention, the
amount of peaking is controlled in response to a signal representative of
the distance between a pixel of an expanded/compressed video signal and a
corresponding pixel of an original video signal.
DESCRIPTION OF THE DRAWING
FIG. 1 shows a block diagram of an encoder for a compatible widescreen
television system;
FIGS. 2-4 depict signal waveforms helpful in understanding the operation of
the system of FIG. 1;
FIGS. 5 and 6 illustrate aspects of the system of FIG. 1 in greater detail;
and
FIGS. 7 and 7a-7d illustrate aspects of apparatus in accordance with the
principles of the present invention.
A brief overview of the system to be described will be helpful. A system
intended to transmit wide aspect ratio pictures, e.g., 5:3, through a
standard, e.g., NTSC, channel should achieve a high quality picture
display by a widescreen receiver, while greatly reducing or eliminating
observable degradations in a standard 4:3 aspect ratio display. The use of
signal compression techniques on the side panels of a picture takes
advantage of the horizontal overscan region of a standard NTSC television
receiver display, but may sacrifice image resolution in the side panel
regions of a reconstructed widescreen picture. Since compression in time
results in an expansion in the frequency domain, only low frequency
components would survive processing in a standard television channel,
which exhibits a smaller bandwidth compared with that required for a
widescreen signal. Thus, when the compressed side panels of a compatible
widescreen signal are expanded in a widescreen receiver, there results a
noticable difference between the resolution or high frequency content of
the center portion of a displayed widescreen picture and the side panels,
unless steps are taken to avoid this effect.
The Figures and associated description describe a system for developing a
widescreen signal capable of being processed through a standard NTSC
channel. The system also permits a widescreen receiver to recover a
widescreen picture with good picture quality across the entire display.
As will be seen from the encoder of FIG. 1, the use of spatial compression
allows low frequency side panel information to be squeezed into the
horizontal overscan region of a standard NTSC signal. The high frequency
side panel information is spectrally shared with the standard NTSC signal
through the video transmission channel, in a manner transparent to a
standard receiver, through the use of an alternate subcarrier modulation
technique.
Before discussing the compatible widescreen encoding system of FIG. 1,
reference is made to signal waveforms A and B of FIG. 2. Signal A is a 5:3
aspect ratio widescreen signal that has been converted to a standard NTSC
compatible signal with a 4:3 aspect ratio as indicated by signal B.
Widescreen signal A includes left and right side panel portions each
associated with intervals TS which typically are of equal duration, and a
center panel portion associated with an interval TC. Widescreen signal A
has been converted to NTSC signal B by compressing certain side panel
information completely into the horizontal overscan regions associated
with time intervals TO. The NTSC signal has an active line interval TA
(approximately 52.5 microseconds duration) which encompasses overscan
intervals TO, a display time interval TD which contains the video
information to be displayed, and a total horizontal line time interval TH
of approximately 63.556 microseconds duration. Intervals TA and TH are the
same for both the widescreen and NTSC signals. It has been found that
almost all consumer television receivers have an overscan interval which
occupies at least 4% of the total active line time TA, i.e., 2% overscan
on the left and right sides. At a sampling rate of 4.times.f.sub.sc (where
f.sub.sc is the frequency of the color subcarrier), each horizontal line
interval contains 910 pixels (picture elements) of which 754 constitute
the active horizontal line image information to be displayed.
Returning to FIG. 1, a widescreen camera 10 provides a widescreen color
signal with R, G, B components and a wide aspect ratio of 5:3 in this
example. A widescreen camera is essentially identical to a standard NTSC
camera except that a widescreen camera has a greater aspect ratio and a
greater video bandwidth. The video bandwidth of a widescreen camera is
proportional to the product of its aspect ratio and the total number of
lines per frame, among other factors. Assuming constant velocity scanning
by the widescreen camera, an increase in its aspect ratio causes a
corresponding increase in its video bandwidth as well as horizontal
compression of picture information when the signal is displayed by a
standard television receiver with a 4:3 aspect ratio. For these reasons,
it is necessary to modify the widescreen signal for full NTSC
compatibility.
