HD Tips & Techniques—The Impact of Codecs on Broadcast

Should you go with AVC-Intra or MPEG-2 Long GOP?
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ORLANDO, FLA. As camera manufacturers embrace file-based production solutions, broadcasters have been introduced to a bevy of new codecs. These generally break down into two categories—Intraframe and Long-GOP compression.

Sony, with their optical disc and solid-state XDCAM-HD and EX camera products, has supported Long-GOP compression based on MPEG-2 algorithms. A sequence of I, B and P frames forms a "group of pictures" (GOP) that constitute a stream of moving video. I-frames contain all the data necessary to independently rebuild an image, while B and P frames contain less data and derive their information in part from the surrounding frames. I-frames usually occur every 6th, 7th, 12th or 15th frame, depending on the codec specifics. Sony's single-layer XDCAM-HD format employs three compression settings: 18 Mbps, 25 Mbps and 35 Mbps. The 18 Mbps and 35 Mbps use variable bit rate encoding, while the 25 Mbps rate is a constant bitrate that is essentially the same as HDV. Sony has also introduced XDCAM-HD422, a 50 Mbps version recorded onto dual-layer optical media.

Panasonic has championed Intraframe compression and argued against Long-GOP encoding. Each and every frame is unique in Intraframe encoding schemes and not dependent on adjacent frames. Panasonic's initial P2 solid-state products recorded HD signals with DCT-based DVCPRO HD compression at 100 Mbps. P2 has since expanded to include the new professional AVC-Intra codec at two data rates: 100 Mbps and 50 Mbps. AVC is part of the MPEG-4 family of codecs and a cousin of H.264. It's newer and more efficient than MPEG-2 or DCT codecs, so 100 Mbps using AVC-Intra yields improved image quality over DVCPRO HD at the same bitrate.

EDIT FRIENDLY

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Panasonic's AJ-HPX camcorder (L) supports AVC-Intra while Sony's PMW-EX1 camcorder supports MPEG-2 Long GOP. Historically, Intraframe codecs have been considered more edit system-friendly, because an edit can be easily made at each frame. NLE software engineers have developed a workaround to make Long-GOP compression editable by evening out the "lumpiness" of the data stream. When back-to-back I-frame edits are made, data is discarded to lower the total data rate. Conversely, when B or P-frames are edited in succession, artificial I-frames have to be created. This increases the bitrate at those edits without any actual increase in information. A cuts-only sequence in an Intraframe format, like AVC-Intra or DVCPRO HD, would be lossless, while the same sequence using Long-GOP media would be slightly lossy.

Most NLEs can handle Long-GOP media, complete with effects, in real-time, but have to process (render) the new GOP structure if the output is to be recorded over FireWire back onto a Long-GOP recording device. Apple Final Cut Pro engages in a "conforming" process necessary to rebuild a proper I/B/P cadence. This is more extensive with HDV and 25 Mbps XDCAM-HD because of the constant data rate, but relatively minor when the variable 35 Mbps setting has been used. On the other hand, if your NLE is configured with a hardware capture card (Matrox Axio, Avid Mojo DX, AJA KONA, Grass Valley HDSTORM, etc.), such rendering may not be needed, as the video is played out as full bandwidth HD or SD through the I/O card.

PROCESSING INTENSIVE

The Long-GOP versus I-frame debate is not new, but the truth of the matter is that I-frame codecs are not without their own issues. Panasonic's AVC-Intra is very computationally intensive to decode, so only the latest computer platforms should be used if you intend to work in this format. Hardware from Apple or HP is easily up to the task of editing either Long-GOP or advanced Intraframe formats. Broadcasters considering AVC-Intra as the house ENG standard might find it a challenge if their news photographers are using laptops on ENG mobile units. In this case, a ruggedized computer, such as the 1 Beyond HD OctoFlex Rugged or Panasonic's AJ-HPM110 P2 Mobile solid-state recorder/player, might be a better solution.

The best native support for these new codecs can be found in Avid Media Composer and Newscutter, as well as Grass Valley EDIUS 5. Since either NLE brand supports MXF files, media from P2 cards or XDCAM-HD discs can be directly accessed or at most, copied and rewrapped into a different flavor of MXF. By comparison, Apple Final Cut Pro is based on QuickTime. It offers matching codecs for XDCAM-HD, EX and DVCPRO HD, but FCP must first copy the files and rewrap them as QuickTime MOV files. This is made possible through additional software that can be downloaded from Sony or Panasonic. Apple is lagging behind in native support for AVC-Intra, which FCP must first transcode into Apple's ProRes 422 codec—a process that takes a bit longer than a simple file copy.

These workflows differ greatly with each camera recording format and the brand of NLE you intend to use in your broadcast operation, so it's best to evaluate these two items as a synergistic system. Mix and match the right parts and you'll get on the air faster, more reliably and with less operator stress.

Oliver Peters is an independent editor/colorist and a frequent contributor to Videography magazine. He may be contacted at www.oliverpeters.com.