Adobe Media Encoder CC - Manual (Page 35)

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Encoding quick start and basics
Last updated 12/15/2014
Key frames
Key frames are complete video frames (or images) that are inserted at consistent intervals in a video clip. The frames
between the key frames contain information on changes that occurs between key frames.
Note: Key frames are not the same as keyframes, the markers that define animation properties at specific times.
By default, Adobe Media Encoder automatically determines the key frame interval (key frame distance) to use based
on the frame rate of the video clip. The key frame distance value tells the encoder how often to re-evaluate the video
image and record a full frame, or key frame, into a file.
If your footage has a lot of scene changes or rapidly moving motion or animation, then the overall image quality may
benefit from a lower key frame distance. A smaller key frame distance corresponds to a larger output file.
When you reduce the key frame distance value, raise the bitrate for the video file to maintain comparable image quality.
Image aspect ratio and frame size
As with the frame rate, the frame size for your file is important for producing high-quality video. At a specific bitrate,
increasing the frame size results in decreased video quality.
The image aspect ratio is the ratio of the width of an image to its height. The most common image aspect ratios are 4:3
(standard television), and 16:9 (widescreen and high-definition television).
Pixel aspect ratio
Most computer graphics use square pixels, which have a width-to-height pixel aspect ratio of 1:1.
In some digital video formats, pixels aren’t square. For example, standard NTSC digital video (DV), has a frame size of
720x480 pixels, and it’s displayed at an aspect ratio of 4:3. This means that each pixel is non-square, with a pixel aspect
ratio (PAR) of 0.91 (a tall, narrow pixel).
Interlaced versus noninterlaced video
Interlaced video consists of two fields that make up each video frame. Each field contains half the number of horizontal
lines in the frame; the upper field (Field 1) contains all of the odd-numbered lines, and the lower field (Field 2) contains
all of the even-numbered lines. An interlaced video monitor (such as a television) displays each frame by first drawing
all of the lines in one field and then drawing all of the lines in the other field. Field order specifies which field is drawn
first. In NTSC video, new fields are drawn to the screen 59.94 times per second, which corresponds to a frame rate of
29.97 frames per second.
Noninterlaced video frames are not separated into fields. A progressive-scan monitor (such as a computer monitor)
displays a noninterlaced video frame by drawing all of the horizontal lines, from top to bottom, in one pass.
Adobe Media Encoder deinterlaces video before encoding whenever you choose to encode an interlaced source to a
noninterlaced output.
High-definition (HD) video
High-definition (HD) video refers to any video format with pixel dimensions greater than those of standard-definition
(SD) video formats. Typically, standard-definition refers to digital formats with pixel dimensions close to those of
analog TV standards, such as NTSC and PAL (around 480 or 576 vertical lines, respectively). The most common HD
formats have pixel dimensions of 1280x720 or 1920x1080, with an image aspect ratio of 16:9.