The color video signal processed by the encoder system of FIG. 1 contains
both luminance and chrominance signal components. The luminance and
chrominance signals contain both low and high frequency information, which
in the following discussion will be referred to as "lows" and "highs",
respectively.
The wide bandwidth widescreen color video signals from camera 10 are
matrixed in a unit 12 to derive luminance component Y and color difference
signal components I and Q from the R, G, B color signals. The wideband Y,
I, Q signals are sampled at a four-times chrominance subcarrier rate
(4.times.f.sub.sc) and are converted from analog to digital (binary) form
individually by separate analog-to-digital converters (ADC) in an ADC unit
14 before being filtered individually by separate horizontal low pass
filters in a filter unit 16 to produce filtered signals YF, IF and QF.
These signals are each of the form indicated by waveform A in FIG. 2.
Luminance signal YF is bandwidth limited by filter 16 to CEF.times.4.2
MHz, or approximately 5 MHz, where CEF is the center panel expansion
factor. This is necessary so that, after subsequent time expansion as will
be discussed, the bandwidth of the center panel signal is reduced to 4.2
MHz, the bandwidth of an NTSC video signal. For a similar reason, signals
IF and QF are bandwidth limited by filter 16 to CEF.times.500 KHz, or
approximately 600 KHz. Filter unit 16 therefore exhibits a luminance
cutoff frequency of approximately 5.0 MHz, and an I and Q cutoff frequency
of approximately 600 KHz.
The bandwidths of the Y, I and Q filters of unit 16 are related to the
center panel expansion factor, which in turn is a function of the
difference between the width of an image displayed by a widescreen
receiver and the width of an image displayed by a standard receiver. The
image width of a widescreen display with a 5:3 aspect ratio is 1.25 times
greater than the image width of a standard display with a 4:3 aspect
ratio. This factor of 1.25 is a preliminary center panel expansion factor
which must be adjusted to account for the overscan region of a standard
receiver, and to account for an intentional slight overlap of the boundary
regions between the center and side panels as will be explained. These
considerations dictate a CEF of 1.19.
The wideband signals from filter unit 16 are processed by a side-center
panel signal separator and processor 18 to produce three groups of output
signals: YE, IE and QE; YO, IO and QO; and LH, RH, IH and QH. The first
two groups of signals (YE, IE, QE and YO, IO, QO) are processed in a first
channel which develops a signal containing a full bandwidth center panel
component, and side panel luminance lows compressed into horizontal
overscan regions. The third group of signals (LH, RH, IH, QH) is processed
in a second channel which develops a signal containing side panel highs.
When the output signals from the two channels are combined, an NTSC
compatible widescreen signal with a 4:3 display aspect ratio is produced.
Details of circuits comprising unit 18 will be shown and discussed in
connection with FIGS. 5 and 6.
Signals YE, IE and QE contain complete center panel information and exhibit
the same format, as indicated by signal YE in FIG. 3. Briefly, signal YE
is derived from signal YF as follows. Widescreen wideband signal YF from
signal YF as follows. Widescreen wideband signal YF from unit 16 contains
pixels 1-754 occuring during the active line interval of the widescreen
signal, containing side and center panel information. The wideband center
panel information (pixels 75-680) is extracted as a center panel luminance
signal YC via a time de-multiplexing process. Signal YC is time expanded
by the center panel expansion factor of 1.19 (i.e., 5.0 MHz.div.4.2 MHz)
to produce NTSC compatible center panel signal YE. Signal YE exhibits an
NTSC compatible bandwidth (0-4.2 MHz) due to the time expansion by factor
1.19. Signal YE occupies picture display interval TD (FIG. 2) between
overscan regions TO (pixels 1-14 and 741-754). Signals IE and QE are
developed from signals IF and QF, respectively, and are similarly
processed in the manner of signal YE.
