Understanding AVCHD - TvTechnology

Understanding AVCHD

The codec differs from H.264/AVC in several ways.
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When DVCAM, DVCPRO and DVCPRO50 were introduced, manufacturers positioned these proprietary formats as “professional” compared to the “consumer” DV format. After working with all four formats, it became clear that differences were confined to their tape recording system. DV, DVCAM, DVCPRO and DVCPRO50 all use the same video codec. (DVCPRO50 employs dual 25Mb/s DV codecs.)

AVCHD, developed jointly by Panasonic and Sony, is a proprietary version of H.264/AVC. Specifically, AVCHD employs both the H.264 Main Profile (MP) and High Profile (HP). (See Figure 1.) The HP codec provides important image quality advantages over the MP codec. Thus, although AVCHD is marketed as a single codec, it uses a pair of codec profiles. (The HP codec is downward compatible with the MP codec.) Moreover, although AVCCAM and NXCAM are marketed as professional formats, both use the same AVCHD HP codec. As you can see, understanding AVCHD, AVCCAM and NXCAM is more complex than understanding DVCAM, DVCPRO and DVCPRO50.

Baseline Profile

The lowest profile used by an HD camera is BP. BP supports only the less efficient context-adaptive variable-length coding (CAVLC). Level 3.1 supports 720p30 at up to 14Mb/s, while Level 3.2 and Level 4.0 support 720p60 at up to 20Mb/s — although at such a low data rate, only 720p30 would be visually acceptable. Level 4.1 supports 720p60 at up to 50Mb/s.

Main Profile

MP offers the next performance level. MP supports both CAVLC and the more efficient context-adaptive binary-arithmetic coding (CABAC). MP also supports B-slices in addition to I- and P-slices. Because B data packets provide H.264 with its greatest encoding efficiency, MP decreases the probability of compression artifacts upon rapid motion. AVCHD uses MP and higher profiles.

A B-reference is generated when two motion vectors are defined from the displacement between the Current Block and Reference Blocks. With H.264, “bi” means two vectors — not two directions as it does for MPEG-2.

Several levels may be used with MP. Level 4.0 supports 720p59.94 and 1080i59.94 up to 20Mb/s (17Mb/s), while Level 4.1 supports data rates up to 50Mb/s (22Mb/s to 24Mb/s). The ability of Levels 4.0 and 4.1 to support 1080i59.94 means that 23.976fps can be recorded after applying 2:3 pulldown. This capability also means that 1080p29.97 can be recorded as 1080i59.94/29.97PsF because its frame rate is equal to the 29.97fps used by 1080i59.94.

High Profile

HP offers all the capabilities of MP (CABAC coding and B-slices) plus an optional capability that greatly improves codec efficiency — the ability to dynamically switch between 8 × 8 and 4 × 4 submacroblocks during compression. Image areas with high detail are compressed using 4 × 4 pixel blocks, while areas with low detail are compressed using 8 × 8 pixel blocks. The latter generates less data; therefore, more bandwidth is available for data from areas with fine detail.

During encoding, each 16 × 16 pixel macroblock is partitioned into four 8 × 8 submacroblocks and 16 4 × 4 submacroblocks. (See Figure 2.) The encoder can switch among working with 16 × 16 blocks, 8 × 8 blocks and 4 × 4 blocks. When predictions are made for 16 × 16 macroblocks, four modes are used. (See Figure 3.) When predictions are made for 8 × 8 submacroblocks, nine modes are used. (See Figure 4.) Canon AVCHD camcorders were the first to use HP H.264. Shooters quickly found MP software decoders were unable to decode Canon recordings.

An HP encoder supports 720p59.94 and 1080i59.94 using multiple levels. Level 4.0 supports data rates up to 20Mb/s (17Mb/s). Level 4.1, used by AVCHD, AVCCAM and NXCAM, supports data rates up to 50Mb/s (22Mb/s to 24Mb/s). Blu-ray employs Level 4.1 using a video data rate up to 40Mb/s.

Level 4.2, available in camcorders using AVCHD 2.0, supports a data rate up to 50Mb/s (28Mb/s) for 1080p59.94. When AVCHD is recorded on a DVD, the disc's maximum spin speed limits the data rate to 17Mb/s. Therefore, when you shoot either MP or HP Level 4.1, or HP Level 4.2, you will not be able to archive to a DVD.

GOP structure

Each frame is encoded as one or more I-, P- and B-slices. Typically, every half-second, an H.264 encoder outputs an I-frame — a picture with all intra-encoded slices.

Audio encoding

H.264/AVC encodes stereo audio using ACC or LPCM audio. AVCHD audio is restricted to AC-3 Dolby Digital 2.0 stereo or 5.1 surround. (NXCAM camcorders record un-compressed audio using PCM audio sampled at 48kHz.)

Steve Mullen is the owner of Digital Video Consulting.