CA1327648C - Apparatus for converting an rgb signal into a composite video signal and its use in providing computer generated video overlays - Google Patents
Apparatus for converting an rgb signal into a composite video signal and its use in providing computer generated video overlaysInfo
- Publication number
- CA1327648C CA1327648C CA000598032A CA598032A CA1327648C CA 1327648 C CA1327648 C CA 1327648C CA 000598032 A CA000598032 A CA 000598032A CA 598032 A CA598032 A CA 598032A CA 1327648 C CA1327648 C CA 1327648C
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- signal
- video signal
- composite video
- video
- rgb
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- H—ELECTRICITY
- H01—ELECTRIC ELEMENTS
- H01F—MAGNETS; INDUCTANCES; TRANSFORMERS; SELECTION OF MATERIALS FOR THEIR MAGNETIC PROPERTIES
- H01F38/00—Adaptations of transformers or inductances for specific applications or functions
- H01F38/42—Flyback transformers
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- G—PHYSICS
- G09—EDUCATION; CRYPTOGRAPHY; DISPLAY; ADVERTISING; SEALS
- G09G—ARRANGEMENTS OR CIRCUITS FOR CONTROL OF INDICATING DEVICES USING STATIC MEANS TO PRESENT VARIABLE INFORMATION
- G09G1/00—Control arrangements or circuits, of interest only in connection with cathode-ray tube indicators; General aspects or details, e.g. selection emphasis on particular characters, dashed line or dotted line generation; Preprocessing of data
- G09G1/06—Control arrangements or circuits, of interest only in connection with cathode-ray tube indicators; General aspects or details, e.g. selection emphasis on particular characters, dashed line or dotted line generation; Preprocessing of data using single beam tubes, e.g. three-dimensional or perspective representation, rotation or translation of display pattern, hidden lines, shadows
- G09G1/14—Control arrangements or circuits, of interest only in connection with cathode-ray tube indicators; General aspects or details, e.g. selection emphasis on particular characters, dashed line or dotted line generation; Preprocessing of data using single beam tubes, e.g. three-dimensional or perspective representation, rotation or translation of display pattern, hidden lines, shadows the beam tracing a pattern independent of the information to be displayed, this latter determining the parts of the pattern rendered respectively visible and invisible
- G09G1/16—Control arrangements or circuits, of interest only in connection with cathode-ray tube indicators; General aspects or details, e.g. selection emphasis on particular characters, dashed line or dotted line generation; Preprocessing of data using single beam tubes, e.g. three-dimensional or perspective representation, rotation or translation of display pattern, hidden lines, shadows the beam tracing a pattern independent of the information to be displayed, this latter determining the parts of the pattern rendered respectively visible and invisible the pattern of rectangular co-ordinates extending over the whole area of the screen, i.e. television type raster
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- G—PHYSICS
- G09—EDUCATION; CRYPTOGRAPHY; DISPLAY; ADVERTISING; SEALS
- G09G—ARRANGEMENTS OR CIRCUITS FOR CONTROL OF INDICATING DEVICES USING STATIC MEANS TO PRESENT VARIABLE INFORMATION
- G09G1/00—Control arrangements or circuits, of interest only in connection with cathode-ray tube indicators; General aspects or details, e.g. selection emphasis on particular characters, dashed line or dotted line generation; Preprocessing of data
- G09G1/28—Control arrangements or circuits, of interest only in connection with cathode-ray tube indicators; General aspects or details, e.g. selection emphasis on particular characters, dashed line or dotted line generation; Preprocessing of data using colour tubes
- G09G1/285—Interfacing with colour displays, e.g. TV receiver
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- H—ELECTRICITY
- H04—ELECTRIC COMMUNICATION TECHNIQUE
- H04N—PICTORIAL COMMUNICATION, e.g. TELEVISION
- H04N9/00—Details of colour television systems
- H04N9/64—Circuits for processing colour signals
- H04N9/641—Multi-purpose receivers, e.g. for auxiliary information
-
- G—PHYSICS
- G09—EDUCATION; CRYPTOGRAPHY; DISPLAY; ADVERTISING; SEALS
- G09G—ARRANGEMENTS OR CIRCUITS FOR CONTROL OF INDICATING DEVICES USING STATIC MEANS TO PRESENT VARIABLE INFORMATION
- G09G2340/00—Aspects of display data processing
- G09G2340/12—Overlay of images, i.e. displayed pixel being the result of switching between the corresponding input pixels
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- Engineering & Computer Science (AREA)
- Computer Hardware Design (AREA)
- Radar, Positioning & Navigation (AREA)
- Remote Sensing (AREA)
- Physics & Mathematics (AREA)
- General Physics & Mathematics (AREA)
- Theoretical Computer Science (AREA)
- Power Engineering (AREA)
- Signal Processing (AREA)
- Multimedia (AREA)
- Processing Of Color Television Signals (AREA)
- Color Television Systems (AREA)
- Controls And Circuits For Display Device (AREA)
Abstract
ABSTRACT OF THE DISCLOSURE
A circuit for converting RGB to composite video. The circuit is particularly useful in a video overlay application. Notch filters are used both in the video path and keying path. The notch filters are centered at frequencies equal to the dot clock frequency of the RGB divided by integers where the result of this division falls within the chroma subcarrier spectrum.
A circuit for converting RGB to composite video. The circuit is particularly useful in a video overlay application. Notch filters are used both in the video path and keying path. The notch filters are centered at frequencies equal to the dot clock frequency of the RGB divided by integers where the result of this division falls within the chroma subcarrier spectrum.
Description
132764~
BACKGROUND OF THE INVENTION
1. Field of th~ Inventi()n.
Ths invention relates to the field of converting RGB signals into a composite video signal, and its use in video overlays.
BACKGROUND OF THE INVENTION
1. Field of th~ Inventi()n.
Ths invention relates to the field of converting RGB signals into a composite video signal, and its use in video overlays.
2. Prior Art.
Some commercially available computers, particularly personal computers, provide circuitry which permits the merger of a composite video signal (e.g., NTSC
signal, more specifically Proposed Standard ElA RS-170A) with a computer generated video graphics display, typically red, green, blue (RGB) signals. The RGB signals may represent a video overlay such as text intended to be displayed with the eomposite video signal. In a typical application an NTSC signal from a broadcasting station, video disk or tape, or other source of a composite video signal is merged with the signal representing the video overlay to provide, for exampls, titles or subtitles over a background image represented by ths composite signal.
