Anatomy of a video file

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You’re shopping for a camcorder, and you’re swamped by a sea of letters, numbers, and indecipherable acronyms—AVCHD, H.264/MPEG-4 AVC, MPEG-2. What do they all mean and which ones should you care about? Here's the lowdown.

Tale of two formats: MPEG4 vs. AVCHD

Each of the above labels describes a video format. Fortunately, most people have a simple choice: MPEG-4 or AVCHD. You decide between convenience (MPEG-4) and a wider variety of features (AVCHD). However, as camcorders, computers, video players, and editors grow more powerful, AVCHD format is gradually becoming as convenient to shoot, edit, and preview as MPEG-4, making it the clear choice for a growing number of video enthusiasts.

MPEG-4 is a standard format from the Moving Picture Experts Group and has been around for more than 20 years. The current version is officially called H.264/MPEG-4 AVC, but it's usually shortened to a snappy MPEG-4 (pronounced M-Peg-4). Social networking sites, video editors, and video players—including QuickTime—all work with MPEG-4 files. Thus, the MPEG-4 standard is popular on pocket point-and-shoot models, and with people who want to quickly shoot video and pop it onto YouTube or Facebook, which folks can view on their computers, tablets, and smartphones.

AVCHD (pronounced by its initials) stands for Advanced Video Coding High Definition. It arrived on the scene in 2006, is more fully featured than MPEG-4 and is rapidly gaining broad acceptance. Think of AVCHD as a kind of MPEG-4 "Plus’’. That’s because AVCHD is a container format that includes the MPEG-4 format, but tosses a lot of other stuff into the mix, like coding for audio, writing to different media including DVD and Blu-Ray discs, and Digital Rights Management, such as copy protection. AVCHD even gives you media presentation tools so you can add menu items, make slides, and add subtitles directly from your camcorder. With AVCHD format, you don’t need to export your video file to an editor to perform simple editing tasks. Unfortunately, AVCHD is an “all-or-nothing” format. You can’t buy an AVCHD camcorder and decide to shoot using just its MPEG-4 part. If the camcorder offers only AVCHD, you have to shoot in AVCHD format.

AVCHD has other downsides. You need more computing oomph for the video editors and players to crank through AVCHD files, even if they can do it natively. You need a minimum dual-core processor and 2GB of RAM, but it would be better if you had a quad-core with 4GB of RAM. And you need relatively new software if you want to run and edit AVCHD files natively. Final Cut Pro X can process AVCHD files natively, but with the previous version, Final Cut Pro 7, you first have to transcode the file (translate the file bit-by-bit) into ProRes, a set of video compression formats developed by Apple for use in post production. The ProRes family of intermediate codecs are used for editing, but not as a final format for publishing video. Transcoding slows down file imports.

In 2009, Apple introduced the iFrame video format. You can transfer iFrame files directly into iMovie, no transcoding needed. However, few consumer camcorders offer the option to shoot video in iFrame. iFrame video is only 960-by-540 resolution, yielding only a half a megapixel per frame, only one quarter the resolution of Full HD.

You'll likely find AVCHD on more traditional camcorders that have larger lenses and higher end features than pocket camcorders, such as powerful optical zooms and a wider range of focus. However, we’re starting to see traditional models that let you switch between AVCHD and standalone MPEG-4, including Canon’s Vixia HF M50 and R30 series.

Video settings

For each video format, your camcorder usually offers several profiles of four settings: resolution, frame rate, scan method (interlaced or progressive), and bit rate (in megabits per second, or mbps). These offer a trade-off between video quality and file size. You can increase video quality by raising the resolution, frame rate, and bit-rate, but you generate a larger, more unwieldy file.

You often see “aspect ratio” mentioned in this context. This means the number of horizontal pixels divided by the number of vertical pixels in a frame. HD video is in a 16:9 aspect ratio, also called widescreen.

Camcorder specifications cryptically sum up these settings with confusing terminology that looks like this example: 1080/60p 17Mbps. Let's break down what this means.

Resolution This is the number of pixels in each video frame. A frame is one of many still images you view in quick succession to get the illusion of motion. If you’ve ever riffled the pages of an animation flip-book, it’s the same concept, with each page representing a frame.

Video resolution is usually stated as the number of pixels horizontally by the number of pixels vertically. HD camcorders offer at least two resolutions: 1920-by-1080 (Full HD) and 1280-by-720. These are usually shortened to just their vertical measures, such as 1080 and 720.

Frame rate Continuing with the flip-book example, if you flip the pages slowly, you see a series of still images. But speed up the page-flipping, and those images start to run together in a fluid unbroken motion. The number of frames per second that you see is called the frame rate. For video, common frame rates are 30fps and 60fps. The faster the frame rate, the smoother fast-motion scenes will appear. When filming high-speed events like a NASCAR race or a football game, when given the same choice of scan method (interlaced or progressive, see below), choose the higher frame rate number. For example, if you have a choice between 1080/60i, 1080/30p, and 720/60p, choose 720/60p.

Scan method With scan method, you have two choices: interlaced and progressive. Progressive is the newer, better method, and it’s steadily pushing interlaced video out of the picture. Interlaced was developed in the 1920s to handle display limitations of the cathode-ray tube (CRT), that large glass monitor that made older television sets so heavy. Nimbler, solid-state flat-panel screens (LCDs or Liquid Crystal Displays) have largely replaced CRTs, letting progressive-scan video move to the fore.

Progressive scan video looks smoother and crisper than interlaced, with fewer artifacts. Two common camcorder video settings are 1080/60i (interlaced) and 720/60p (progressive). For shooting fast motion events, you’re usually better off filming in 720/60p, the progressive mode setting, even though the 720 resolution is half of the 1080 setting. The 720 setting gives you only 0.9 megapixels per frame compared with more than two megapixels for the 1080 resolution.

Until recently, if you wanted to shoot at the top available video quality, you had to choose between 1080/60i and 720/60p. Now we’re starting to see the best of both worlds. Last year the industry upgraded the AVCHD format to include 1080/60p, and some of this year’s new camcorders—such as the Panasonic HCs X900, V700, and V500 series models—offer it.

Bit rate The more you compress a video stream, the lower the bit rate, which speeds up file transfers and uses less memory. But lower bit rates yield lower video quality. For some video formats, such as AVCHD, your camcorder usually offers several bit rate levels, so you can choose your best trade-off between file size and video quality. For AVCHD, typical bit-rate levels include 24, 17- 13- and 9-megabits per second.

Camcorder shopping is easier than ever

Shopping for complex electronic equipment has never been a cakewalk. But if you're in the market for a new camcorder, your task will take a lot less effort than in the past. After a long transition away from standard definition (SD), mainstream high-definition (HD) camcorders are cheaper to buy and easier to work with.

We’re also seeing camcorder and computer makers, video software developers, and social network sites all syncing up more readily on standards. Video formats now consist of two main players: MPEG-4 and AVCHD. For video resolution, most casual shooters will be fine sticking with 1920-by-1080. For storage, SD memory is about the only game in town. Pretty much all camcorders use USB 2.0 and HDMI to let you pipe your video to your computer and TV.

Plus, it’s become much easier to quickly do what you want to do with your new camcorder. More than ever, you can shoot, edit, and post video in the same format, allowing an unbroken workflow from the time you capture that candid camera moment to when you toss it up onto YouTube. You already do that with your smartphone and pocket camcorder. It’s about time you can enjoy the same seamless operation with a traditional camcorder.

[Bryan Hastings is a freelance writer in the San Francisco Bay Area.]

This story, "Anatomy of a video file" was originally published by Macworld.

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