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In August 1908, a white mob attacked Springfield, Illinois’s Black community, destroying homes and businesses and lynching two Black men. National outrage over the violence helped spur the movement that created the NAACP the following year.
MORE →Reflects the personal views, recollections, and perspective of the author, Mike Davis.
This is a personal recollection on the Move fire on May 13, 1985
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In a raster scan display, the vertical blanking interval (VBI), also known as the vertical interval or VBLANK, is the time between the end of the final visible line of a frame or field[1][2] and the beginning of the first visible line of the next frame or field. It is present in analog television, VGA, DVI and other signals. Here the term field is used in interlaced video, and the term frame is used in progressive video and there can be a VBI after each frame or field.[3][4] In interlaced video a frame is made up of 2 fields.[5] Sometimes in interlaced video a field is called a frame which can lead to confusion.[6]
In raster cathode-ray tube (CRT) displays, the blank level is usually supplied during this period to avoid painting the retrace line—see raster scan for details; signal sources such as television broadcasts do not supply image information during the blanking period. Digital displays usually will not display incoming data stream during the blanking interval even if present.
The VBI was originally needed because of the inductive inertia of the magnetic coils which deflect the electron beam vertically in a CRT; the magnetic field, and hence the position being drawn, cannot change instantly. Additionally, the speed of older circuits was lower than today’s. For horizontal deflection, there is also a pause between successive lines, to allow the beam to return from right to left, called the horizontal blanking interval. Modern CRT circuitry does not require such a long blanking interval, and thin panel displays require none, but the standards were established when the delay was needed (and to allow the continued use of older equipment). Blanking of a CRT may not be perfect due to equipment faults or brightness set very high; in this case a white retrace line shows on the screen, often alternating between fairly steep diagonals from right to left and less-steep diagonals back from left to right, starting in the lower right of the display.
In analog television systems the vertical blanking interval can be used for datacasting (to carry digital data), since nothing sent during the VBI is displayed on the screen; various test signals, VITC timecode, closed captioning, teletext, CGMS-A copy-protection indicators, and various data encoded by the XDS protocol (e.g., the content ratings for V-chip use) and other digital data can be sent during this time period.
In U.S. analog broadcast television, line 19 was reserved for a Ghost-canceling reference and line 21 was reserved for NABTS captioning data. Teletext contemplated the use of line 22 for data transmission.[citation needed]
The pause between sending video data is sometimes used in real time computer graphics to modify the frame buffer, or to provide a time reference for when switching the source buffer for video output can happen without causing a visible tear. This is especially true in video game systems, where the fixed frequency of the blanking period might also be used to derive in-game timing.
On many consoles there is an extended blanking period, as the console opts to paint graphics on fewer lines than the television would natively allow, permitting its output to be surrounded by a border. On some very early machines such as the Atari 2600, the programmer is in full control of video output and therefore may select their own blanking period, allowing arbitrarily few painted lines. On others such as the Nintendo Entertainment System, a predefined blanking period could be extended.
Most consumer VCRs use the known black level of the vertical blanking pulse to set their recording levels. The Macrovision copy protection scheme inserts pulses in the VBI, where the recorder expects a constant level, to disrupt recording to videotapes.
While digital video interconnects (such as DVI and HDMI) generally do have a "vertical blanking" part of the datastream, they are unable to carry closed caption text or most of the other items that, in analog TV interconnects, are transmitted during the "vertical blanking interval". This can lead to closed captioning § Digital television interoperability issues.
Source: Wikipedia. Article content is retrieved live through the MediaWiki API.
In a raster scan display, the vertical blanking interval (VBI), also known as the vertical interval or VBLANK, is the time between the end of the final visible line of a frame or field and the beginning of the first visible line of the next frame or field. It is present in analog television, VGA, DVI and other signals. Here the term field is used in interlaced video, and the term frame is used in progressive video and there can be a VBI after each frame or field. In interlaced video a frame is made up of 2 fields. Sometimes in interlaced video a field is called a frame which can lead to confusion. In raster cathode-ray tube (CRT) displays, the blank level is usually supplied during this period to avoid painting the retrace line—see raster scan for details; signal sources such as television broadcasts do not supply image information during the blanking period. Digital displays usually will not display incoming data stream during the blanking interval even if present. The VBI was originally needed because of the inductive inertia of the magnetic coils which deflect the electron beam vertically in a CRT; the magnetic field, and hence the position being drawn, cannot change instantly. Additionally, the speed of older circuits was lower than today’s. For horizontal deflection, there is also a pause between successive lines, to allow the beam to return from right to left, called the horizontal blanking interval. Modern CRT circuitry does not require such a long blanking interval, and thin panel displays require none, but the standards were established when the delay was needed (and to allow the continued use of older equipment). Blanking of a CRT may not be perfect due to equipment faults or brightness set very high; in this case a white retrace line shows on the screen, often alternating between fairly steep diagonals from right to left and less-steep diagonals back from left to right, starting in the lower right of the display. In analog television systems the vertical blanking interval can be used for datacasting (to carry digital data), since nothing sent during the VBI is displayed on the screen; various test signals, VITC timecode, closed captioning, teletext, CGMS-A copy-protection indicators, and various data encoded by the XDS protocol (e.g., the content ratings for V-chip use) and other digital data can be sent during this time period. In U.S. analog broadcast television, line 19 was reserved for a Ghost-canceling reference and line 21 was reserved for NABTS captioning data. Teletext contemplated the use of line 22 for data transmission. The pause between sending video data is sometimes used in real time computer graphics to modify the frame buffer, or to provide a time reference for when switching the source buffer for video output can happen without causing a visible tear. This is especially true in video game systems, where the fixed frequency of the blanking period might also be used to derive in-game timing. On many consoles there is an extended blanking period, as the console opts to paint graphics on fewer lines than the television would natively allow, permitting its output to be surrounded by a border. On some very early machines such as the Atari 2600, the programmer is in full control of video output and therefore may select their own blanking period, allowing arbitrarily few painted lines. On others such as the Nintendo Entertainment System, a predefined blanking period could be extended. Most consumer VCRs use the known black level of the vertical blanking pulse to set their recording levels. The Macrovision copy protection scheme inserts pulses in the VBI, where the recorder expects a constant level, to disrupt recording to videotapes.
