ITU-R BT.601 is the designation for an entire set of standards that defines rules for converting analog (PAL or NTSC) television signals to digital and back. The standard was formalized by the International Telecommunications Union (ITU) and has since been used as a guideline for nearly all digital consumer video formats. This standard is also commonly referred to as simply Rec.601, or even by its older name of CCIR 601.
The most commonly used portion of ITU-R BT.601 is the Resolution of 720x576 for PAL video and 720x480 for NTSC. This Resolution is achieved by sampling at a Frequency of 13.5MHz (13,500,000 samples per second) across horizontal lines 53.333µs (.000053333 seconds) long. Since the actual line length for PAL and NTSC TVs is 52µs and 52.666µs respectively. That means a standard ITU-R BT.601 Frame includes either PAL video with an Active Area (the part of the Frame containing visible pixels) of 702x576 or NTSC video with an active area of 711x480. In reality many real-world implementations of this conversion use either a different resolution, such as 640x480/576, or a slighly different line length, resulting in the active area of the picture to use all 720 Pixels.
The best known application of ITU-R BT.601 is probably DVD-Video, where frame sizes of 720x480 and 720x576 are standard. However, like all implementations, the actual conversion back to an analog signal is dependent on the player, and may not follow the ITU specification. Likewise, video encoding for video often uses all 720 Pixels for a standard 4:3 image. For either Widescreen (Anamorphic) or 4:3 Letterbox DVDs it may be necessary using the additional pixels to maintain the source's Aspect ratio.
DV, used in most Standard Definition (SD) digital camcorders, uses the recommended resolution of 720x480/576, but uses the entire width of the frame for the picture. When using DV as a source for other formats that respect the ITU-R BT.601 specifications with regard to line length it may be desirable to resize to match the active area of your TV's picture and add borders to the sides.
One of the biggest advantages of ITU-R BT.601's relatively high horizontal Samplerate is that it scales well for 16:9 (Widescreen) video. While 640 samples (pixels) per line would probably be plenty for standard PAL and NTSC, the additional 80 pixels allow wider images, intended to be stretched for display on Widescreen displays. Due to the Bandwidth limitations, the connectors designed for 4:3 SD video transmission (Composite Video and S-Video) aren't the best choice for Widescreen TV's, including EDTVs, which are also designed around the standard ITU-R BT.601 resolutions. Instead, it's preferable to use Composite Video connections for analog or any of the standard Digital display connectors (DVI, HDMI, etc, ...).
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