Video resolution standards primer

Understanding the video resolution standards for both standard definition and HD formats helps make informed video conferencing decisions.

Video conferencing, like any other transmitted medium, needs a standard format understood by the devices on each side of the transmission. For video, a standard set of video resolutions defines how large of an image each party can expect. While a vendor of a proprietary telepresence solution could set whatever it wanted in terms of resolution, interoperability with other platforms demands each follow defined video properties.

CIF (Common Intermediate Format or Common Interchange Format)

CIF,  known as either Common Intermediate Format or Common Interchange Format, is the standard bearer for most Web conferencing solutions and consumer video chat products. CIF itself is defined as having an image resolution of 352x288, pushing images at 30 frames per second. The full CIF standard is a similar resolution to standard definition television broadcasts.

The CIF standard also includes variants to support resolutions. QCIF, or quarter CIF, offers a 176×144 resolution by halving both the horizontal and vertical resolutions of the base CIF. The standard also includes SQCIF, or Sub Quarter CIF, which delivers an even smaller 128x96 resolution to support smaller resolution camera and low bandwidth video transmissions. The standard also extends upward to support higher quality video broadcasts. 4CIF offers double the image quality at a resolution twice that of CIF at 704x480, while 16CIF offers four times the image size with a 1408×1152 resolution. Like the analog television signals they are based on, CIF uses roughly 4:3 aspect ratio, meaning that for every four horizontal lines, there are three vertical columns, producing a nearly square video image.

High definition video resolution

The introduction of high definition (HD) video has added both new choices in video transmission and some confusion in comparing video conferencing products. HD video, not surprisingly, adopts the formats found in HDTV broadcasts and has three primary standards: 720p, 1080i and 1080p. In all three cases, the number in each reflects in the number of horizontal lines in the resolution, with the letter denoting either progressive or interlaced scanning. All HD video formats utilize a 16:9 aspect ratio, producing a rectangular, widescreen image. So 720p offers a resolution of 1280x720 and progressive scanning, while 1080i delivers video resolutions of 1920×1080, but uses an interlaced scanning method for display. Likewise, 1080p offers the same 1920x1080 resolution with progressive scan.

Progressive and interlaced scanning

Progressive and interlaced scanning are two approaches for literally drawing the video on the screen. Progressive scanning works like most people expect a video display to work, drawing each line of the display, from top to bottom, then going back to the top and repeating. Interlacing actually divides the process into two steps, first drawing the odd numbered lines on the display, then following up by displaying the odd numbered lines. While progressive scanning eliminates a flicker effect attributed to interlacing, interlacing also allows for a higher resolution image to be sent with the same amount of bandwidth signal.

While high definition television broadcasts adhere to these three standards, video conferencing products are now offering quarter high definition (QHD) as an alternative to CIF. A definitive answer on video resolution for QHD varies, with different sources citing either 960x540 or 640x360, which translates to halving both the horizontal and vertical dimensions of 1080p or 720p respectively.

This was first published in December 2010

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