OltQn, in the prior art, the signal representing the overlay image is storad in a frame buffer. The portion of this overlay image in which the background image from thls composite signal is to show thr~ugh is assigned a ~key color~ in tha buffer which is distinct from any other color in the overlay image. Th~s buffer is thenscannlsd out synchronously with the composite video signal and ~he contents of the buffer are compared, pixel by pixel, with the ~key color~ When the comparator indicates that the porlion of thls buffer being scanned out contains the ~key color~, a switch sel~cts the composite video signal to be the output signal. On the othar hand, when other than the ~key color~ is scanned in the buffer (indicating that the overlay irnage is being scanned) a signal from the comparator (key or keying signal) causlss the switch to select thls contents of the buffer. This arrangement : ` ' 13276~8 permits the overlay image to be shown in all colors which the buf~er can store, except for the ~key color~.
The frame buffer stores the overlay signal in digital torm (e.g., RGB or an index to an RGB lookup table). The RGB signal is converted to a composite 5 overlay signal be~ore being coupled to ths switch, thus, the switch selects between first and second analog signals. In another prior art arrangem~nt, the composite video signal is converted to an RGB signal and th3 switch selects between first and second RGB signals.
Commercially available integrated circuits are used to perform the above 10 described functions, such as MC 1378 and TDA 3301, both manufactured by Motorola Semiconductor, Inc. Phase locked loops are used to synchronize the scanning of the frame buffer with the composite video signal.
In many applications it is desirable to provide both merged RGB signals and a merged composite video signal. This a110ws a user to record the composRe 15 video signal while monitoring the merged' images on an RGB monitor. One prior art circuit for providing both output signals first converts the composite video into an RGB signal and then merges two RGB signals. Ths results of this merger provide first output RGB signals. A second composite output signal is provided by converting the merged RGB signal into a composite video signal. This 20 arrangement provides a relativeiy poor second video output signal that results from the second conversion~ Note that thers is, Tn effect, a ~serial" double conv~rsion, and the second conversion provides relatively poor video because there is unavoidable signal distortion from sach conversion step~ As will ba seen with the present invention a double merging (not a double, serial conversion) is25 used to soive this problem and ther~by providing both high quality RGB and composRe signal outputs~
- 132764~
Video artifacts are known to occur where computer generated images are displayed on composite vid~o monitors. These arti~acts are caused in part by high fre~uency video signals ~chroma crosstalk~ that occur at ths color ref~rence fre~uency of 3.58MHz for NTSC signals. There are numerous filters used that 5 attempt to remove these artifacts. Oflen, a 3.58MHz notch filter is used to prefilter the luminance component of an NTSC signal~ Applicant believes that lowpass fiRers may be used in the prior art to remove all chroma signals, for exanlple, above 3.0MHz. This eliminates the ar~ifacts but at the cost of destroying some of the image sharpness.
R has been found that when tha frequoncy of tha dot clock associatcd with RGB signals is not a harmonic of tho color reference frequency, color artifacts appear when the signal is converted to composite video despite tho presence of a3.58MH2 notch filter. The present inve~tion solves this problem by using a~dWonal notch fllters including notch filters to filter the keying signal.
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An improvement in an apparatus which converts RGB signals into a composite video signal is disclosed. Notch filters are used centered at certain 5 frequencies. These frequ~ncies are equal to the dot clock frequency of the RGBsignal divided by integers provided that thQ frequency resulting from this division ~alls within the chroma subcarrier spectrum.
This improvement is used in an apparatus ~or providing video output signals containing a composite video signal merged with ~irst RGB video overlay signals.10 A keying signal generation means provides a keying signal indicating when theimage represented by the video overlay signal overlies the image represented by the composite video signal. A first converter converts the video composite signal into second RGB signals. A second converter is used to convert the first RGB
signals into a composite overlay signal using notch filters to ~iRer the luminance 15 signal in the conversion process. Merging occurs in both a first and a secondswitch A first switch under control of the keying signal selects between the first and second RGB signals. In a second switch the keying signal, filtered by notch filters, is used to select between the composite video s;gnal and 1he composite overlay signal.
Accordingly, in one aqpect the present invention resides in ~n apparatus for c~nverting red, green and blue (RGB) video signals having a dot ~requency of M into a composite video signal having a color referenc~ frequency N, where ~1 is not a harmonic of N, said apparatus having terminals for receiving filters for at least one of the components Y, R-Y
and B-Y of the composit~ video signal, where Y is the luminance component of the composite video signal, an improvement comprising:
a filter coupled to receive and filter said luminance component of said composite video signal, said filter having a frequency characteristic which has a notch center~d at a frequency approximately equal to M divided by an integer, and where said frequency is within thl3 chroma subcarrier spectrum of said composite video signal.
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`` 1327648 In another aspect, the pre~ent lnvention resid~s in an apparatus for convorting an RG~ video ~ignal having a dot frequency of approximately 12.272 MHz into a composite video signal having a color reference frequency of approximately 3.58 MHz, said apparatus having terminals for receiving filters for at least one of the components Y, R-Y and B-Y
of 1he composite video signal, where Y is the luminance component of the composite video signal, an improvement comprising:
a first notch filter coupled to receive and filter said luminance component of said composite video signal, said nrst notch nlter having a notch centered at approximalely 4~09 MHz;
a second notch filter coupled to receive and filter said luminance component of said composite vidso signal, said second notch filter having a notch c~ntered at a frequency of approximately 3.07 MHz.
Other aspect3 o~ the present invention will be apparent fro~ the detailed description of the invention.
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Flgure 1 is an electrical schematic of the presently preferred embodiment of the invention.
Figure 2 is a general block diagram illustrating one aspect of the present invention.
FigurQ 3a is a flow diagram illustrating the steps used to implement one 10 aspect of the present invention~
Figure 3b is a flow diagram illustrating a specific example for dat~rmining the freiquency of notches tor notch fiiters based on the flow diagram of Figure 2a~
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", .~;... ..
