June 93 - MAKING BETTER QUICKTIME MOVIES
MAKING BETTER QUICKTIME MOVIES
QuickTime 1.5 makes it easier than ever to make CD-playable movies. These tips on
capturing, compressing, and playing back movies will help you use the new Apple
Compact Video compressor to its best advantage, creating movies that will play well
off a standard CD-ROM drive on a Macintosh LC computer.
QuickTime introduced the world of digital video to the Macintosh and enabled a whole new
category of multimedia content: movies. With QuickTime, it's simple to play back movies in any
application and to exchange movies between applications using the standard cut/copy/paste
mechanism. But no one ever said it was going to be easy to create them!
What makes movie creation tricky is the tradeoffs required to get QuickTime movies to play off a
CD-ROM drive, the most effective distribution medium for digital video. Uncompressed, full-screen,
full-motion video requires a data transfer rate of about 27,000 kilobytes/second, yet a typical CD-
ROM drive has a data transfer rate of only 100 kilobytes/second. QuickTime solves this problem by
using video compression, which requires you to make tradeoffs between frame size, frame rate, image
quality, and sound quality when making a movie.
The tips in this article will help you make the right tradeoffs to produce high-quality movies that will
play off CD-ROM. You'll also find tips on capturing digital video, using the MovieShop utility on
this issue's CD to produce compressed movies, and playing back what you've created. I assume you're
already familiar with the basics of movie making with QuickTime.
TIPS ON TRADEOFFS
Making the right tradeoffs is the key to producing better QuickTime movies. Depending on your
target platform, to get smooth playback you may need to limit the frame size and rate, minimize the
differences between frames, and trade off audio for video quality.
Before you get started, you need to decide which Macintosh platform your movie will play on.
Obviously, a Macintosh Quadra 950 with a double-speed CD-ROM drive can play much larger,
higher-quality movies than a Macintosh LC with a standard CD drive. Table 1 shows some common
platforms and their capabilities. (See the following sections for more on frame size and rate.)
Common QuickTime Platforms
- Built in CD Drive
- Maximum Movie Frame Size, Rate
- 1, 4, 8, 16, 24
- 320 x 240, 24 fps
- Power user, professional
- 1, 4, 8, 16
- 320 x 240, 15 fps
- Consumer, home
- 1, 4, 8, 16
- 240 x 180, 12 fps
- 160 x 120, 15 fps
- Loyal customers
For the purposes of this article, our target platform is the Macintosh LC II with an AppleCD SC
CD-ROM drive (transfer rate of 100 kilobytes/second). Movies created for this platform should play
back well on virtually every color Macintosh, covering as much of the installed base as possible.
However, keep in mind that machines that use NuBus video, such as the Macintosh II, won't have
the playback performance of the LC II. You should always test your movies on the platforms they'll
The frame size determines how large the movie will be on the screen. The larger the frame size, the
greater the number of pixels that have to be updated every frame. This can be a problem for less
powerful machines, so you often need to limit the frame size to get smooth playback.
Frame sizes are typically specified by horizontal and vertical pixel measurements. Some common
frame sizes for digital video are shown in Table 2.
Table 2 Common Frame Sizes for Digital Video
|Frame Size||Description||Pixels/Frame||Capability Required
|640 x 480||Full-screen||307,200||Hardware acceleration
|320 x 240||Quarter-screen||76,800||Fast CPU like a Macintosh Quadra
|240 x 180||Eighth-screen||43,200||Apple Compact Video compressor
|160 x 120||Sixteenth-screen||19,200||Apple Video compressor
Note that full-screen movies are practical only with hardware acceleration, and for quarter-screen
movies you need a fast CPU like a Macintosh Quadra. With our LC II platform and the Apple
Compact Video compressor made available by QuickTime 1.5, we can create eighth-screen movies,
which have more than twice the screen area of the "postage-stamp movies" possible with QuickTime
1.0's Apple Video compressor.