Signals YO, IO and QO provide the low frequency side panel information
("lows") which is inserted into the left and right horizontal overscan
regions. Signals YO, IO and QO exhibit the same format, as indicated by
signal YO in FIG. 3. Briefly, signal YO is derived from signal YF as
follows. Widescreen signal YF contains left panel information associated
with pixels 1-84 and right panel information associated with pixels
671-754. As will be discussed, signal YF is low pass filtered to produce a
luminance lows signal with a 0-700 KHz bandwidth, from which signal a left
and right side panel lows signal is extracted (signal YL' in FIG. 3) via a
time de-multiplexing process. Luminance lows signal YL' is time compressed
to produce side panel lows signal YO with compressed low frequency
information in the overscan regions associated with pixels 1-14 and
741-754. The compressed side lows signal exhibits an increased BW
proportional to the amount of time compression. Signals IO and QO are
developed from signals IF and QF respectively, and are similarly processed
in the manner of signal YO.
Signals YE, IE, QE and YO, IO, QO are combined by a side-center signal
combiner 28, e.g. a time multiplexer, to produce signals YN, IN and QN
with an NTSC compatible bandwidth and a 4:3 aspect ratio. These signals
are of the form of signal YN shown in FIG. 3. Combiner 28 also includes
appropriate signal delays for equalizing the transit times of the signals
being combined. Such equalizing signal delays, are also included elsewhere
in the system as required to equalize signal transit times.
Chrominance signals IN and QN are quadrature modulated on a subcarrier SC
at the NTSC chrominance subcarrier frequency, nominally 3.58 MHz, by a
modulator 30. The modulated signal is lowpass filtered in the vertical (V)
and temporal (T) dimensions by means of a 2-D (two dimensional) filter 32
before being applied to a chrominance signal input of an NTSC encoder 36.
Luminance signal YN is bandstop filtered in the horizontal (H), vertical
(V) and temporal (T) dimensions by means of a 3-D (three dimensional)
filter 34 before being applied to a luminance input of encoder 36.
Filtering luminance signal YN and chrominance color difference signals IN
and QN serves to assure that luminance-chrominance crosstalk will be
significantly reduced after subsequent NTSC encoding. Luminance filter 34
also bandstop filters the luminance signal in the spectral region where
the luminance side panel highs will be modulated, as will be discussed.
HVT bandstop filter 34 in FIG. 1 removes upwardly moving diagonal frequency
components from luminance signal YN. These frequency components are
similar in appearance to chrominance subcarrier components and are removed
to make a hole in the frequency spectrum into which modulated chrominance
side panel highs and luminance side panel highs will be inserted. The
removal of the upwardly moving diagonal frequency components from
luminance signal YN does not visibly degrade a displayed picture because
it has been determined that the human eye is substantially insensitive to
these frequency components. Filter 34 exhibits a cut-off frequency of
approximately 1.5 MHZ so as not to impair luminance vertical detail
information.
VT bandpass filter 32 reduces the chrominance bandwidth so that modulated
chrominance side panel information can be inserted into the hole created
in the luminance spectrum by filter 34. Filter 32 reduces the vertical and
temporal resolution of chrominance information such that static and moving
edges are slightly blurred, but this effect is of little or no consequence
due to the insensitivity of the human eye to such effect.
An output signal C/SL from encoder 36 contains NTSC compatible information
to be displayed, as derived from the center panel of the widescreen
signal, as well as compressed side panel lows (both luminance and
chrominance) derived from the side panels of the widescreen signal and
situated in the left and right horizontal overscan regions not seen by a
viewer of an NTSC receiver display. The compressed side panel lows in the
overscan region represent one constituent part of the side panel
information for a widescreen display. The other constituent part, the side
panel highs, is developed as follows.
Processor 18 develops signals LH (left side panel luminance highs), RH
(right side panel luminance highs), IH (I highs) and QH (Q highs) in the
side panel highs signal processing channel.
In FIG. 4, a signal YH', derived from widescreen signal YF, contains left
panel high frequency information associated with left panel pixels 1-84,
and right panel high frequency information associated with right panel
pixels 671-754. The high frequency information encompasses a bandwidth of
from 700 KHz to 5.0 MHz in this example. For each horizontal line, the
left side panel highs component between pixels 1-84 of signal YH' is time
expanded by a side expansion factor (thereby reducing its bandwidth
accordingly) and is mapped into the center panel location occupied by
pixels 85-670 to produce one component LH (FIG. 4) of the side panel
information. Simultaneously, for each horizontal line, the right side
panel highs component between pixels 671-754 of signal YH' is also time
expanded and mapped into the center panel location occupied by pixels
85-670 to produce another simultaneous component RH (FIG. 4) of the side
panel information. Simultaneously occurring signals RH and LH each exhibit
a reduced bandwidth due to the side panel expansion factor (6.96), which
is the ratio of the expanded side panel width to the original side panel
width.