D. . AILED DESCRlPTlOt~ OF THE PRESENT INVENTION 1 3 2 7 6 ~ 8 The presently preferre!d embodiment of the present invention is an apparatus for providing output video signals which represent a composite video 5 signal merged with a video overlay signal. This preferred embodiment as well as other aspects of the present invention are described below. In the following description numerous specific details are set forth such as specific ~requencies in order to provide a thorough understanding of the present invantion. It will be obvious, however, to one skilled in the art that these specific details need not be 10 employed to practice the prèsent invention. In other instances, well-known cirwits are shown in block diagram form in ordar not to unnacessarily obscure the present invention.
Referring first to Figur~ 2, a computer 53 is shown which includes a memory or buffer 54~ This computer may bQ any one of sQveral comm~rcially available 15 computers including personal computers. Buffer 54 is typically the random-access memory (RAM) of the computer, and for purposes of discussion, this bufferis re~rred to as a ~frame buffer~ in that it tores one frame of video inforrnation.
Digital signals reprassnting color data such as red, grecn, blue (RGB) data is storQd for each piXQI of thQ display. In some cases indexes are stored in the frame 20 Wer; these indexes point to colors in a color look-up table.
It i~ sometimes necessary to convert the digital (RGB) video signals into a composite video signal. This is typically done by reading the data from the buffer ~4 in synchronous wHh the color reference frequency associated wi)h the composite video signal. The RCB signals are then converted to a composite video 25 format such as that used in the U.S~A~ (NTSC). The conver~er 56 of Figure 2 performs this function. Comm~rcialq~ available circuits are used to convert the RGB signals to NTSC signals such as the Motorola, Inc. MC1377. These .
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co..,mercially available circuits include taps which allow filtsrs to bs addsd to lhe R-Y, B-Y and Y(luminance) signals. Typically, a 3.58MHz notch Siltsr 58 is used to filter the luminance signal to reducs color artifacts that result whsn Y, R-Y and B-Y ars convsrtsd into a composit~ signal. Thsse color artifacts rssult ~rom high frequency luminance signals that fall within the capturs range (i.e., the chromasubcarrier spectrum) o~ the color decoder circuit of ths NTSC receivsr (approximately between 3.0 to 4.2MHz). Although the luminance signal is intended to carry only monochromatic information, th~ color decodsr will intsrpret any energy in ths chroma subcarrier spectnJm to be chroma information and will display color artifacts (i.e., wrong colors) from luminance information within this range of frequencies. As will bs seen with the teachings of lhs present invention, additional notch fiRers are used such as filters 59 and 60 shown coupled in series with the filter 58 to eliminate luminance energy in the chroma spsctrum that will resua in a visible color artifact.
The chroma subcarrier spsctrum may be considersd to be the fr~quency band over which visible artifacts are procluced on a display. This, to some extent, will vary from display-to-display and as a function of Individual eyesight. For pres~nt purposes, th~ chroma subcarrier spectrum is assumed to be between 3.0-4.2MHz for an NTSC signal.
The r~te at which the pixel data is read from the frame buffer 54 and couplsd onto line 55 for convsrsion by the convener 56 is referred to as the dot clock frequ~ncy. This is shown as frequency M in Figure 2. The period of this frequoncy corresponds to the horizontal line frequency of the composits vidso si~nal less the horizontal blanking interval divided by the number of pixsls stored 2~ for each line in buffer 54. This number may not be a harmonic of the color reference frequsncy assodated with the composite video signal (N=3.58MHz for ~ --, i . ;.. . .. .... . . ..
13276~8 NTSC), but i~ typically is phased locked to that ~requency (e.g., 12.272 MHz is 24/7 of 3.5~MHz).
The present invention teaches the use of notch filters to remove certain frequencies from the luminance component of the composite signal which resutt 5 from the dot clock frequency.
More specifically, referring to Figure 3a, first the dot ck)ck frequency is determined as shown by block 61. Next, this frequency is divided by the integers1, 2, 3, 4, etc. Each of ~he quotients ars then examined to determine if they fall within a predetermined frequency window (block 63~. This is the window defined 10 by the chroma subcarrier reference spectrum. For an NTSC signal having a color referencc frequency o~ 3.85MHz, the range is approximately between 3-4.2MHz.
A notch fiRer Is used for each of the quotients that fall within the predetermined window. Each of these filtsrs has a notch centered at the frequency equal to thequotient.
A typical sxampl~ is shown in Figure 3b. Assume that the dot clock fre~u~ncy is equal to 12.272MHz as shown by block 65. This number is divided by 1, 2, 3, 4, etc~ giving the quotients 12.272, 6.136, 4.09, 3.07, etc., as shown in block 66~
Next, as shown by block 67, each of the quotients are examined to determine if they fall within the range of 3~0 - 4.2 MH~ For the integer 1, 12~272 falls outside this window~ This also true for the inleger 2~ For the integers 3 and 4, th~ quotients fall within th6 range of 3.~4.2MHz. For integers 5 and largQr, thequotients fall below the lower fr0quency of the window and hence, notch filters are not required for these frequencies.
Now, as shown by block 68, notch filters ar~ used wh~re th0 characteristic of the filt~r has a notch centered at 4.09MHz and 3.07MHz. ~eferr~ng to Figuro 2, the filter 59 ha~ e~ notch centered at 3.07MHz and the filter 60 has a notch - . . ;. ,. .. .. ~
centered at 4 . O9MHz . These fllter~ ~re used ln add~tiOn to the often used notchl f~lter h~vln~ a notch located ~t th~
color reference frequency at 3.58MHz.
The use of the notch filters 59 and 60 has been found to remove artifacts not remov~d by filter 58. As will be seen in th~ pressntly preferred embodiment, thenotch filters are also used to filter the keying signal associated with a video overlay.
The notch filters used in the presently preferred embodiment are ordinary filters fabricated from discrete, passive components. The filters provid~
approximate~ 35dB att~nuation at their centQr frequency and have a Q of approximately 1.9.
PRESENTLY PREFERRED EMBODIMENT OF THE INVENTION
Rsferring to Figure 1, a computer 10 is illustrated which may b~ any one of a plurality of commercially available computers such as the APPIQ 11 computer.