For the Apple Compact Video compressor to function optimally, the frame size should be a multiple
of 4 in each dimension. This is because the compression algorithm uses a 4-pixel by 4-pixel cell.
The frame rateis the number of frames displayed in each second of the movie, typically described in
frames per second (fps). The frame rate to use for a movie depends on the frame rate of the source
material, whether film or videotape. For the smoothest results, you should use a frame rate of which
the source material frame rate is a multiple, but this may only be possible if you have hardware
acceleration or a fast CPU. Still, an acceptable compromise is available if your platform is limited.
The frame rate of source material in the NTSC video format is approximately 30 fps. Much source
material is shot using film at 24 fps and then transferred onto videotape. Frame rates to use for
movies based on these types of source material are shown in Table 3. Other video standards such as
PAL and SECAM have different frame rates; if your movie is based on one of these types of source
material, you'll have to compensate accordingly. For our target platform, we can use 12 fps with good
Common Frame Rates for Digital Video
If Your Source Material Is NTSC Video:
|Frame Rate||Description||Capability Required
|30 fps||Full-motion||Hardware acceleration
|15 fps||Half-motion||Fast CPU like a Macintosh Quadra
|12 fps||Half-film rate||Apple Compact Video compressor
|10 fps||Third-motion||Apple Video compressor
If Your Source Material Is Film:
|Frame Rate||Description||Capability Required
|24 fps||Full-motion||Hardware acceleration
|12 fps||Half-motion||Apple Compact Video compressor
|10 fps||Third-video||rate Apple Video compressor
There are a couple of minor quirks having to do with frame rate that you should be aware of when
you make a movie. First, you'll note that I said the NTSC frame rate isapproximately 30 fps. For
reasons lost in the dawn of television, the NTSC frame rate is actually 29.97 fps. If you assume the
frame rate is 30 fps, long movies can lose synchronization between sound and video over time, since
there are fewer video frames than expected. For example, if you digitized 100 seconds of video, you
would expect to get 3000 frames, but you would really only get 2997 frames in that period of time.
The GrabGuy utility and the HyperCard® Movie Making Stack (found on theQuickTime 1.5
Developer CDand on this issue's CD) automatically take care of this problem, but if you find sound
sync drifting over time on long movies, you may need to duplicate a video frame every 1000 frames
to get things back in sync.
The second item to note involves transferring 24-fps film to 30-fps video. On videotape, each frame
is composed of two fields, one containing the odd scan lines and the other containing the even scan
lines. These fields are interlaced to produce the frame. When film is transferred to video, six extra
frames are "made up" every second. Typically, once every four frames, two adjacent film frames are
put into the two fields of a single video frame to form a fifth frame. Figure 1 shows how this works.
These made-up frames have a blurred look when digitized.
Figure 1 Making Up Extra Frames When Film Is Transferred to Video
You can use a couple of different methods to digitize only the original film frames and skip the blurry
made-up frames. If you have a capture system that can grab individual video fields, you can set it to
capture at 12 fps, and it will skip the duplicate fields and give the original 12 film frames each second.
Or simply capture at 30 fps and throw away every fifth frame, yielding the original 24 film frames.
Frame differencingis the technique used by QuickTime of storing and updating only the pixels that
differ from the previous frame, so that much less data has to be stored and displayed. For example, in
Figure 2 the frame on the right contains only the information needed to update the areas of the
screen that differ from the frame on the left. As a consequence, less data has to be stored on disc for
the second frame and it takes less time to draw. This in turn allows larger frame sizes and frame
rates, giving better-quality movies.
Figure 2 Example of Frame Differencing
When you're using the Apple Compact Video compressor, it's a good idea to create movies in which
not much changes from one frame to the next, since frame differencing is one way the compressor
achieves lower data transfer rates. Here are some things to keep in mind to get the most benefit from
- When possible, use source material with constant backgrounds and solid colors --
especially all-black and all-white areas -- to reduce the difference between frames.
- When possible, use videos of "talking heads." These are great candidates for frame
differencing, since typically only the lips and head move.
