Television History. Date / Place E. Nemer - 1

Transcription

1 Television History Television to see from a distance Earlier Selenium photosensitive cells were used for converting light from pictures into electrical signals Real breakthrough invention of CRT AT&T Bell Labs had the first television 18 fps, 2 x 3 inch screen, 2500 pixels 1935 TV broadcasting started Date / Place E. Nemer - 1

2 Television History 1927, Hoover made a speech in Washington while viewers in NY could see, hear him Date / Place E. Nemer - 2

3 Color refers to how we perceive a narrow band of electromagnetic energy source, object, observer The Human Visual system transforms light energy into sensory experience of sight Date / Place E. Nemer - 3

4 Image Formation cornea, sclera, pupil, iris, lens, retina, fovea Transduction retina, rods, and cones Processing optic nerve, brain Date / Place E. Nemer - 4

5 The retina contains two types of photoreceptors, rods and cones. The rods are more numerous, some 120 million, and are more sensitive than the cones. However, they are not sensitive to color. The 6 to 7 million cones provide the eye's color sensitivity and they are much more concentrated in the central yellow spot known as the macula. In the center of that region is the " fovea centralis ", a 0.3 mm diameter rod-free area with very thin, densely packed cones. Retina Fovea Date / Place E. Nemer - 5

6 Transform light to neural impulses Receptors signal bipolar cells Bipolar cells signal ganglion cells Axons in the ganglion cells form optic nerve Optic nerve Ganglion Bipolar cells Cones Rods Date / Place E. Nemer - 6

7 Tri-stimulus Theory 3 types of cones (6 to 7 million of them) Red (64%), Green (32%), Blue (2%) Each type most responsive to a narrow band red and green absorb most energy, blue the least Light stimulates each set of cones differently, and the ratios produce sensation of color Date / Place E. Nemer - 7

8 Color Perception Hue distinguishes named colors, e.g., RGB dominant wavelength of the light Saturation how far color is from a gray of equal intensity Brightness (lightness) perceived intensity White Grays Tints Tones Shades Pure colors Black Date / Place E. Nemer - 8

9 Visual Perception: Temporal Resolution The eye (or/and brain) can retain the sensation of an image for a short time even after the actual image is removed. This allows the display of a video as successive frames as long as the frame interval is shorter than the persistence period, The eye will see a continuously varying image in time. When the frame interval is too long, the eye observes frame flicker. Date / Place E. Nemer - 9

10 Visual Perception: Temporal Resolution The minimal frame rate (frames/ second or fps or Hz) required to prevent frame flicker depends on display brightness, viewing distance. Higher frame rate is required with closer viewing and brighter display. For TV viewing: fps For Movie viewing: 24 fps For computer monitor: > 70 fps Date / Place E. Nemer - 10

11 Visual Perception: Temporal Resolution Similar to frame merging, the eye can fuse separate lines into one complete frame, as long as the spacing between lines is small enough. The maximum vertical spacing between lines depends on the viewing distance, the screen size, and the display brightness. For common viewing distance and TV screen size, lines per frame was deemed a good norm. Date / Place E. Nemer - 11

12 Merging Pixels Similarly, the eye can fuse separate pixels in a line into one continuously varying line, as long as the spacing between pixels is sufficiently small. Date / Place E. Nemer - 12

13 Interlacing The brighter the still image presented to the viewer, the shorter the persistence of vision. If the space between pictures is longer than the period of persistence of vision then the image flickers. One way to avoid it is to have 2 "flashes" per frame, interlacing creates the 2 flashes : a single frame is scanned twice. The first includes the odd lines, the next the even ones. Date / Place E. Nemer - 13

14 NTSC - Interlacing NTSC has 525 vertical lines. However lines number 248 to 263 and 511 to 525 are typically blanked to provide time for the beam to return to the upper left hand corner for the next scan. Notice that the beam does not return directly to the top, but zig-zags a bit. Date / Place E. Nemer - 14

