In progressive video scanning, successive scanning lines are vertically aligned from frame to frame. For example, in the 1080p/60 standard (1080 scanning lines per frame, 60 frames per second) line 1 of frame 2 is scanned at the same vertical location as is line 1 of frame 1, and so are lines 1 of frames 3, 4, etc.
An interlaced signal comprises a succession of even and odd fields, 2 successive fields making a frame. For example, in the 1080i/60 standard (1080 lines interlaced, 60 fields per second), each field is composed of 540 lines, and there are 60 fields or 30 frames per second. A frame comprises one even field and one odd field in succession. For interlaced video, line 1 of field 2 will be vertically located between line 1 and line 2 of field 1, and so will be line 1 of field 4, while line 1 of field 3 will be vertically co-incident with line 1 of field 1.
A progressive scan signal comprises horizontal lines drawn consecutively in sequence. Reference may be made to “even” and “odd” lines of a progressive scan video signal, where even lines may comprise the second line in every other consecutive line (i.e., lines 2, 4, 6 etc.), while odd lines comprise the first line and every other consecutive line (i.e., lines 1, 3, 5, etc.). In cases in which a progressive scan video signal is derived from an interlaced scan video signal, even lines may be referred to being derived from even fields, and the odd lines may be referred to being derived from odd fields.
Interlaced video has been used since the early days of television, and has been slowly fallen into disfavor, and replaced by progressive scan for more and more applications, mostly because of the specific needs of computer or computer-like displays. More recent television displays such as LCDs or DLPs, can only be driven with a progressive scan. In these cases, interlaced signals have to be de-interlaced, which is costly and difficult.
The main advantages of progressive scan (as compared to interlaced scanning) are 1) the absence of interlace artifacts, such as line twitter, serrations, flicker and the like, 2) ease of processing from a camera to a display, and 3) ease of digital processing, such as bandwidth or bit-recompression. The main disadvantage of progressive scan is an increase by a ratio of two of bandwidth requirements as compared to an interlaced standard having the same field rate as the comparable progressive scan standard.
Digital compression systems require a high bit count (equivalent to large bandwidth) to process a progressive scan video signal with an acceptable quality. The requirements for bandwidth are increasing daily, and the available bandwidth provided by present communication channels is limited. Already, more than 50% of the Internet traffic is occupied by video.
Broadcasters are no longer satisfied with present HDTV standards (1080i and 720p) it would like to move to higher scan rates (e.g., “4K” which is approximately 2000×4000 pixels) or higher frame rates (e.g., 120 Hz). Furthermore, interlaced standards, which in the past employed analog transmission, reduced the bandwidth requirements by two but at the cost of image quality (artifacts). Interlacing does not present such an advantage in compression systems and is not computer-friendly.
Compression standards are slowly improving, becoming more efficient in packing increasing data loads into increasingly over-worked channels, but an improvement by a ratio of two in bandwidth utilization (Mb/s for a constant channel) occurs only every 8 to 10 years or so.
A need has arisen, therefore, to reduce the bandwidth requirements of a transmitting video signals by reducing the amount of information presented to the transmission path without degrading the image quality.