Signals LH and RH are time multiplexed with signals IH and QH by a
luminance-chrominance multiplexer 42, to simultaneously produce side panel
highs signal components X and Z. Signal component X is produced by
inserting left highs luminance component LH (pixels 85-670) between the
left and right side panel highs of color difference signal IH. Similarly,
signal component Z is simultaneously produced by inserting right highs
luminance component RH (pixels 85-670) between the left and right side
panel highs of color difference signal QH.
Signals X and Z, containing the side panel highs information, each exhibit
a 0-700 KHz bandwidth and are quadrature modulated onto a horizontally
synchronized alternate subcarrier signal ASC by means of a quadrature
modulator 43. The frequency of alternate subcarrier signal ASC is chosen
to insure adequate separation (e.g. 20-30 db) of side and center
information, and to have insignificant impact upon an image displayed by a
standard NTSC receiver. In this embodiment signal ASC exhibits a frequency
of 2.368 MHz.
The frequency of 2.368 MHz chosen for alternate subcarrier signal ASC is an
interlace frequency at an odd multiple of one half of the horizontal line
rate, ie., 301.times.f.sub.H /2. This alternate subcarrier frequency
produces a fine, virtually imperceptible cross-hatch interference pattern
which does not compromise the quality of a displayed picture, compared to
a more serious "moving stripes" interface pattern which would be produced
by a non-interlace subcarrier frequency. The 2.368 MHz subcarrier
frequency advantageously resides in the frequency spectrum substantially
symmetrically between the luminance vertical detail band and the modulated
chrominance band, as shown in FIG. 11. As a result, the modulated side
panel highs information occupies a .+-.700 KHz bandwidth between the
vertical detail and chrominance frequency bands.
Quadrature modulation advantageously permits two narrowband signals to be
transmitted simultaneously. Expanding the side panel highs signals results
in their bandwidth being reduced, consistent with the narrowband
requirements of quadrature modulation. The more the bandwidth is reduced,
the less likely it is that interference between the carrier and modulating
signals will result. It is also noted that the described technique of time
multiplexing the luminance and chrominance side panel highs to produce
signals X and Z before quadrature modulation advantageously requires only
one subcarrier rather than two. Furthermore, since the DC component of the
side panel information is compressed into the overscan region, the energy
of the modulating signal, and therefore the potential interference of the
modulating signal, are greatly reduced.
To reduce the likelihood of interference produced by the quadrature
modulated signal, the signal from modulator 43 is attenuated by attenuator
44, which exhibits a signal gain of 0.25, before being bandpass filtered
along diagonal axes in the vertical-temporal (V-T) plane by bandpass
filter 46. The action of attenuator 44 has been found to reduce the
visibility of certain types of interference caused by uncorrelated
modulated side highs when viewed on a standard NTSC receiver. The
attenuation achieved by network 44 can also be produced by adjusting the
weighting coefficients of filter 46. A filtered quadrature modulated
output signal SH from filter 46, containing the side panel highs, is
combined with signal C/SL in combiner 40 to produce a widescreen NTSC
compatible signal NTSC. Signal NTSC is converted to analog form by a
digital-to-analog converter (DAC) 54 before being applied to an RF
modulator and transmitter network 55 for broadcast via an antenna 56.
The encoded NTSC compatible widescreen signal broadcast by antenna 56 is
intended to be received by both NTSC receivers and widescreen receivers.
FIGS. 5 and 6 illustrate certain portions of the encoder system of FIG. 1
in more detail.
FIG. 5 illustrates apparatus included in processor 18 of FIG. 1 for
developing signals YE, YO, LH and RH from wideband widescreen signal YF.
Signal YF is horizontally low pass filtered by a filter 510 with a cutoff
frequency of 700 KHz to produce low frequency luminance signal YL, which
is applied to one input of a subtractive combiner 512. Signal YF is
applied to another input of combiner 512 and to time de-multiplexing
apparatus 516 after being delayed by a unit 514 to compensate for the
signal processing delay of filter 510. Combining delayed signal YF and
filtered signal YL produces high frequency luminance signal YH at the
output of combiner 512.