The computer includes a m~mory which is used as a frame buffer. A user may enler into this frame buffer signals representing an image (overlay image) which is displayed in conjunction with anothar imagQ. This other image is illustrated as an NTSC (extQmal) video signal (composite video signal) applied to line 30. Ths signal on line 30 may be receivcd from a TV broa~casting station, video disk, video tape, anothsr computQr, or other source of compositQ video signal.
l h~ extemal video signal on line 30 is shown coupled to a circuit 11 within computer 10. This arcuit provides synchronization signals to determine ths rate at which data is r~ad from ths frame buffcr onto lines 14. This is the dot clockfr~quency M pr~viously discuss~d in conjunction with Figure 2. This clot clock is coupl~d to the comparator 12 on lin~ 16 as illustrated in FigurQ 1. - `
~5 GQn~ralb~ for a video overlay, the frame buffer in the computer 10 stores the ovQrlay imagQ in any color except for ons color referrQd to the as th~ key color.
Th~ It9y color is storQd in all other pixel locations. The key color is coupl~d to th~
13276~8 conlparator 12 by lines 13 als illustrated in Figur~ 1. On a pixe~by-pixel basis, the key color is compared to the contents of th~ ~rame buffer as the color data is read from the frame buffer. When the key color is not the same color as contained in the frame buffer, the overlay is display~d and th~ comparator 12 provides a k~ying 6 signal on line 45. This is a common prior art technique. (In some cases, indices to colors in a color lookup 1able are stored in the ~rame buffer.) The comparison shown in Figure 1 within comparator 12 is a digital comparison. The digital RGB signals (both th~ signals on line 14 and th3 key color) can be com/erted to analog form and the comparison done by comparing 10 two analog signals. As illustrated the digital signals on lines 14 (four bits for each color) are converled to analog form (one analog signal for red, one ~or green and one for blue) by convcrters 15 and thsn coupled to switch 22 and conn3ctor 32.
This conversion, howev~r, is not don~ ~or the comparison function.
The keying signal on line 45 controls a pulse width modulated signal 1~ generated by an ordinary gating means ~modulator 46) so as to provide blending (fading) between the ov~rlay image and th~ external vid~o image~ Registers 50 and 51, writabl~ by the computer 10, each store a 4 bit code that indicates thc level of blending for each state, 0 or 1, of kcying signal 45~ A multiplex~r 52,controlled by the keying signal 45, selects between ths output codes of registsrs 20 50 and 51 and couples tho selectcd cod~ through lines 70 to the modulator 46. A
signal of approximately 28MHz is couplQd to tho modulator 46 as the pulse width clock and the modulator 46 generates a pulsQ width modulated signal ot a duty cycle specified by th~ code on lines 70. Th~ presently preferred embodiment supports pulse width modulation duty cycles of 0~/100%, læ5~o/87.5%, :
25 25Yo/75%, 50%l50%, 75%l25%, 87.5%/12.5%, and 100%/0%.
The pulso width modulated signal generated by modulator 46 is tho swTtching signal used by the present invcntion to select between the ov~rlay . .
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image and the extemal video image. Since the pulse width modulation can provide a rapid switching between the two images, it is possible to creata the illusion of a weighted blending (fading) between the two images. For example, itthe duty cycle of the pulse width modulation is 25%/75%, then a blending of 25%
5 of one image and 75% of the other image is achieved. The two fade registers 50and 5t provide for a different blend weighting for each state of the keying signal.
For simple keying with ~lading turned of~ the code in fade register 50 is set to100%/0% blending and the code in fade register 51 is set to 0/J100% blending.
To reduce aliasing of th~ pulse width modulation of modulator 46 with image t 0 patterns in the overlay image, the pulse width modulation phase can be inverted each video field, and inverted again each video frame. This causes the overlay image and external video imags keying pattern to alternate by video line and by video frame for an enhanced blending eff~ct.
The keying signal is coupled to switch 22 and, after being filtered by a cutoff 15 filter 47, is coupled to filters 48 and 49~ The filters 48 and 4g have the center frequenciesoftheirnotcheslocatedat 4.09M~z and 3.07MHz a~ taught by the present invention and described in coniunction with Figures 2 and 3. The filtering of the keyin~ signal has been found to reduce artifacts~
In accordance with the present inv~ntion, two merging switches are used to 20 mQrgs the extarnal video signal on line 30 with the overlay signal. In a first circuit 18, the extemal video signal is converted from its NTSC format (or other composite forrnat) to RGB signals by a converter 20. The RGB signals resulting from this conversion (three analog signals) are connected to a first switch 22 via lines 24.
Switch 22 selects between the ou1put of the converter 20 and the RGB signals 25 representing the video overlay from the computer 10. The outputs of switch 22 are merged RGB analog signals on lines 26. That is, the signals on line 26 contain, 11 ~` '""' .
fro"l a viewing standpoint, th~ external video image overlayed with the overlay image ~rom the computer 10~
Gircuit 18 may be a commercially available part, such as Motorola TDA3301. For this commercial part the switch 22 selects either the signals on lines 14 or 24 (not a blend o~ both) and lor this reason the output of a pulse width modulator 46 is used to provide the ~ading.
A second circuit 31 includes a converter 32 which converts the RGB
signals on lines 72 to an NTSC signal on line 40. The switch 33 selects between the composite signals on lines 30 and 40. The merged output composits signal on 1 0 line 34 represents the same image as the ima~e represented by the signals on line 26.
The circuit 32 may be a commQrcially available arcuit such as th~ Motorola MC1378. For this àrcuit, the switch 33 is not ~discrete~, that is, depQnding on the bvel of the control signal to ths switch, both composite input signals can be 1 5 simultaneously selected, thsreby providing blending between the signals. For this reason, the keying signal from the pulse width modulator 46 i~ tirst coupl~d to a cutoff filter 47 which sffectively convsrts the pulse width modulated signal to an analog control signal for thQ switch 33. This keying signal is then coupled to filters 48 and 49 as discussed in conjunction with Figure 2 and 3. The output of the filter 20 4g provides a control signal for the switch 33~
The drcuit of Figure 1 provides both an RGB and composite video output signals (lines 26 and 34, rsspectiveh"). Importantly, it should be noted that noportion of the signals on lines 26 have been twice converted and the same is true ~or th~ NTSC signal on line 34. In the prior art, the double merging provided by25 switchQs 22 and 33 was not employed The convertsr 32 provides temlinals to which filters may be coupl~d. The finers 35, 36 and 37 are ordinary flltels which are coupled to the R-Y, B-Y and Y
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terminals. Additionally, as dilscussed previously, with the present invention, additional notch filters 38 ancl 39 are used to filter the luminance component of the compositesignal. ThesefiltersprovidenotcheSCenteredat 4.09 and 3.07MHz for an NTSC signal where the dot Glock frequency is cqual to 12.272MHz.
