HDV
HDV was designed to offer existing video production environments a cost-conscious upgrade path from standard-definition (SD) to high-definition (HD) video. Since HDV operates at the same recorded datarate (25 Mbit/s) as DV, HDV recorders share the same physical (MiniDV) tape transport as existing DV equipment. For the camera, the main expense is concentrated in the optics and imaging electronics. Compared to HD video equipment built on more professional standards (such as HDCAM and DVCPRO HD), HDV enjoys a tremendous cost advantage. HDV camcorders open high-definition video acquisition to consumers, amateur videographers, and low-budget TV production.
Although 1080i HDV and DV share the same (DV) tape format and the same recorded datarate, they use different video compression technology. The DV codec uses strictly an intraframe (spatial) scheme, whereas HDV uses the MPEG-2 video format, including both intraframe(spatial compression) and interframe coding. This allows HDV to achieve its higher spatial resolution at the target bitrates of 19.7 Mbit/s (720p) and 25 Mbit/s (1080i). Compared to more expensive HDCAM and DVCPRO HD equipment, HDV suffers from significantly more spatial and temporal (motion) artifacts.
As a consequence of interframe (temporal) compression, HDV editing is more complex, and introduces some editing distortion near splice points (due to the interdependencies between video frames). Compared to conventional SD DV, HDV offers much higher spatial resolution and higher overall fidelity, so most observers are willing to accept these problems. While standard definition MPEG-2 broadcasts typically use only 2 to 4 Mbit/s, the 25 Mbit/s of HDV enables both high-definition video storage and a lower degree of compression artifacts, with much less visible artifacts. HDV audio uses lossy compression to reduce the audio bitrate to 384 kbit/s. DV audio uses uncompressed 16-bit PCM at 1536 kbit/s. As a result, HDV audio is technically inferior, although MPEG-1 at 384 kbit/s is regarded as 'perceptually lossless.'
Canon has started to ship consumer camcorders that are capable of 24 progressive frames per second. That is, 24 progressive 1080 frames are captured per second, each of which is stored as two coded fields in a 1080i bitstream. This allows decoders to display the progressive frames as full resolution 1080p frames at 24 frames per second or to use "2:3 pull-down" display to show it on a 60-field per second interlaced display.
Since HDV and DV use the same DV25 tape transport, at the same linear speed, recording times for DV and HDV are identical. That is, a 60 minute MiniDV cassette can store 60 minutes of either DV or HDV footage. As of yet, no HDV cameras can record HDV at LP speed, so the maximum record time on one tape is 80 minutes, as opposed to 120 with an 80 minute tape at LP.
Because HDV uses the same tape form factor as DV, users should be able to use any high quality MiniDV tape in their HDV camcorder. However, because HDV has a lower tolerance for drop-outs because of its long-GOP compression, many HDV users purchase either "master" quality Mini-DV tapes or specially formulated HDV tapes.
HDV compression
Although HDV and DV share the same tape format and the same recorded datarate, they use completely different video compression technology. The DV codec is strictly an intraframe (spatial) compression. Each DV video frame is recorded as an independent picture, with a fixed bit allocation and uniform placement on the videotape. The HDV codec is based on MPEG-2 video compression, which employs both intraframe and interframe (temporal) techniques. Interframe compressors store only a fraction of the frames in a video as independent pictures -- called I frames -- and encode the remaining frames as changes relative to them.
Consequently, HDV frames vary in size depending on their prior and future neighbors. In HDV 1080i, one in every 12 (25fps) or 15 (30fps) frames is an I frame. In HDV 1080p, one in every 12 (25 FPS) or 15 (24 or 30 FPS) frames is an I frame. In HDV 720p, one in every 6 (24, 25, or 30 FPS) or 12 (50 or 60 FPS) frames is an I frame.
MPEG-2 video enables HDV to achieve a much higher compression ratio than DV, but at the cost of motion-induced artifacts in scenes of complex motion. The artifacts are a limitation of the compression technology and bitrate allocated to the video bitstream. Motion artifacts are imperceptible for static shots and gentle pans, but may become increasingly detracting as motion complexity increases. For example, a moving riverbed may exhibit regions of picture breakup, depending on its portion of the total screen area. It is important to view these limitations in the proper context. Lighting, chroma content, camera motion, etc all play a role in the potential for artifacts.
Dropouts or errors in the compressed video bitstream affect HDV much more severely than DV. This is an unavoidable characteristic of interframe compression. Since frame data affects multiple frames (and not just the one it originated from), a dropout will impact all dependent neighbors. Frame-accurate editing is also made more difficult by the MPEG-2 codec. Any modifications to the video sequence require the surrounding group of frames to undergo a complete (and lossy) decompression/recompression cycle. However, virtually all professional non-linear editing software is now designed to work flawlessly with HDV.
For all its limitations, HDV is quite stunning on HD displays. Although free of motion-induced artifacts, DV tends to look fuzzy when scaled up to HD resolutions. Subjectively, most observers are willing to accept HDV's visual artifacts in exchange for a more detailed picture.
Resolution and aspect ratio
In HDV, the video frame is defined to have an aspect ratio of 16:9. Permitted resolutions are 720p and 1080i.
HDV 720p uses a resolution of 1280x720 square pixels. HDV 1080i uses a resolution of 1440×1080 pixels, but is still displayed with an aspect ratio of 16:9 (like SD widescreen formats, it uses a pixel aspect ratio of 1.33 instead of 1.0).