Delayed signal YF and signals YH and YL are applied to separate inputs of
de-multiplexing apparatus 516, which includes de-multiplexing (DEMUX)
units 518, 520 and 521 for respectively processing signals YF, YH and YL.
De-multiplexing units 518, 520 and 521 respectively derive full bandwidth
center panel signal YC, side panel highs signal YH' and side panel lows
signal YL' as illustrated in FIGS. 3 and 4.
Signal YC is time expanded by a time expander 522 to produce signal YE,
while time expanders 524 and 526 expand signal YH' to produce signals LH
and RH, respectively. Signal YC is time expanded with a center expansion
factor sufficient to leave room for the left and right horizontal overscan
regions. The center expansion factor (1.19) is the ratio of the intended
width of signal YE (pixels 15-740) to the width of signal YC (pixels
75-680) as shown in FIG. 3. Signal YH' is expanded with a side expansion
factor to produce signal LH. The side expansion factor (6.97) is the ratio
of the intended width of signal LH (pixels 85-670) to the width of the
left panel component of signal YH' (pixels 1-84) as shown in FIG. 4.
Signal RH is produced by a similar process.
Signal YL' is compressed with a side compression factor by a time
compressor 528 to produce signal YO. The side compression factor (0.166)
is the ratio of the intended width of signal YO (e.g. left pixels 1-14) to
the width of the corresponding portion of signal YL' (e.g. left pixels
1-84) as shown in FIG. 3. Time expanders 522, 524 and 526 and time
compressor 528 can be of the type shown in FIG. 7, as will be discussed.
Signals IE, IH, IO and QE, QH, QO are respectively developed from signals
IF and QF in a manner similar to that by which signals YE, YH' and YO are
developed by the apparatus of FIG. 5. In this regard reference is made to
FIG. 6, which illustrates apparatus for developing signals IE, IH and IO
from signal IF. Signals QE, QH and QO are developed from signal QF in a
similar manner.
Wideband widescreen signal IF, after being delayed by a unit 614, is
coupled to de-multiplexing apparatus 616 and is also subtractively
combined with low frequency signal IL from a low pass filter 610 in a
subtractive combiner 612 to produce high frequency signal IL. Delayed
signal IF and signals IH and IL are respectively de-multiplexed by
de-multiplexers 618, 620 and 621 associated with de-multiplexing apparatus
616 to produce signals IC, IH and IL'. Signal IC is time expanded by an
expander 622 to produce signal IE, and signal IL' is time compressed by a
compressor 628 to produce signal IO. Signal IC is expanded with a center
expansion factor similar to that employed for signal YC as discussed, and
signal IL' is compressed with a side compression factor similar to that
employed for signal YL', also as discussed.
In connection with the arrangements of FIGS. 5 and 6 it is noted that, e.g.
in FIG. 5, filtering of the input signal prior to, rather than after,
being applied to de-multiplexer 716 advantageously avoids unwanted signal
edge transients in output signals LH, RH and YO. Specifically,
de-multiplexer 716 produces output signals with sharp, well defined output
transitions which would be distorted (e.g. smeared) by filtering the
output signals from de-multiplexer 716.
FIG. 7 illustrates raster mapping apparatus which can be used for the time
expanders and compressors of FIGS. 5 and 6. In this regard, reference is
made to the waveforms of FIG. 7a which illustrates the mapping process.
FIG. 7a shows an input signal waveform S with a center portion between
pixels 84 and 670 which is intended to be mapped into pixel locations
1-754 of an output waveform Y by means of a time expansion process. End
point pixels 84 and 670 of waveform S map directly into end point pixels 1
and 754 of waveform Y. Intermediate pixels do not map directly on a 1:1
basis due to the time expansion, and in many cases do not map on an
integer basis. The latter case is illustrated wherein, for example, pixel
location 85.33 of input waveform S corresponds to integer pixel location 3
of output waveform Y. Thus pixel location 85.33 of signal S contains an
integer part (85) and a fractional part DX (.33), and pixel location 3 of
waveform Y contains an integer part (3) and a fractional part (0).