For some video signals artifacts may not appear because of the very nature of the image/colors defined by the signal (e.g., black and white signal). In these cases, it may be desirable to disable particularly filters 37, 38 and 39 to maximize bandwidth. A filter enable signal is shown coupled to these filters to allow, for example, the manual selection/deselection of the filters.
Thus, an apparatus has been described which provides improved conversion of a computer generated RGB signal to a composite signal. In the presently preferred embodiment, this improved conversion is used as part of a video overlay apparatus which has both an NTSC and RGB outputs.
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Some commercially available computers, particularly personal computers, provide circuitry which permits the merger of a composite video signal (e.g., NTSC
signal, more specifically Proposed Standard ElA RS-170A) with a computer generated video graphics display, typically red, green, blue (RGB) signals. The RGB signals may represent a video overlay such as text intended to be displayed with the eomposite video signal. In a typical application an NTSC signal from a broadcasting station, video disk or tape, or other source of a composite video signal is merged with the signal representing the video overlay to provide, for exampls, titles or subtitles over a background image represented by ths composite signal.
OltQn, in the prior art, the signal representing the overlay image is storad in a frame buffer. The portion of this overlay image in which the background image from thls composite signal is to show thr~ugh is assigned a ~key color~ in tha buffer which is distinct from any other color in the overlay image. Th~s buffer is thenscannlsd out synchronously with the composite video signal and ~he contents of the buffer are compared, pixel by pixel, with the ~key color~ When the comparator indicates that the porlion of thls buffer being scanned out contains the ~key color~, a switch sel~cts the composite video signal to be the output signal. On the othar hand, when other than the ~key color~ is scanned in the buffer (indicating that the overlay irnage is being scanned) a signal from the comparator (key or keying signal) causlss the switch to select thls contents of the buffer. This arrangement : ` ' 13276~8 permits the overlay image to be shown in all colors which the buf~er can store, except for the ~key color~.
The frame buffer stores the overlay signal in digital torm (e.g., RGB or an index to an RGB lookup table). The RGB signal is converted to a composite 5 overlay signal be~ore being coupled to ths switch, thus, the switch selects between first and second analog signals. In another prior art arrangem~nt, the composite video signal is converted to an RGB signal and th3 switch selects between first and second RGB signals.
Commercially available integrated circuits are used to perform the above 10 described functions, such as MC 1378 and TDA 3301, both manufactured by Motorola Semiconductor, Inc. Phase locked loops are used to synchronize the scanning of the frame buffer with the composite video signal.
In many applications it is desirable to provide both merged RGB signals and a merged composite video signal. This a110ws a user to record the composRe 15 video signal while monitoring the merged' images on an RGB monitor. One prior art circuit for providing both output signals first converts the composite video into an RGB signal and then merges two RGB signals. Ths results of this merger provide first output RGB signals. A second composite output signal is provided by converting the merged RGB signal into a composite video signal. This 20 arrangement provides a relativeiy poor second video output signal that results from the second conversion~ Note that thers is, Tn effect, a ~serial" double conv~rsion, and the second conversion provides relatively poor video because there is unavoidable signal distortion from sach conversion step~ As will ba seen with the present invention a double merging (not a double, serial conversion) is25 used to soive this problem and ther~by providing both high quality RGB and composRe signal outputs~
- 132764~
Video artifacts are known to occur where computer generated images are displayed on composite vid~o monitors. These arti~acts are caused in part by high fre~uency video signals ~chroma crosstalk~ that occur at ths color ref~rence fre~uency of 3.58MHz for NTSC signals. There are numerous filters used that 5 attempt to remove these artifacts. Oflen, a 3.58MHz notch filter is used to prefilter the luminance component of an NTSC signal~ Applicant believes that lowpass fiRers may be used in the prior art to remove all chroma signals, for exanlple, above 3.0MHz. This eliminates the ar~ifacts but at the cost of destroying some of the image sharpness.
R has been found that when tha frequoncy of tha dot clock associatcd with RGB signals is not a harmonic of tho color reference frequency, color artifacts appear when the signal is converted to composite video despite tho presence of a3.58MH2 notch filter. The present inve~tion solves this problem by using a~dWonal notch fllters including notch filters to filter the keying signal.
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An improvement in an apparatus which converts RGB signals into a composite video signal is disclosed. Notch filters are used centered at certain 5 frequencies. These frequ~ncies are equal to the dot clock frequency of the RGBsignal divided by integers provided that thQ frequency resulting from this division ~alls within the chroma subcarrier spectrum.
This improvement is used in an apparatus ~or providing video output signals containing a composite video signal merged with ~irst RGB video overlay signals.10 A keying signal generation means provides a keying signal indicating when theimage represented by the video overlay signal overlies the image represented by the composite video signal. A first converter converts the video composite signal into second RGB signals. A second converter is used to convert the first RGB
signals into a composite overlay signal using notch filters to ~iRer the luminance 15 signal in the conversion process. Merging occurs in both a first and a secondswitch A first switch under control of the keying signal selects between the first and second RGB signals. In a second switch the keying signal, filtered by notch filters, is used to select between the composite video s;gnal and 1he composite overlay signal.
Accordingly, in one aqpect the present invention resides in ~n apparatus for c~nverting red, green and blue (RGB) video signals having a dot ~requency of M into a composite video signal having a color referenc~ frequency N, where ~1 is not a harmonic of N, said apparatus having terminals for receiving filters for at least one of the components Y, R-Y
and B-Y of the composit~ video signal, where Y is the luminance component of the composite video signal, an improvement comprising:
a filter coupled to receive and filter said luminance component of said composite video signal, said filter having a frequency characteristic which has a notch center~d at a frequency approximately equal to M divided by an integer, and where said frequency is within thl3 chroma subcarrier spectrum of said composite video signal.
