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ITU-R BT.601

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 simplyRec.601, or even by its older name ofCCIR 601.

Resolution
The most commonly used portion of ITU-R BT.601 is theResolution of 720x576 forPAL video and 720x480 forNTSC. ThisResolution is achieved by sampling at aFrequency of 13.5MHz (13,500,000 samples per second) across horizontal lines 53.333µs (.000053333 seconds) long. Since the actual line length forPAL andNTSC TVs is 52µs and 52.666µs respectively. That means a standard ITU-R BT.601Frame includes either PAL video with anActive Area (the part of theFrame 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 720Pixels.

DVD
The best known application of ITU-R BT.601 is probablyDVD-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 theITU specification. Likewise, video encoding for video often uses all 720Pixels for a standard4:3 image. For eitherWidescreen (Anamorphic) or4:3 Letterbox DVDs it may be necessary using the additional pixels to maintain the source'sAspect ratio.

DV
DV, used in mostStandard Definition (SD) digital camcorders, uses the recommended resolution of 720x480/576, but uses the entire width of the frame for the picture. When usingDV 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.

Widescreen Video
One of the biggest advantages of ITU-R BT.601's relatively high horizontalSamplerate is that it scales well for16:9 (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 onWidescreen displays. Due to theBandwidth limitations, the connectors designed for 4:3SD video transmission (Composite Video andS-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 useComposite Video connections for analog or any of the standard Digital display connectors (DVI,HDMI, etc, ...).

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