15 Date / Place E. Nemer - 15

16 NTSC Standard NTSC (National Television System Committee) TV standard is mostly used in North America and Japan. It uses the familiar 4:3 aspect ratio (i.e., the ratio of picture width to its height) and uses 525 scan lines per frame at 30 frames (actually 29.95) per second (fps). NTSC follows the interlaced scanning system, and each frame is divided into two fields, with lines/field. The horizontal sweep frequency is 525x29:97 /sec ~15,734 lines/sec, so that each line is swept out in 1/15,734 sec ~63:6µsec. Since the horizontal retrace takes 10.9 µsec, this leaves 52.7 µsec for the active line signal during which image data is displayed Date / Place E. Nemer - 16

17 NTSC Standard a) Vertical retrace takes place during 20 lines reserved for control 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 is blanked for horizontal retrace and sync. The non-blanking pixels are called active pixels. c) Since the horizontal retrace takes 10.9 sec this leaves 52.7 sec for the active line signal during which image data is displayed. d) Pixels often fall in-between the scan lines. Therefore, even with non-interlaced scan, NTSC TV is only capable of showing about 340 (visually distinct) lines, i.e., about 70% of the 485 specified active lines. With interlaced scan, this could be as low as 50%. Date / Place E. Nemer - 17

18 NTSC B& W To Color In the most basic form, color television could simply be implemented by having cameras with three filters (red, green and blue) and then transmitting the three color signals over wires to a receiver with three electron guns and three drive circuits. Unfortunately, this idealized view is not compatible with the previously allocated 6 MHz bandwidth of a TV channel. It is also not compatible with previously existing monochrome receivers. Date / Place E. Nemer - 18

19 NTSC Color Encoding Therefore, modern color TV is structured to preserve all the original monochrome information -- and just add on the color information on top. To do this, one signal, called luminance (Y) has been chosen to occupy the major portion (0-4 MHz) of the channel. Y contains the brightness information and the detail. Y is the monochrome TV signal. Consider the model of a scene being filmed with three cameras. One camera has a red filter, one camera a green filter and one camera a blue filter. Date / Place E. Nemer - 19

20 NTSC Color Encoding Assume that the cameras all adjusted so that when pointed at "white" they each give equal voltages. To create the Y signal, the red, green and blue inputs to the Y signal must be balanced to compensate for the color perception misbalance of the eye. The governing equation is: For example, in order to produce "White" light to the human observer there needs to be 11 % blue, 30 % red and 59% green (=100%). Date / Place E. Nemer - 20

21 Color Model and Modulation of NTSC NTSC uses the YIQ color model, and the technique of quadrature modulation is employed to combine (the spectrally overlapped part of) I (in-phase) and Q (quadrature) signals into a single chroma signal C: This modulated chroma signal is also known as the color subcarrier, whose magnitude is (I 2 +Q 2), and phase is tan 1 (Q/I). The frequency of C is F sc ~3.58 MHz. The NTSC composite signal is a further composition of the luminance signal Y and the chroma signal as: Date / Place E. Nemer - 21

22 Color Model and Modulation of NTSC NTSC assigns a bandwidth of 4.2 MHz to Y, and only 1.6 MHz to I and 0.6 MHz to Q, due to humans insensitivity to color details (high frequency color changes). Interleaving Y and C signals in the NTSC spectrum. Date / Place E. Nemer - 22

23 The television bandwidth is 6 MHz. The sub-carrier for the color is 3.58 MHz off the carrier for the monochrome information. The sound carrier is 4.5 MHz off the carrier for the monochrome information. There is a gap of 1.25 MHz on the low end and 0.25 MHz on the high end to avoid cross talk with other channels. Date / Place E. Nemer - 23

24 Modulation of NTSC In NTSC Luminance is AM VSB, the Chroma is QAM I&Q, and the Aural FM. Date / Place E. Nemer - 24

25 Modulation of NTSC Date / Place E. Nemer - 25

26 Date / Place E. Nemer - 26

27 Other Color Coding Schemes: YUV PAL video standard Based on CIE model Y is luminance UV are chrominance YUV from RGB Y =.299R +.587G +.114B U = (B - Y) V = (R - Y) U-V plane at Y=0.5 From Date / Place E. Nemer - 27