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`` 1327648 In another aspect, the pre~ent lnvention resid~s in an apparatus for convorting an RG~ video ~ignal having a dot frequency of approximately 12.272 MHz into a composite video signal having a color reference frequency of approximately 3.58 MHz, said apparatus having terminals for receiving filters for at least one of the components Y, R-Y and B-Y
of 1he composite video signal, where Y is the luminance component of the composite video signal, an improvement comprising:
a first notch filter coupled to receive and filter said luminance component of said composite video signal, said nrst notch nlter having a notch centered at approximalely 4~09 MHz;
a second notch filter coupled to receive and filter said luminance component of said composite vidso signal, said second notch filter having a notch c~ntered at a frequency of approximately 3.07 MHz.
Other aspect3 o~ the present invention will be apparent fro~ the detailed description of the invention.
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Flgure 1 is an electrical schematic of the presently preferred embodiment of the invention.
Figure 2 is a general block diagram illustrating one aspect of the present invention.
FigurQ 3a is a flow diagram illustrating the steps used to implement one 10 aspect of the present invention~
Figure 3b is a flow diagram illustrating a specific example for dat~rmining the freiquency of notches tor notch fiiters based on the flow diagram of Figure 2a~
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D. . AILED DESCRlPTlOt~ OF THE PRESENT INVENTION 1 3 2 7 6 ~ 8 The presently preferre!d embodiment of the present invention is an apparatus for providing output video signals which represent a composite video 5 signal merged with a video overlay signal. This preferred embodiment as well as other aspects of the present invention are described below. In the following description numerous specific details are set forth such as specific ~requencies in order to provide a thorough understanding of the present invantion. It will be obvious, however, to one skilled in the art that these specific details need not be 10 employed to practice the prèsent invention. In other instances, well-known cirwits are shown in block diagram form in ordar not to unnacessarily obscure the present invention.
Referring first to Figur~ 2, a computer 53 is shown which includes a memory or buffer 54~ This computer may bQ any one of sQveral comm~rcially available 15 computers including personal computers. Buffer 54 is typically the random-access memory (RAM) of the computer, and for purposes of discussion, this bufferis re~rred to as a ~frame buffer~ in that it tores one frame of video inforrnation.
Digital signals reprassnting color data such as red, grecn, blue (RGB) data is storQd for each piXQI of thQ display. In some cases indexes are stored in the frame 20 Wer; these indexes point to colors in a color look-up table.
It i~ sometimes necessary to convert the digital (RGB) video signals into a composite video signal. This is typically done by reading the data from the buffer ~4 in synchronous wHh the color reference frequency associated wi)h the composite video signal. The RCB signals are then converted to a composite video 25 format such as that used in the U.S~A~ (NTSC). The conver~er 56 of Figure 2 performs this function. Comm~rcialq~ available circuits are used to convert the RGB signals to NTSC signals such as the Motorola, Inc. MC1377. These .
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co..,mercially available circuits include taps which allow filtsrs to bs addsd to lhe R-Y, B-Y and Y(luminance) signals. Typically, a 3.58MHz notch Siltsr 58 is used to filter the luminance signal to reducs color artifacts that result whsn Y, R-Y and B-Y ars convsrtsd into a composit~ signal. Thsse color artifacts rssult ~rom high frequency luminance signals that fall within the capturs range (i.e., the chromasubcarrier spectrum) o~ the color decoder circuit of ths NTSC receivsr (approximately between 3.0 to 4.2MHz). Although the luminance signal is intended to carry only monochromatic information, th~ color decodsr will intsrpret any energy in ths chroma subcarrier spectnJm to be chroma information and will display color artifacts (i.e., wrong colors) from luminance information within this range of frequencies. As will bs seen with the teachings of lhs present invention, additional notch fiRers are used such as filters 59 and 60 shown coupled in series with the filter 58 to eliminate luminance energy in the chroma spsctrum that will resua in a visible color artifact.
The chroma subcarrier spsctrum may be considersd to be the fr~quency band over which visible artifacts are procluced on a display. This, to some extent, will vary from display-to-display and as a function of Individual eyesight. For pres~nt purposes, th~ chroma subcarrier spectrum is assumed to be between 3.0-4.2MHz for an NTSC signal.
The r~te at which the pixel data is read from the frame buffer 54 and couplsd onto line 55 for convsrsion by the convener 56 is referred to as the dot clock frequ~ncy. This is shown as frequency M in Figure 2. The period of this frequoncy corresponds to the horizontal line frequency of the composits vidso si~nal less the horizontal blanking interval divided by the number of pixsls stored 2~ for each line in buffer 54. This number may not be a harmonic of the color reference frequsncy assodated with the composite video signal (N=3.58MHz for ~ --, i . ;.. . .. .... . . ..
13276~8 NTSC), but i~ typically is phased locked to that ~requency (e.g., 12.272 MHz is 24/7 of 3.5~MHz).
The present invention teaches the use of notch filters to remove certain frequencies from the luminance component of the composite signal which resutt 5 from the dot clock frequency.
More specifically, referring to Figure 3a, first the dot ck)ck frequency is determined as shown by block 61. Next, this frequency is divided by the integers1, 2, 3, 4, etc. Each of ~he quotients ars then examined to determine if they fall within a predetermined frequency window (block 63~. This is the window defined 10 by the chroma subcarrier reference spectrum. For an NTSC signal having a color referencc frequency o~ 3.85MHz, the range is approximately between 3-4.2MHz.
A notch fiRer Is used for each of the quotients that fall within the predetermined window. Each of these filtsrs has a notch centered at the frequency equal to thequotient.
A typical sxampl~ is shown in Figure 3b. Assume that the dot clock fre~u~ncy is equal to 12.272MHz as shown by block 65. This number is divided by 1, 2, 3, 4, etc~ giving the quotients 12.272, 6.136, 4.09, 3.07, etc., as shown in block 66~
Next, as shown by block 67, each of the quotients are examined to determine if they fall within the range of 3~0 - 4.2 MH~ For the integer 1, 12~272 falls outside this window~ This also true for the inleger 2~ For the integers 3 and 4, th~ quotients fall within th6 range of 3.~4.2MHz. For integers 5 and largQr, thequotients fall below the lower fr0quency of the window and hence, notch filters are not required for these frequencies.
