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Lesson 2. About Digital Video Editing > Measuring frame size and resolution

Measuring frame size and resolution

Several attributes of frame size are important when editing video digitally: pixel and frame aspect ratio, clip resolution, project frame size, and bit depth. A pixel (picture element) is the smallest unit that can be used to create a picture; you can't accurately display anything smaller than a pixel.

Aspect ratio

The aspect ratio of a frame describes the ratio of its width to its height in the dimensions of a frame. For example, the frame aspect ratio of NTSC video is 4:3, whereas some motion-picture frame sizes use the more elongated aspect ratio of 16:9.

A frame using a 4:3 aspect ratio (left), and a frame using the 16:9 aspect ratio (right)

Some video formats use a different aspect ratio for the pixels that make up the frame. When a video using non-square pixels (i.e., pixels that are taller than they are wide, or wider than they are tall) is displayed on a square-pixel system, or vice versa, shapes and motion appear stretched. For example, circles are distorted into ellipses.

Frame with square pixels (left), frame with tall horizontal pixels (center), and center frame again displayed using square pixels (right)

Non-square pixels

Premiere provides support for a variety of non-square pixel aspect ratios, including DV's Wide screen (Cinema) pixel aspect ratio of 16:9 and the Anamorphic pixel aspect ratio of 2:1.

When you preview video with non-square pixel aspect ratios on your computer screen, Premiere displays a corrected aspect ratio on the computer monitor so that the image is not distorted. Motion and transparency settings, as well as geometric effects, also use the proper aspect ratio, so distortions don't appear after editing or rendering your video.

Frame size

In Premiere, you specify a frame size for playing back video from the Timeline and, if necessary, for exporting video to a file. Frame size is expressed by the horizontal and vertical dimensions, in pixels, of a frame; for example, 640 by 480 pixels. In digital video editing, frame size is also referred to as resolution.

In general, higher resolution preserves more image detail and requires more memory (RAM) and hard disk space to edit. As you increase frame dimensions, you increase the number of pixels Premiere must process and store for each frame, so it's important to know how much resolution your final video format requires. For example, a 720 × 480 pixel (standard DV) NTSC frame contains 345,600 pixels, while a 720 × 576 PAL image contains 414,720 pixels. If you specify a resolution that is too low, the picture will look coarse and pixelated; specify too high a resolution and you'll use more memory than necessary. When changing the frame size, keep the dimensions proportional to the original video clip.

If you plan to work with higher resolutions and/or you are concerned about your CPU's processing capabilities, you can specify one or more scratch disks for additional RAM and hard disk space. For more information, see “Setting up Premiere's scratch disks” in Chapter 1 of the Adobe Premiere 6.0 User Guide.

Overscan and safe zones

Frame size can be misleading if you're preparing video for television. Most NTSC consumer television sets enlarge the picture; however, this pushes the outer edges of the picture off the screen. This process is called overscan. Because the amount of overscan is not consistent across all televisions, you should keep action and titles inside two safe areas—the action-safe and title-safe zones.

The action-safe zone is an area approximately 10% less than the actual frame size; the title-safe zone is approximately 20% less than the actual frame size. By keeping all significant action inside the action-safe zone and making sure that all text and important graphic elements are within the title-safe zone, you can be sure that the critical elements of your video are completely displayed. You'll also avoid the distortion of text and graphics that can occur toward the edges of many television monitors. Always anticipate overscan by using safe zones, keeping important action and text within them, and testing the video on an actual television monitor.

You can view safe zones in the Monitor window's Source view, Program view, or both.

Safe zones in the Program view: A. Title-safe zone B. Action-safe zone

Safe zones are indicated by dotted lines in Premiere's Title window.

A. Title-safe zone B. Action-safe zone C. Overscan area D. Perimeter of frame

For more information on customizing safe zones in the Monitor and Title windows, see Lesson 8, “Creating a Title” in this book.

Bit depth

A bit is the most basic unit of information storage in a computer. The more bits used to describe something, the more detailed the description can be. Bit depth indicates the number of bits set aside for describing the color of one pixel. The higher the bit depth, the more colors the image can contain, which allows more precise color reproduction and higher picture quality. For example, an image storing 8 bits per pixel (8-bit color) can display 256 colors, and a 24-bit color image can display approximately 16 million colors.

The bit depth required for high quality depends on the color format that is used by your video-capture card. Many capture cards use the YUV color format, which can store high-quality video using 16 bits per pixel. Before transferring video to your computer, video-capture cards that use YUV convert it to the 24-bit RGB color format that Premiere uses. For the best RGB picture quality, you should:

  • Save source clips and still images with 24 bits of color (although you can use clips with lower bit depths).

  • If the clip contains an alpha channel mask, save it from the source application using 32 bits per pixel (also referred to as 24 bits with an 8-bit alpha channel, or millions of colors). For example, QuickTime movies can contain up to 24 bits of color with an 8-bit alpha channel, depending on the exact format used.

Internally, Premiere always processes clips using 32 bits per pixel regardless of each clip's original bit depth. This helps preserve image quality when you apply effects or superimpose clips.

If you're preparing video for NTSC, you should keep in mind that although both 16-bit YUV and 24-bit RGB provide a full range of color, the color range of NTSC is limited by comparison. NTSC cannot accurately reproduce saturated colors and subtle color gradients. The best way to anticipate problems with NTSC color is to preview your video on a properly calibrated NTSC monitor during editing.

For more information, see “Previewing on another monitor” in the Adobe Premiere 6.0 Technical Guides found in the Support area on the Adobe Web site (www.adobe.com/support/techdocs/topissuespre.htm).

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