28 YCrCb original Subset of YUV that scales and shifts the chrominance values into range 0..1 Y = 0.299R G B Cr = ((B-Y)/2) Cb = ((R-Y)/1.6) Y Cb Cr Date / Place From E. Nemer - 28

29 Modulation of Luminance and Chrominance Luminance is the "monochrome" part of the TV signal. It officially takes up the first 4 MHz of the 6 MHz bandwidth of the TV signal. However, in practice, the signal is usually band-limited to 3.2 MHz. Two signals are then created to carry the chrominance (C) information. One of these signals is called "Q" and the other is called "I". They are related to the R, G and B signals by: Date / Place E. Nemer - 29

30 Modulation of Luminance and Chrominance The positive polarity of Q is purple, the negative is green. The positive polarity of I is orange, the negative is cyan. Thus, Q is often called the "green-purple" or "purple-green" axis information and I is often called the "orange-cyan" or "cyan-orange" axis information. The human eye is more sensitive to spatial variations in the "orange-cyan" than it is for the "green purple". Thus, the "orange-cyan" or I signal has a maximum bandwidth of 1.5 MHz and the "green purple" only has a maximum bandwidth of 0.5 MHz. Date / Place E. Nemer - 30

31 Modulation of Luminance and Chrominance Now, the Q and I signals are both modulated by a 3.58 MHz carrier wave. However, they are modulated out of 90 degrees out of phase.(qam) These two signals are then summed together to make the C or chrominance signal. The nomenclature of the two signals aids in remembering what is going on. The I signal is In-phase with the 3.58 MHz carrier wave. The Q signal is in Quadrature (i.e. 1/4 of the way around the circle or 90 degrees out of phase, or orthogonal) with the 3.58 MHz carrier wave. Date / Place E. Nemer - 31

32 Modulation of Luminance and Chrominance New chrominance signal (formed by Q and I) has the interesting property that the magnitude of the signal represents the color saturation, and the phase of the signal represents the hue. Phase = Arctan (Q/ I) = hue Magnitude = sqrt (I 2 + Q 2 ) = saturation Date / Place E. Nemer - 32

33 Bandwidth of Chrominance Signals With real video signals, the chrominance component typically changes much slower than luminance Furthermore, the human eye is less sensitive to changes in chrominance than to changes in luminance The eye is more sensitive to the orange- cyan range (I) (the color of face!) than to green- purple range (Q) The above factors lead to I: bandlimitted to 1.5 MHz and Q: bandlimitted to 0.5 MHz Date / Place E. Nemer - 33

34 Multiplexing of Luminance and Chrominance Position the bandlimited chrominance at the high end of the luminance spectrum, where the luminance is weak, but still sufficiently lower than the audio (at 4.5 MHz). The two chrominance components (I and Q) are multiplexed onto the same subcarrier using QAM. The resulting video signal including the baseband luminance signal plus the chrominance components modulated to f c is called composite video signal. Date / Place E. Nemer - 34

35 Transmitter Block Diagram Date / Place E. Nemer - 35

36 Color Decoder Date / Place E. Nemer - 36

37 Decoding NTSC Signals The first step in decoding the composite signal at the receiver side is the separation of Y and C. After the separation of Y using a low-pass filter, the chroma signal C can be demodulated to extract the components I and Q separately. To extract I: 1. Multiply the signal C by 2 cos(f sc t), i.e., 2. Apply a low-pass filter to obtain I and discard the two higher frequency (2F sc ) terms. Similarly, Q can be extracted by first multiplying C by 2 sin(f sc t) and then low-pass filtering. Date / Place E. Nemer - 37

38 Block diagrams of TV receivers Date / Place E. Nemer - 38

39 Comparison of Analog Broadcast TV Systems Date / Place E. Nemer - 39