Now, as shown by block 68, notch filters ar~ used wh~re th0 characteristic of the filt~r has a notch centered at 4.09MHz and 3.07MHz. ~eferr~ng to Figuro 2, the filter 59 ha~ e~ notch centered at 3.07MHz and the filter 60 has a notch - . . ;. ,. .. .. ~
centered at 4 . O9MHz . These fllter~ ~re used ln add~tiOn to the often used notchl f~lter h~vln~ a notch located ~t th~
color reference frequency at 3.58MHz.
The use of the notch filters 59 and 60 has been found to remove artifacts not remov~d by filter 58. As will be seen in th~ pressntly preferred embodiment, thenotch filters are also used to filter the keying signal associated with a video overlay.
The notch filters used in the presently preferred embodiment are ordinary filters fabricated from discrete, passive components. The filters provid~
approximate~ 35dB att~nuation at their centQr frequency and have a Q of approximately 1.9.
PRESENTLY PREFERRED EMBODIMENT OF THE INVENTION
Rsferring to Figure 1, a computer 10 is illustrated which may b~ any one of a plurality of commercially available computers such as the APPIQ 11 computer.
The computer includes a m~mory which is used as a frame buffer. A user may enler into this frame buffer signals representing an image (overlay image) which is displayed in conjunction with anothar imagQ. This other image is illustrated as an NTSC (extQmal) video signal (composite video signal) applied to line 30. Ths signal on line 30 may be receivcd from a TV broa~casting station, video disk, video tape, anothsr computQr, or other source of compositQ video signal.
l h~ extemal video signal on line 30 is shown coupled to a circuit 11 within computer 10. This arcuit provides synchronization signals to determine ths rate at which data is r~ad from ths frame buffcr onto lines 14. This is the dot clockfr~quency M pr~viously discuss~d in conjunction with Figure 2. This clot clock is coupl~d to the comparator 12 on lin~ 16 as illustrated in FigurQ 1. - `
~5 GQn~ralb~ for a video overlay, the frame buffer in the computer 10 stores the ovQrlay imagQ in any color except for ons color referrQd to the as th~ key color.
Th~ It9y color is storQd in all other pixel locations. The key color is coupl~d to th~
13276~8 conlparator 12 by lines 13 als illustrated in Figur~ 1. On a pixe~by-pixel basis, the key color is compared to the contents of th~ ~rame buffer as the color data is read from the frame buffer. When the key color is not the same color as contained in the frame buffer, the overlay is display~d and th~ comparator 12 provides a k~ying 6 signal on line 45. This is a common prior art technique. (In some cases, indices to colors in a color lookup 1able are stored in the ~rame buffer.) The comparison shown in Figure 1 within comparator 12 is a digital comparison. The digital RGB signals (both th~ signals on line 14 and th3 key color) can be com/erted to analog form and the comparison done by comparing 10 two analog signals. As illustrated the digital signals on lines 14 (four bits for each color) are converled to analog form (one analog signal for red, one ~or green and one for blue) by convcrters 15 and thsn coupled to switch 22 and conn3ctor 32.
This conversion, howev~r, is not don~ ~or the comparison function.
The keying signal on line 45 controls a pulse width modulated signal 1~ generated by an ordinary gating means ~modulator 46) so as to provide blending (fading) between the ov~rlay image and th~ external vid~o image~ Registers 50 and 51, writabl~ by the computer 10, each store a 4 bit code that indicates thc level of blending for each state, 0 or 1, of kcying signal 45~ A multiplex~r 52,controlled by the keying signal 45, selects between ths output codes of registsrs 20 50 and 51 and couples tho selectcd cod~ through lines 70 to the modulator 46. A
signal of approximately 28MHz is couplQd to tho modulator 46 as the pulse width clock and the modulator 46 generates a pulsQ width modulated signal ot a duty cycle specified by th~ code on lines 70. Th~ presently preferred embodiment supports pulse width modulation duty cycles of 0~/100%, læ5~o/87.5%, :
25 25Yo/75%, 50%l50%, 75%l25%, 87.5%/12.5%, and 100%/0%.
The pulso width modulated signal generated by modulator 46 is tho swTtching signal used by the present invcntion to select between the ov~rlay . .
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image and the extemal video image. Since the pulse width modulation can provide a rapid switching between the two images, it is possible to creata the illusion of a weighted blending (fading) between the two images. For example, itthe duty cycle of the pulse width modulation is 25%/75%, then a blending of 25%
5 of one image and 75% of the other image is achieved. The two fade registers 50and 5t provide for a different blend weighting for each state of the keying signal.
For simple keying with ~lading turned of~ the code in fade register 50 is set to100%/0% blending and the code in fade register 51 is set to 0/J100% blending.
To reduce aliasing of th~ pulse width modulation of modulator 46 with image t 0 patterns in the overlay image, the pulse width modulation phase can be inverted each video field, and inverted again each video frame. This causes the overlay image and external video imags keying pattern to alternate by video line and by video frame for an enhanced blending eff~ct.
The keying signal is coupled to switch 22 and, after being filtered by a cutoff 15 filter 47, is coupled to filters 48 and 49~ The filters 48 and 4g have the center frequenciesoftheirnotcheslocatedat 4.09M~z and 3.07MHz a~ taught by the present invention and described in coniunction with Figures 2 and 3. The filtering of the keyin~ signal has been found to reduce artifacts~
In accordance with the present inv~ntion, two merging switches are used to 20 mQrgs the extarnal video signal on line 30 with the overlay signal. In a first circuit 18, the extemal video signal is converted from its NTSC format (or other composite forrnat) to RGB signals by a converter 20. The RGB signals resulting from this conversion (three analog signals) are connected to a first switch 22 via lines 24.
Switch 22 selects between the ou1put of the converter 20 and the RGB signals 25 representing the video overlay from the computer 10. The outputs of switch 22 are merged RGB analog signals on lines 26. That is, the signals on line 26 contain, 11 ~` '""' .
fro"l a viewing standpoint, th~ external video image overlayed with the overlay image ~rom the computer 10~
Gircuit 18 may be a commercially available part, such as Motorola TDA3301. For this commercial part the switch 22 selects either the signals on lines 14 or 24 (not a blend o~ both) and lor this reason the output of a pulse width modulator 46 is used to provide the ~ading.
