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The HDV specification embraces two types of high definition recording, 720p at a video bit rate of 19Mbps, and 1080i at a rate of 25Mbps. A 1080/60i HDV signal (1440 x 1080) is made up of 1080 lines of vertical resolution (number of lines from the top of the screen to the bottom), each line containing 1440 pixels of horizontal resolution, displayed at a rate of 60 interlaced fields, or 30 frames, per second.
Standard definition DV records a 480/60i signal (720 x 480) in NTSC countries, which includes the U.S. These resolutions render the HDV uncompressed video bit rate at roughly 4.5x that of consumer DV. In order to reduce the uncompressed video bit rate to DV’s compressed 25Mbps, HDV adopts the same powerful MPEG-2 compression format that is used for digital broadcasts and DVDs. MPEG-2 for HDV combines intra-frame (intra = within frames) compression, used in DV recording with, the more efficient, inter-frame (inter = between frames) compression. Inter-frame compression organizes frames into groups of pictures (GOPs) or frames, where one GOP is equivalent to about 15 frames, or a half-second of video. Each GOP begins with one intra-frame (I-frame) containing a complete frame of video, similar to a frame of DV video, followed by predictive and bi-directional frames which encode only the changes in the incoming video relative to the complete I-frame. By exploring redundancies within each frame, in addition to between frames, the MPEG codec is capable of compression ratios in excess of 20:1, compared to only 5:1 for DV.
We always record in 1080i.
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