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Much more about 720p24 HDV, Part 3

Apr 10, 2006 3:16 PM, Steve Mullen

In this newsletter, we will take a closer look at HDV color sampling. The Tables below show the total number of bits recorded per second for both HDV 720p24 and DVCPRO HD 720p24.

We can see that frames of HDV and DVCPRO HD have the same number of bits: 265,420,800. Why would one format (4:2:2) discard luminance resolution while another format (4:2:0) would discard chrominance resolution? The answer, well known to television engineers, is that visually our eyes do not perceive a loss in chrominance resolution nearly as well as a loss in luminance resolution. If you have to discard data, which you must during compression, it is logical to eliminate the chroma first because it is the least noticeable loss.

Of course, nothing is ever that simple. We must differentiate between a format’s color-sampling specification and the actual chroma information carried by a format. For example, the Panasonic HVX200 employs the DVCPRO HD format that uses 4:2:2 color sampling. DVCPRO HD 720p requires 480 horizontal chroma samples and 720 vertical chroma samples, while 1080i DVCPRO HD requires 640 horizontal chroma samples and 1080 vertical chroma samples. (Some consider DVCPRO HD to use 2.7:1.3:1.3 color sampling.)

The HVX200’s 960x540 CCDs easily meet the requirements for 480 and 640 horizontal chroma samples. (Pixel-shift does not increase chroma resolution.) Unfortunately, these CCDs cannot provide the 720 or 1080 vertical chroma samples required for traditional 4:2:2 video. This vertically undersampled chroma is not the equivalent of DVCPRO HD 4:2:2 from a Varicam.

Many would argue that the real benefit of a 4:2:2 format is manifest when it is edited natively — as is DV. These folks point out that increased chroma resolution is mandatory for image manipulation during editing — specifically color correction and chroma keying. This claim is both true and false.

For extremely demanding work, the best option is to convert both 4:2:0 and 4:2:2 video to a 10-bit, uncompressed, “intermediate” file. Unfortunately, if you want to do so, you’ll need to own a very fast and very large RAID. For this reason, it is much more practical to convert to a “mathematically lossless” intermediate codec. These include the "Visually Perfect" CFHD codec from CineForm, the HQ codec from Canopus, and the DnxHD codec from Avid. These codecs typically do not require a RAID, and with today’s computers they support realtime effects. The Apple Intermediate Codec, because it is an I-frame-only MPEG-2 codec is not, despite its name, a lossless intermediate codec. Moreover, while AIC maintains image quality with progressive video, it performs less well with interlaced video.

If you want to edit natively, then shooting DVCPRO HD offers several benefits over shooting HDV. Not only is the 4:2:2 chroma sampling an advantage, when editing using FCP you get the benefit of multiple-stream, realtime effects. Moreover, because DVCPRO, like DV, is an intraframe codec, exporting to DVCPRO HD is many times faster than exporting to HDV tape. (For these reasons, some FCP editors transcode HDV to the 720p DVCPRO HD codec — with the understanding the DVCPRO HD codec slightly reduces horizontal resolution and is not mathematically lossless.) For more information on DVCPRO HD, consult these two CineForm white papers.

Despite the many claims that color correction and chroma-keying must be done with 4:2:2 video, native HDV shooters should keep in mind that there has been a decade of successful, native DV (4:1:1) editing. Moreover, long export times are not the function of the shooting codec. No matter what codec your camcorder uses, when you export an MPEG-2, AVC, or VC-1 file for an HD DVD, it is going to require a long wait. Moreover, HD video will eventually be subsampled to 4:2:0 when the final HD product is distributed over the air, cable, satellite, or on optical disc. Bottom line, there is little reason for HDV shooters to worry about any of the issues raised by those who shoot formats that do not employ MPEG-2 compression.

© 2008 Penton Media, Inc.