A second circuit 31 includes a converter 32 which converts the RGB
signals on lines 72 to an NTSC signal on line 40. The switch 33 selects between the composite signals on lines 30 and 40. The merged output composits signal on 1 0 line 34 represents the same image as the ima~e represented by the signals on line 26.
The circuit 32 may be a commQrcially available arcuit such as th~ Motorola MC1378. For this àrcuit, the switch 33 is not ~discrete~, that is, depQnding on the bvel of the control signal to ths switch, both composite input signals can be 1 5 simultaneously selected, thsreby providing blending between the signals. For this reason, the keying signal from the pulse width modulator 46 i~ tirst coupl~d to a cutoff filter 47 which sffectively convsrts the pulse width modulated signal to an analog control signal for thQ switch 33. This keying signal is then coupled to filters 48 and 49 as discussed in conjunction with Figure 2 and 3. The output of the filter 20 4g provides a control signal for the switch 33~
The drcuit of Figure 1 provides both an RGB and composite video output signals (lines 26 and 34, rsspectiveh"). Importantly, it should be noted that noportion of the signals on lines 26 have been twice converted and the same is true ~or th~ NTSC signal on line 34. In the prior art, the double merging provided by25 switchQs 22 and 33 was not employed The convertsr 32 provides temlinals to which filters may be coupl~d. The finers 35, 36 and 37 are ordinary flltels which are coupled to the R-Y, B-Y and Y
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terminals. Additionally, as dilscussed previously, with the present invention, additional notch filters 38 ancl 39 are used to filter the luminance component of the compositesignal. ThesefiltersprovidenotcheSCenteredat 4.09 and 3.07MHz for an NTSC signal where the dot Glock frequency is cqual to 12.272MHz.
For some video signals artifacts may not appear because of the very nature of the image/colors defined by the signal (e.g., black and white signal). In these cases, it may be desirable to disable particularly filters 37, 38 and 39 to maximize bandwidth. A filter enable signal is shown coupled to these filters to allow, for example, the manual selection/deselection of the filters.
Thus, an apparatus has been described which provides improved conversion of a computer generated RGB signal to a composite signal. In the presently preferred embodiment, this improved conversion is used as part of a video overlay apparatus which has both an NTSC and RGB outputs.
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Claims (2)
1. In an apparatus for converting red, green and blue (RGB) video signals having a dot frequency of M into a composite video signal having a color reference frequency N, where M is not a harmonic of N, said apparatus having terminals for receiving filters for at least one of the components Y, R-Y
and B-Y of the composite video signal, where Y is the luminance component of the composite video signal, an improvement comprising:
a filter coupled to receive and filter said luminance component of said composite video signal, said filter having a frequency characteristic which has a notch centered at a frequency approximately equal to M divided by an integer, and where said frequency is within the chroma subcarrier spectrum of said composite video signal.
and B-Y of the composite video signal, where Y is the luminance component of the composite video signal, an improvement comprising:
a filter coupled to receive and filter said luminance component of said composite video signal, said filter having a frequency characteristic which has a notch centered at a frequency approximately equal to M divided by an integer, and where said frequency is within the chroma subcarrier spectrum of said composite video signal.
2. In an apparatus for converting an RGB video signal having a dot frequency of approximately 12.272MHz into a composite video signal having a color reference frequency of approximately 3.58 MHz, said apparatus having terminals for receiving filters for at least one of the components Y, R-Y and B-Y
of the composite video signal, where Y is the luminance component of the composite video signal, an improvement comprising:
a first notch filter coupled to receive and filter said luminance component of said composite video signal, said first notch filter having a notch centered at approximately 4.09 MHz;
a second notch filter coupled to receive and filter said luminance component of said composite video signal, said second notch filter having a notch centered at a frequency of approximately 3.07 MHz.
of the composite video signal, where Y is the luminance component of the composite video signal, an improvement comprising:
a first notch filter coupled to receive and filter said luminance component of said composite video signal, said first notch filter having a notch centered at approximately 4.09 MHz;
a second notch filter coupled to receive and filter said luminance component of said composite video signal, said second notch filter having a notch centered at a frequency of approximately 3.07 MHz.
Applications Claiming Priority (2)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| US24960388A | 1988-09-26 | 1988-09-26 | |
| US249,603 | 1988-09-26 |
Publications (1)
| Publication Number | Publication Date |
|---|---|
| CA1327648C true CA1327648C (en) | 1994-03-08 |
Family
ID=22944207
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CA000598032A Expired - Lifetime CA1327648C (en) | 1988-09-26 | 1989-04-27 | Apparatus for converting an rgb signal into a composite video signal and its use in providing computer generated video overlays |
Country Status (4)
| Country | Link |
|---|---|
| JP (1) | JPH02121596A (en) |
| KR (1) | KR910005694A (en) |
| CA (1) | CA1327648C (en) |
| GB (1) | GB2223143B (en) |
Families Citing this family (3)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| IT1243883B (en) * | 1990-10-31 | 1994-06-28 | Grafite Srl | COLOR GRAPHIC CONVERSION DEVICE PARTICULARLY FOR THE CONVERSION OF VIDEO SIGNALS COMING FROM GRAPHIC STATIONS INTO TELEVISION SIGNALS |
| EP0697689B1 (en) * | 1994-08-11 | 2001-09-12 | International Computers Limited | Integrated computer and television with remote control of cursor position |
| KR100762609B1 (en) * | 2001-07-25 | 2007-10-04 | 삼성토탈 주식회사 | Polypropylene-syndiotactic polystyrene resin composition |
-
1989
- 1989-04-06 GB GB8907804A patent/GB2223143B/en not_active Expired - Fee Related
- 1989-04-27 CA CA000598032A patent/CA1327648C/en not_active Expired - Lifetime
- 1989-08-26 KR KR1019890012202A patent/KR910005694A/en not_active Application Discontinuation
- 1989-09-01 JP JP1224734A patent/JPH02121596A/en active Pending
Also Published As
| Publication number | Publication date |
|---|---|
| JPH02121596A (en) | 1990-05-09 |
| KR910005694A (en) | 1991-03-30 |
| GB2223143A (en) | 1990-03-28 |
| GB8907804D0 (en) | 1989-05-17 |
| GB2223143B (en) | 1993-01-13 |
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