Beyond HDV: AVCHD High-definition Compression
Although AVCHD first appeared in consumer camcorders, a variationAVC-Intrawill soon show up in broadcast cameras from Panasonic. Given that the MPEG-4 technology that fuels AVCHD is roughly twice as efficient as the MPEG-2 technology used in HDV, the new Sony-Panasonic standard will likely supplant HDV in the prosumer space. For this reason, it''s a standard you need to know about, even though you may not use it for a while.
AVCHD stands for Advanced Video Codec High Definition, and it''s based upon the AVC codec, a joint standard of the ITU (International Telecommunications Union) and ISO (International Standardization Organization) groups. It's also called H.264. AVC/H.264 is an advanced subset of MPEG-4 compression (MPEG-4 Part 10 to be specific), so sometimes the terms are used interchangeablyalthough in most instances, references to MPEG-4 are to the older codec, not AVC.
While AVCHD is relatively new, AVC is an established standardparticularly in the streaming space, where, with Apple''s backing, it serves as the primary codec for iPod video. AVC is also starting to displace MPEG-2 in the cable TV and satellite TV markets, and it's one of the three technologies available for HD DVDs (along with MPEG-2 and Microsoft''s VC1).
The AVCHD specification itself supports scalable frame sizes from 720x480 up to 1920x1080 (including 1080/24p) at both the 4:3 and 16:9 aspect ratios. Like HDV, AVCHD video uses the 4:2:0 sampling format, which is superior to the 4:1:1 used in DV camcorders. The format supports either PCM (1-7.1 channels) or Dolby Digital AC-3 audio (1 to 5.1 channels) at data rates up to 640kbps (for AC3) and 1.5Mbps (for PCM). AVCHD uses an MPEG-2 transport stream “wrapper,” and it is scalable up to 18Mbps.
The initial AVCHD devices are all “tapeless,” using either mini-DVD recorders, SD or Memory Stick memory cards, or hard drives. Interestingly, when AVCHD was announced, the focus seemed to be optical media-based camcorders that could store high-definition video for immediate playback on Blu-ray players, although the format seems to have evolved into a more general-purpose acquisition format.
Like HDV, AVCHD is a “long-GOP” format that uses a mixture of intraframes (also called I-frames) as well as bidirectional (B) and predictive (P) frames. This sophisticated scheme enhances compression efficiency, maintaining quality at lower bit rates, but also makes AVCHD harder to edit.
Interestingly, Panasonic (without Sony's involvement) also recently introduced the strictly professional AVC-Intra format, which uses 4:2:2 color sampling and intraframe-only encoding, but still reportedly cuts the data rate of DVCPRO HD in half. It uses only intraframes, so it should be much easier to edit than long-GOP AVCHD. This format is expected to debut as an optional encoder board for Panasonic''s AJ-HPX2000 P2 HD camcorder, and it is expected to double the capacity of the P2 card.
Today, however, most of the MPEG-4-recording camcorders have been single-CCD consumer-class AVCHD camcorders, with the notable exception of Panasonic''s 3CCD AG-HSC1U AVCHD camcorder. Panasonic bills it as “the world''s smallest professional 3CCD AVCHD camcorder.” The unit, which I''ll overview in the next issue of HDV@Work, has pro-level features such as a microphone input and optical image stabilization and costs $1,995 at B&H Photo Video. Still, this is a tiny camcorder, smaller than a 12oz. can of Coke, and doesn''t seem to threaten the installed base of prosumer/professional camcorders such as the Canon XH A1 and Sony's V1 and Z1.
This is a good thing, because the editing software isn''t there yet, at least on the professional side. Neither Apple Final Cut Pro nor Adobe Premiere Pro supports the new format, although Sony Vegas 7.0e does. Canopus/Grass Valley has a tool that lets you convert AVCHD into a format you can input into Edius, and ProCoder should directly import AVCHD. (Editor's note: On June 27, Apple announced Final Cut Pro 6.0.1, which introduces log and transfer support for the AVCHD format.)
In the consumer space, support is more extensive, as you would expect given the consumer focus of the initial camcorders. Both Ulead VideoStudio 11 Plus and Pinnacle Studio 11 support AVCHD, and for a future issue of HDV@Work, I''ll try to burn a Blu-ray disc from an AVCHD source using the latest beta version of Studio 11. Nero Digital also supports AVCHD in Nero Ultra Edition Enhanced, and several vendors, including Cyberlink, have announced plans to support AVCHD during 2007.
This adds up to a standard that''s intriguing but probably not immediately relevant to most producers. So why the fuss? Because MPEG-2 has been around since the early 1990s and hasn''t been the compression quality leader since before Bill Clinton left office. With the support of the two key standards organizations, AVCHD is the clear heir apparent. Shrinking hard drives and dropping prices for SD and other solid-state memory devices probably also presage the demise of tape-based formats (such as HDV, because it's technically described as a standard), as much as I''ll hate that day personally. These dynamics make the future seem a lot like AVCHD, don't they?