Sample Rate Selection: SDTV, EDTV, HDTV
SDTV Sample Rate Selection Line-locked sampling of analog R´G´B´ or YUV video signals is done. This technique produces a static orthogonal sampling grid in which samples on the current scan line fall directly beneath those on previous scan lines and fields. Another important feature is that the sampling is locked in phase so that one sample is coincident with the 50% amplitude point of the falling edge of analog horizontal sync (0x0). This ensures that different sources produce samples at nominally the same positions in the picture. Making this feature common simplifies conversion from one standard to another. For 480i and 576i video systems, several Y sampling frequencies were initially examined, including four times Fsc. However, the fourtimes Fsc sampling rates did not support the requirement of simplifying international exchange of programs, so they were dropped in favor of a single common sampling rate. Because the lowest sample rate possible (while still supporting quality video) was a goal, a 12 MHz sample rate was preferred for a long time, but eventually was considered to be too close to the Nyquist limit, complicating the filtering requirements. When the frequencies between 12 MHz and 14.3 MHz were examined, it became evident that a 13.5 MHz sample rate for Y provided some commonality between 480i and 576i systems. Cb and Cr, being color difference signals, do not require the same bandwidth as the Y, so may be sampled at one-half the Y sample rate, or 6.75 MHz. The “4:2:2” notation now commonly used originally applied to NTSC and PAL video, implying that Y, U and V were sampled at 4×, 2×, and 2× the color subcarrier frequency, respectively. The “4:2:2” notation was then adapted to BT.601 digital component video, implying that the sampling frequencies of Y, Cb and Cr were 4×, 2×, and 2× 3.375 MHz, respectively. “4:2:2” now commonly means that the sample rate of Cb and Cr is one-half that of Y, regardless of the actual sample rates used. With 13.5 MHz sampling, each scan line contains 858 samples (480i systems) or 864 samples (576i systems) and consists of a digital blanking interval followed by an active line period. Both the 480i and 576i systems use 720 samples during the active line period. Having a common number of samples for the active line period simplifies the design of multistandard equipment and standards conversion. With a sample rate of 6.75 MHz for Cb and Cr (4:2:2 sampling), each active line period contains 360 Cr samples and 360 Cb samples. With analog systems, problems may arise with repeated processing, causing an extension of the blanking intervals and softening of the blanking edges. Using 720 digital samples for the active line period accommodates the range of analog blanking tolerances of both the 480i and 576i systems. Therefore, repeated processing may be done without affecting the digital blanking interval. Blanking to define the analog picture width need only be done once, preferably at the display or upon conversion to analog video. Initially, BT.601 supported only 480i and 576i systems with a 4:3 aspect ratio (720 × 480i and 720 × 576i active resolutions). Support for a 16:9 aspect ratio was then added (960 × 480i and 960 × 576i active resolutions) using an 18 MHz sample rate.
EDTV Sample Rate Selection ITU BT.1358 defines the progressive SDTV video signals, also known as 480p or 576p, or Enhanced Digital Television (EDTV). The sample rate is doubled to 27 MHz (4:3 aspect ratio) or 36 MHz (16:9 aspect ratio) in order to keep the same static orthogonal sampling grid as that used by BT.601.
HDTV Sample Rate Selection ITU BT.709 defines the 720p, 1080i, and 1080p video signals, respectively. With HDTV, a different technique was used—the number of active samples per line and the number of active lines per frame is constant regardless of the frame rate. Thus, in order to keep a static orthogonal sampling grid, each frame rate uses a different sample clock rate.