Vertical Interval Timecode (VITC, pronounced "vitsee") is a form of SMPTE timecode encoded on one scan line in a video signal. These lines are typically inserted into the vertical blanking interval of the video signal. With one exception, VITC contains the same payload as SMPTE linear timecode (LTC), embedded in a new frame structure with extra synchronization bits and an error-detection checksum. The exception is that VITC is encoded twice per interlaced video frame, once in each field, and one additional bit (the "field flag") is used to distinguish the two fields. A video frame may contain more than one VITC code if necessary, recorded on different lines. This is often used in production, where different entities may want to encode different sets of time-code metadata on the same tape. As a practical matter, VITC can be more 'frame-accurate' than LTC, particularly at very slow tape speeds on analog formats. LTC readers can lose track of code at slow jog speeds whereas VITC can be read frame-by-frame if need be. Conversely, at high speeds (FF/rew.), the VITC is often unreadable due to image distortions, so the LTC is often used instead. Some VCRs have an auto selection between the two formats to provide the highest accuracy. VITC is 90 bits long: 32 bits of time code, 32 bits of user data, 18 synchronization bits, and 8 bits of checksum. It is transmitted using non-return-to-zero encoding at a bit rate of 115 times the line rate. (The unused 25 bit times are to leave room for the horizontal blanking interval.) Bit 14 is set to 1 if drop frame numbering is in use; frame numbers 0 and 1 are skipped during the first second of every minute, except multiples of 10 minutes. This converts 30 frame/second time code to the 29.97 frame/second NTSC standard. Bit 15, the color framing bit, is set to 1 if the time code is synchronized to a (color) video signal. The frame number modulo 2 (for NTSC and SECAM) or modulo 4 (for PAL) should be preserved across cuts in order to avoid phase jumps in the chrominance subcarrier. Bits 35, 55, and 75 differ between 25 frame/s time code, and 30/29.97 frame/s. The bits are: Field flag (bit 35 for 29.97/30 frame/s, bit 75 for 25 frame/s): This is an additional least-significant bit for the frame number, distinguishing the two interlaced fields in one video frame. It is set to 0 during the first field of a frame, and to 1 during the second. This bit replaces the "polarity correction" bit in linear timecode. "Binary group flag" bits BGF0 and BGF2 (bits 55 and 75 for 29.97/30 frame/s, bits 35 and 55 for 25 frame/s): These indicate the format of the user bits. Both bits zero indicates no (or unspecified) format. Only BGF0 set indicates four 8-bit characters (transmitted little-endian). The combinations with BGF2 set are reserved. Bit 74 ("Binary group flag 1") was previously unassigned, but is used to indicate that the time code is synchronized to an external clock. If zero, the timecode start time is arbitrary. The checksum in bits 82–89 is a simple bytewise XOR of the previous 82 bits (including the sync bits, so bit 82 is the XOR of bits 74, 66, ..., 2), which can be described as a CRC with generator polynomial x8+1. (Preset to zero, no inversion.) The exact nature of the color frame sequence depends on the video standard being used. In the case of the three main composite video standards, PAL video has an 8-field (4 frame) color frame sequence, and NTSC and SECAM both have 4-field (2 frame) color frame sequences. Preserving the color framing sequence of video across edits and between channels in video effects was an important issue in early analog composite videotape editing systems, as cuts between different color sequences would cause jumps in subcarrier phase, and mixing two signals of different field dominance would result in color artifacts on the part of the signal that was not in sync with the output color frame sequence. To help prevent these problems, SMPTE time code contains a color framing bit, which can be used to indicate that the video material the timecode refers to follows a standard convention regarding the synchronization of video time code and the color framing sequence. If the color framing bit was set in both types of material, the editing system could then always ensure that color framing was preserved by constraining edit decisions between input sources to keep the correct relationship between the timecode sequences, and hence the color framing sequences.
In analog video, blanking occurs between horizontal lines and between frames. In raster scan equipment, an image is built up by scanning an electron beam from left to right across a screen to produce a visible trace of one scan line, reducing the brightness of the beam to zero (horizontal blanking), moving it back as fast as possible to the left of the screen at a slightly lower position (the next scan line), restoring the brightness, and continuing until all the lines have been displayed and the beam is at the bottom right of the screen. Its intensity is then reduced to zero again (vertical blanking), and it is rapidly moved to the top left to start again, creating the next frame. In television, in particular, the vertical blanking interval is long to accommodate the slow equipment available at the time the standard was set. Fast modern electronics allows digital information to be encoded into the signal during the vertical blanking interval; it is not displayed on screen as the beam is blanked, but can be processed by appropriate circuitry.
Blanking may refer to: Blanking (metalworking), a metalworking process to form the rough shape of a sheet metal workpiece Blanking (video) Horizontal blanking interval Vertical blanking interval
Before the 1921 destruction of Tulsa’s Greenwood District, Black residents had created a remarkable center of business and community life. The district included stores, professional offices, entertainment venues and homes owned by Black citizens. Understanding Greenwood means learning what was built—not only what was burned.
MORE →Shirley Chisholm, elected in 1968.