EXPORTING VIDEO- what do all the terms in the dialog box mean?
Data rate: How many kilobits, kilobytes, megabits, or megabytes per second is required for realtime playback of a given video or audio clip. For purposes of comparison, here are some standard data rates: Uncompressed broadcast-quality digital video has a data rate of 18 megabytes (MB) per second. This means that in order to see this type of digital video in realtime at full frame size and without dropping frames, the system playing back the video needs to be able to deliver a steady stream of data at 18 MB/sec - a data rate few standard PCs are able to sustain. Standard DV video, on the other hand, being somewhat compressed, has a data rate of 3.5 MB/sec, which is more reasonable for current PCs. Standard CD-quality audio (44.1 KHz/16 bit/stereo) has a data rate of 150 kilobytes (KB) per second. This is also the delivery speed of a 1x CD-ROM drive.
Compare that to the data rates an Internet connection is able to provide. A 56K modem can deliver a theoretical maximum data rate of 56 kilobits (not kilobytes) per second; this translates to 7 KB/sec (8 kilobits equals 1 kilobyte). A full ISDN connection of 128 kilobits/sec provides a data rate of 16 KB/sec. This means that for a video clip to stream in realtime without downloading via an ISDN line, the clip cannot have a data rate of more than 16 KB/sec. This is less than 1/1000th of the uncompressed broadcast video data rate of 18 MB/sec, which corresponds to a compression ratio of more than 1000 to 1. Even compared to the already-compressed DV data rate, 16 KB/sec video has a compression ratio of more than 200:1. This is a lot of compression, and inevitably the sound and image will be significantly degraded. You can reduce the degradation by reducing the frame rate, so that fewer frame per second go by, and reducing the frame size, so there are fewer pixels to compress, but you can only take that so far before you can't see the video image anymore. After some tests with the Sorenson codec, I decided to aim for an overall data rate of 35KB/sec for the QuickTime clips. With a 56K connection at 7KB/sec, this would give a download time of 5 times the length of the clip - that is, a 3 minute clip should download in 15 minutes, which didn't seem unreasonable.
Keyframes:
Most modern CODECs are perfectly good at inserting keyframes automatically. By setting a keyframe interval of 10 seconds all you are telling your transcoder is that you want to manually force a keyframe every 10 seconds, the codec will still insert it's own in between your manually forced ones.
Forcing more keyframes will actually hurt picture quality and, as you have found, encoding efficiency.
Remember that the 25Mb/s you are getting is actually a higher datarate than your source! (1280x720 HDV is 19Mb/s)
Key frame interval = < 24 K.F., or auto. Lower is better. Key frames are frames per second that the CODEC uses as samples of the kind of video it is compressing.
H.264 is recommended for the highest quality at the lowest possible data rate (or the smallest file).
Frame rate: Frame rate is the number of individual images shown every second. Standard (NTSC) video has a frame rate of 29.97 frames per second (fps), and the standard for film is 24 fps. The European standard (PAL) is 25 fps. If you are not overly concerned with how big your file is, leave the frame rate at the default — “Current” — for the best quality. This will make the frame rate of your encoded file the same as the frame rate of your source material. However, you can choose to use a lower frame rate if you need to reduce bandwidth and CPU requirements for playback. Movies with higher frame rates show motion better but have larger file sizes. If you choose a frame rate that’s lower than the movie’s current frame rate, frames will be deleted. If you choose a number that’s higher than the movie’s current frame rate, existing frames will be duplicated (not recommended, since it increases file size without improving quality). When choosing a frame rate less than that of your source, use a simple fraction of your current frame rate, such as 1/2, 1/3 and so on. For example, if your current frame rate is 30 (29.97), use 15 or 10. But again, for best quality with H.264, leave the setting at “Current.”
Key Frames: Many compressors use “frame differencing” to compress moving images. Frame differencing is the process of determining what information has changed from a starting frame (called a “key frame”) to subsequent frames. The key frame contains all of the information for an image. Subsequent frames contain only the information that has changed. Depending on the compressor you use, you can specify how often you want key frames to occur. If you don’t have enough key frames, the quality of your movie might be lower because most frames are generated from others. On the other hand, more key frames result in a larger movie with a higher data rate. With some compressors, an additional key frame is inserted automatically if too much of the image has changed from one frame to the next. A good rule of thumb for general use is to have one key frame every 5 seconds (multiply the frames per second by 5). If you are creating a file for RTSP streaming and have concerns about the reliability of the delivery network (as with the public Internet), you may want to increase key frame frequency to one key frame every 1 or 2 seconds. To leave the key frame interval up to the compressor, select Automatic. For H.264, we recommend leaving the key frame interval up to the compressor; so you should choose “Automatic” for the best quality result.
Frame Reordering: Some more advanced compressors use “frame reordering” to more efficiently represent movie data. Frame reordering is the concept of allowing frames to be decompressed in a different order than their display order. For almost all cases, leave this box checked for H.264 encoding. The only time you would uncheck this box is if you are creating an H.264 movie that needs to be played back by an application that does not understand frame reordering, i.e., an application that does not yet use the new Frame Reordering APIs of QuickTime 7 or if someone asks you to create your content with “B-frames turned off.” If your audience will play back your movie with QuickTime 7 Player, you should leave the box checked.
Data Rate (bit rate): In general, the higher the data rate, the better the quality, but the bigger the file. In most cases, you’ll want to set a data rate based on the way your movie will be viewed. For example, for streaming to 384K broadband connections, you need to limit the data rate to around 350-360 kilobits per second to leave room for network traffic. If the file will be downloaded for playback, the data rate can be much higher (the higher the data rate, however, the longer a slow-connection user has longer to wait before playback begins). In addition, remember in this dialog you’re setting the video data rate. You also need to leave some room for audio.
For H.264, here are some general guidelines:
* For a frame size of 1920 x 1080 (full high definition), choose a data rate of 7,000-8,000 Kbps.
* For a frame size of 1280 x 720 (commonly-used high definition), choose a data rate of 5,000-6,000 Kbps.
* For a frame size of 640 x 480 (standard definition), choose a data rate of 1,000-2,000 Kbps.
* For a frame size of 320 x 240 (Internet-size content), choose a data rate of 300-500 Kbps.
* For a frame size of 176 x 144 (3G), choose a data rate of 50-60 Kbps for 10-15 fps content, or up to 150-200 Kbps for 24-30 fps content.
As mentioned in the 3G example, it’s important to keep in mind that the data rate of a movie is also affected by other compression options you set, such as the frame rate. So the higher your frame rate, the higher your data rate needs to be. If your data rate requirements aren’t this strict and you’d just like QuickTime to give you a beautiful video, you can let the H.264 codec choose an appropriate data rate for your movie by selecting “Automatic.” The codec will pick it’s own data rate based on the Size you’ve chosen and your choice of Quality on the Quality slider (see below).
Optimized for: If you’ve entered your own data rate rather that choosing an Automatic data rate, you have the option to choose your intended delivery method from the “Optimized for” pop-up menu. This setting tells the codec how much the data rate can vary above and below the data rate you choose. For the best quality, choose Download. If you intend to deliver your movie via CD or DVD, choose CD/DVD where the data rate needs to be somewhat constrained so that the disc reader can keep up with reading and passing the data on to your viewer’s computer. If you intend to deliver your movie via RTSP streaming, choose Streaming where the data rate will be most constrained. This option is available only for compressors that can apply limits, such as H.264.