Horizontal blanking interval
The horizontal blanking interval (HBI), also known as the horizontal retrace interval, is the inactive segment of each horizontal scan line in analog raster-scan video systems, during which the video signal is suppressed to or below black level, allowing the scanning beam in cathode-ray tube (CRT) displays to return from the right edge of the screen to the left without producing visible retrace artifacts.[1][2] This interval ensures precise synchronization between the transmitter and receiver by embedding timing signals that control horizontal deflection.[1][2] In standard analog television formats, the HBI forms part of the composite video signal and occurs at a rate matching the horizontal scan frequency, such as 15,750 Hz in NTSC systems.[2] It consists of key components including a front porch (a brief period at black level before synchronization), a horizontal synchronizing pulse (a sharp negative excursion below black level to trigger deflection reset), and a back porch (which includes the color burst in color systems for chrominance phase reference).[2] For NTSC, the total HBI duration is nominally 10.9 μs (±0.2 μs), representing about 17% of the 63.5 μs horizontal line period, with the sync pulse lasting 4.7 μs (±0.1 μs), the front porch 1.5 μs (±0.1 μs), and the back porch encompassing the remaining time including a 2.5 μs color burst at approximately 20 IRE units.[1][2] Similar structures exist in other standards like PAL, though timings vary slightly (e.g., 12 μs HBI in PAL-B).[1] The HBI plays a critical role in maintaining image stability and quality in legacy broadcast and display technologies, preventing distortions from beam flyback while providing opportunities for ancillary data insertion in some applications, though its primary function remains synchronization.[1][2] Although largely superseded by digital video formats, the concept persists in emulations of analog signals and certain professional video equipment.[3]Fundamentals
Definition
The horizontal blanking interval refers to the inactive portion of a horizontal scan line in raster scanning systems, during which the electron beam in analog cathode-ray tube (CRT) displays—or the equivalent scanning mechanism in digital systems—returns from the right end of the visible line to the left starting point without producing any visible illumination on the screen. This period ensures that the retrace path remains invisible, preventing unwanted streaks or artifacts from appearing during the flyback.[4][5][6] In the context of raster scanning, where an image is formed by sequentially scanning lines from left to right across the display, the horizontal blanking interval serves as a non-visible buffer within the total horizontal line time. It typically accounts for 10-20% of the line duration, allowing sufficient time for the scanning mechanism to reset without interfering with the active video content.[7][8] This interval is distinct from the vertical blanking interval, which addresses the retrace at the frame level between the end of one complete image and the start of the next.[5]Purpose in Raster Scanning
In raster scanning systems, such as those used in cathode-ray tube (CRT) displays, the horizontal blanking interval serves the primary purpose of suppressing the electron beam's intensity during the horizontal retrace period, when the beam returns from the right edge of the screen to the left edge after completing a scan line. This blanking prevents visible artifacts, such as bright streaks or lines, that would otherwise appear on the display due to the beam's movement and potential residual phosphor excitation along the return path.[9][10] By providing this dedicated interval for beam repositioning, the horizontal blanking interval maintains the integrity of the displayed image, ensuring that only the intended visible picture elements contribute to the final output. Without blanking, the retrace would disrupt the uniform black level and introduce unwanted luminance variations, compromising the overall visual quality. This mechanism allows the scanning process to focus exclusively on rendering the active video content during the forward sweep, preserving the fidelity of the rasterized image.[11][9] Furthermore, the horizontal blanking interval facilitates the progressive scanning of successive lines from left to right across the screen, enabling seamless transitions between scan lines without interference from the retrace. This supports the continuous buildup of the complete frame in a top-to-bottom manner, essential for coherent image reproduction in raster-based technologies. The blanking ensures that the beam is accurately reset for each new line, promoting stable and artifact-free line-to-line progression.[10][11]Signal Structure
Components
The horizontal blanking interval consists of three primary components that form its signal waveform structure: the front porch, the horizontal sync pulse, and the back porch. The front porch is a brief period immediately following the end of the active video signal, during which the video level is held at the blanking level to allow the display device to stabilize before the synchronization event. For NTSC, this is nominally 1.5 μs (±0.1 μs).[12] This component ensures a smooth transition without introducing artifacts into the visible image. The horizontal sync pulse follows the front porch and provides a sharp, negative-going transition that triggers the horizontal retrace of the electron beam or scanning mechanism in raster displays. For NTSC, the pulse width is 4.7 μs (±0.1 μs).[12] Positioned centrally within the blanking interval, this pulse is essential for aligning the timing across transmitter and receiver. The back porch occurs after the trailing edge of the sync pulse, serving as a settling period for the signal before the next active video line begins; in color television systems, it includes the colorburst, a short burst of the color subcarrier frequency used for phase and amplitude reference in demodulating chrominance information. For NTSC, the back porch is nominally 4.7 μs, including a 2.5 μs color burst. For PAL, the front porch is 1.65 μs, sync pulse 4.7 μs, and back porch 5.65 μs.[12][13] In analog video representations, the waveform features distinct voltage levels to define these components. The blanking level corresponds to the zero or reference black level, maintaining the signal at a nominal voltage (typically 0 IRE) during the front and back porches to suppress the retrace beam.[14] The sync tip level reaches a negative peak relative to the blanking level (around -40 IRE), creating the precise pulse for synchronization.[12] In contrast, basic digital representations, such as in VGA interfaces, employ binary logic levels where the blanking signal is active low to force outputs to a black or below-black state, and the sync pulse is a low-voltage assertion (e.g., 0 V) amid high-impedance or defined blanking codes, adapting the analog structure to discrete pixel clocks without continuous voltage modulation.[15] The total duration of the horizontal blanking interval is given by the equation: T_{HBI} = T_{FP} + T_{SP} + T_{BP} where T_{HBI} is the horizontal blanking interval, T_{FP} is the front porch duration, T_{SP} is the sync pulse width, and T_{BP} is the back porch duration.[12] This summation encapsulates the non-visible portion of each scan line, supporting the overall raster scanning process by concealing the horizontal retrace.[14]Timing Parameters
The horizontal blanking interval forms part of the total horizontal line duration in raster-scanned video systems, expressed by the formula: total line duration = active video time + blanking interval. This relationship ensures that the display device allocates sufficient time for beam retrace without visible artifacts, with the blanking interval encompassing synchronization and porch periods. In the NTSC standard, the total line duration is 63.556 μs, with the blanking interval nominally 10.9 μs (tolerances ±0.2 μs), representing approximately 17% of the line time (calculated as 10.9 / 63.556 ≈ 0.1715). For PAL, the total line duration is 64 μs, with the blanking interval 12 μs (±0.3 μs), equating to about 19% (12 / 64 = 0.1875). These durations are defined in international standards to maintain compatibility across broadcast systems.| Parameter | NTSC (525-line) | PAL (625-line) |
|---|---|---|
| Total line duration (μs) | 63.556 | 64 |
| Blanking interval (μs) | 10.9 (±0.2) | 12 (±0.3) |
| Percentage blanking | ~17% | ~19% |