In FIG. 7, a pixel counter 710 operating at a 4xf.sub.sc rate provides an
output WRITE ADDRESS signal M representative of pixel locations (1 . . .
754) on an output raster. Signal M is applied to PROM (Programmable Read
Only Memory) 712 which includes a look-up table containing programmed
values depending upon the nature of raster mapping to be performed, eg.,
compression or expansion. In response to signal M PROM 712 provides an
output READ ADDRESS signal N representing an integer number, and an output
signal DX representing a fractional number equal to or greater than zero
but less than unity. In the case of a 6-bit signal DX (2.sup.6 =64),
signal DX exhibits fractional parts 0, 1/64, 2/64, 3/64 . . . 63/64.
PROM 712 permits expansion or compression of a video input signal S as a
function of stored values of signal N. Thus a programmed value of READ
ADDRESS signal N and a programmed value of fractional part signal DX are
provided in response to integer values of pixel location signal M. To
achieve signal expansion, for example, PROM 712 is arranged to produce
signal N at a rate slower than that of signal M. Conversely, to achieve
signal compression, PROM 712 provides signal N at a rate greater than that
of signal M.
Video input signal S is delayed by cascaded pixel delay elements 714a, 714b
and 714c to produce video signals S(N+2), S(N+1) and S(N) which are
mutually delayed versions of the video input signal. These signals are
applied to video signal inputs of respective dual port memories 716a-716d,
as are known. Signal M is applied to a write address input of each of
memories 716a-716d, and signal N is applied to a read address input of
each of memories 716a-716d. Signal M determines where incoming video
signal information will be written into the memories, and signal N
determines which values will be read out of the memories. The memories can
write into one address while simultaneously reading out of another
address. Output signals S(N-1), S(N), S(N+1) and S(N+2) from memories
716a-716d exhibit a time expanded or time compressed format depending upon
the read/write operation of memories 716a-716d, which is a function of how
PROM 712 is programmed.
Signals S(N-1), S(N), S(N+1) and S(N+2) from memories 716a-716d are
processed by a four-point linear interpolator including peaking filters
720 and 722, a PROM 725 and a two point linear interpolator 730, details
of which are shown in FIGS. 7b and 7c. Peaking filters 720 and 722 receive
three signals from the group of signals including signals S(N-1), S(N),
S(N+1) and S(N+2), as shown, as well as receiving a peaking signal PX. The
value of peaking signal PX varies from zero to unity as a function of the
value of signal DX, as shown in FIG. 7d, and is provided by PROM 725 in
response to signal DX. PROM 725 includes a look-up table and is programmed
to produce a given value of PX in response to a given value of DX.
Peaking filters 720 and 722 respectively provide peaked mutually delayed
video signals S'(N) and S'(N+1) to two-point linear interpolator 730 which
also receives signal DX. Interpolator 730 provides a (compressed or
expanded) video output signal Y, where output signal Y is defined by the
expression
Y=S'(N)+DX[S'(N+1)-S'(N)]
The described four-point interpolator and peaking function advantageously
approximates a (sin X)/X interpolation function with good resolution of
high frequency detail.
FIG. 7b shows details of peaking filters 720 and 722, and interpolator 730.
In FIG. 7b, signals S(N-1), S(N) and S(N+1) are applied to a weighting
circuit 740 where these signals are respectively weighted by peaking
coefficients -1/4, 1/2 and -1/4. As shown in FIG. 7c, weighting circuit
740 comprises multipliers 741a-741c for respectively multiplying signals
S(N-1), S(N) and S(N+1) with peaking coefficients -1/4, 1/2 and -1/4.
Output signals from multipliers 741a-741c are summed in an adder 742 to
produce a peaked signal P(N), which is multiplied by signal PX to produce
a peaked signal which is summed with signal S(N) to produce peaked signal
S'(N). Peaking filter 722 exhibits similar structure and operation.
In two point interpolator 730, signals S'(N) is subtracted from S'(N+1) in
a subtractor 732 to produce a difference signal which is multiplied by
signal DX in a multiplier 734. The output signal from multiplier 734 is
summed with signal S'(N) in an adder 736 to produce output signal Y.
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