ECE160Spring 2008Lecture 5Concepts in Video1ECE160 / CMPS182MultimediaLecture 5: Spring 2008Concepts in VideoECE160Spring 2008Lecture 5Concepts in Video2Types of Video Signals Component videoComponent video: Higher-end video systems make use ofthree separate video signals for the red, green, and blueimage planes. Each color channel is sent as a separatevideo signal.(a) Most computer systems use Component Video, withseparate signals for R, G, and B signals.(b) For any color separation scheme, Component Videogives the best color reproduction since there is no“crosstalk“ between the three channels.(c) This is not the case for S-Video or Composite Video,discussed next. Component video, however, requiresmore bandwidth and good synchronization of the threecomponents.ECE160Spring 2008Lecture 5Concepts in Video3Types of Video SignalsComposite Video - 1 SignalComposite video: color (“chrominance") and intensity (“luminance")signals are mixed into a single carrier wave.a) Chrominance is a composition of two color components(I and Q, or U and V).b) In NTSC TV, I and Q are combined into a chroma signal, and acolor subcarrier is then employed to put the chroma signal at thehigh-frequency end of the signal shared with the luminance signal.c) The chrominance and luminance components can be separated atthe receiver end and the two color components can be recovered.d) When connecting to TVs or VCRs, Composite Video uses only onewire and video color signals are mixed, not sent separately.The audio and sync signals are additions to this one signal.Since color and intensity are wrapped into the same signal, someinterference between the luminance and chrominance signals isinevitable.ECE160Spring 2008Lecture 5Concepts in Video4Types of Video SignalsS-Video - 2 SignalsS-Video: as a compromise, (Separated video, or Super-video, e.g., in S-VHS) uses two wires, one for luminanceand another for a composite chrominance signal.• As a result, there is less crosstalk between the colorinformation and the crucial gray-scale information.• The reason for placing luminance into its own part of thesignal is that black-and-white information is most crucialfor visual perception.– In fact, humans are able to differentiate spatial resolution in gray-scale images with a much higher acuity than for the color part ofcolor images.– As a result, we can send less accurate color information thanmust be sent for intensity information - we can only see fairlylarge blobs of color, so it makes sense to send less color detail.ECE160Spring 2008Lecture 5Concepts in Video5Analog Video• An analog signal f(t) samples a time-varyingimage. So-called “progressive" scanning tracesthrough a complete picture (a frame) row-wisefor each time interval.• In TV, and in some monitors and multimediastandards as well, another system, called“interlaced" scanning is used:a) The odd-numbered lines are traced first, and then theeven-numbered lines are traced. This results in “odd"and “even" fields - two fields make up one frame.b) In fact, the odd lines (starting from 1) end up at themiddle of a line at the end of the odd field, and theeven scan starts at a half-way point.ECE160Spring 2008Lecture 5Concepts in Video6InterlaceFirst the solid (odd) lines are traced, P to Q, then R to S, etc., ending at T; then the even field starts at U and ends at V.The jump from Q to R, etc. in Figure 5.1 is called the horizontal retrace,during which the electronic beam in the CRT is blanked out.The jump from T to U or V to P is called the vertical retrace.ECE160Spring 2008Lecture 5Concepts in Video7Interlace• Interlaced scan produces two fields for each frame.(a) The video frame, (b) Field 1, (c) Field 2, (d) Difference of FieldsECE160Spring 2008Lecture 5Concepts in Video8Interlace• Because of interlacing, the odd and even lines aredisplaced in time from each other - generally notnoticeable except when very fast action is taking placeon screen, when blurring may occur.• Since it is sometimes necessary to change the framerate, resize, or even produce stills from an interlacedsource video, various schemes are used to “de-interlace“a) The simplest de-interlacing method consists ofdiscarding one field and duplicating the scan lines of theother field. The information in one field is lost completelyusing this simple technique.b) Other more complicated methods that retaininformation from both fields are also possible.ECE160Spring 2008Lecture 5Concepts in Video9Interlace• Analog video use a small voltage offset fromzero to indicate “black", and another value suchas zero to indicate the start of a line. Forexample, we could use a “blacker-than-black“zero signal to indicate the beginning of a line.Electronic signal forone NTSC scan line.ECE160Spring 2008Lecture 5Concepts in Video10NTSC Video• NTSC (National Television System Committee) TVstandard is mostly used in North America and Japan.It uses the familiar 4:3 aspect ratio (i.e., the ratio ofpicture width to its height) and uses 525 scan lines perframe at 30 frames (actually 29.95) per second (fps).a) NTSC follows the interlaced scanning system, and eachframe is divided into two fields, with 262.5 lines/field.b) The horizontal sweep frequency is 525x29:97 /sec~15,734 lines/sec,so that each line is swept out in 1/15,734sec ~63:6µsec.c) Since the horizontal retrace takes 10.9 sec, this leaves52.7 sec for the active line signal during which imagedata is displayedECE160Spring 2008Lecture 5Concepts in Video11NTSC Video• The effect of “vertical retrace & sync" and“horizontal retrace & sync" on the NTSC videoraster.ECE160Spring 2008Lecture 5Concepts in Video12NTSC Videoa) Vertical retrace takes place during 20 lines reserved forcontrol information at the beginning of each field. Hence,the number of active video lines per frame is only 485.b) Similarly, almost 1/6 of the raster at the left side isblanked for horizontal retrace and sync. The non-blanking pixels are called active pixels.c) Since the horizontal retrace takes 10.9 sec, this leaves52.7 sec for the active line signal during which imagedata is displayed.d) Pixels often fall in-between the scan lines. Therefore,even with non-interlaced scan, NTSC TV is only capableof showing about 340 (visually distinct) lines, i.e., about70% of the 485 specified active lines. With interlacedscan, this could be as low as 50%.ECE160Spring 2008Lecture 5Concepts in Video13NTSC Video• NTSC video is an analog signal with no fixed