- Avoid videos with lots of panning and zooming or with complex backgrounds.
These effects increase the difference between frames and thus decrease the
possibility of compression gains.
- Avoid source material with a lot of video noise, as this increases the difference
Frames can't be differenced indefinitely, however. At regular intervals akey frame--
a frame that refreshes the entire movie area, not just the pixels that differ from the previous frame --
is inserted. You can adjust the interval to achieve the tradeoff between data rate and movie quality
that you desire. Normally, you should have one key frame per second. The more you put in, the
higher the data rate and the better your movie looks but the less compression gain you get from
SOUND SAMPLING RATE
The standard Macintosh sound rate is 22.254 kHz, so you'll get the highest-quality audio by
sampling to this rate. The Sound Manager is also more efficient at this native rate, so the movie will
play back better. However, because your data transfer rate off CD-ROM is limited, using higher
audio rates will decrease the quality of the video. The same constraint applies to stereo and 16-bit
sound, both of which are supported by QuickTime: these formats eat up more bandwidth, so the
video quality may suffer.
If your source is mostly people talking and not music, you can record audio at the alternate sampling
rate of 11.127 kHz. In many cases this will make your video look sharper and give acceptable sound
With most audio sampling hardware, it's best to set the hardware at the sampling rate you desire,
since the hardware will often do filtering to avoid aliasing artifacts. One exception is the
MacRecorder digitizer (from MacroMedia), which doesn't filter at 11.127 kHz. If possible, you
should record at 22.254 kHz on a MacRecorder and downsample the audio to 11.127 kHz when you
compress the movie. One other thing to keep in mind when using a MacRecorder to digitize audio at 22.254 kHz is that
its sample rate tends to drift away from the standard Macintosh rate, so you should always resample it
to 22.254 kHz or 11.127 kHz when you compress the movie.
TIPS ON CAPTURING
The QuickTime 1.5 Developer CD
and this issue's CD contain two utilities for capturing video:
GrabGuy and the HyperCard Movie Making Stack. GrabGuy is an application that does a multipass
grab off a controllable VCR like the Sony µMatic, giving frame-accurate recording. HyperCard and
the Movie Making Stack enable you to get frame-accurate grabs off controllable Pioneer laser discs.
Most video cards also come with software that enables you to grab raw video directly to RAM or
To get the highest possible quality when you're capturing the source material, you should do three
- Start with a clean source.
- Adjust black, white, brightness, and contrast levels.
- Grab at a larger size than you need and scale down when you compress.
I'll discuss these tips one at a time.
START WITH A CLEAN SOURCE
The less video noise, the better compression and the more benefit from frame differencing you'll get,
so you should digitize from the cleanest, highest-quality video source possible. The most common
video formats, in decreasing order of quality, are BetaCam, µMatic/S-VHS/Hi8, laser disc, and VHS.
Beware of tapes that have been duplicated many times or played a lot -- they can be very noisy. If
your digitizing card supports S-Video inputs, use them if you can, as
S-Video delivers better quality than composite video.
Many digitizing cards support one or more settings with regard to black level, white level, brightness,
and contrast. To enhance compression gains from frame differencing, you should adjust the black
level of your card so that black areas in your source digitize as truly black pixels. A frame with truly
black pixels differs much less from the original than the same frame with noisy black pixels. Thus,
refreshing the screen with the noisy frame takes more data than refreshing the screen with the clean
frame. Often what looks like black is quite noisy, so you should experiment with your video card.
The same rule applies to white levels.
GRAB AT A LARGER SIZE THAN YOU NEED
Because many video cards do a poor job of scaling down frames when they grab, it's best to capture
at a large size and let QuickDraw do a filtered scale when you compress. If you're using GrabGuy
you don't need to worry, because it will grab 320 x 240 fields and use QuickDraw to scale them
down. If you're grabbing from laser disc, grab frames at 640 x 480 and scale them down at
To save disc space, you should grab using JPEG compression set to the highest quality instead of
grabbing raw frames. Most of the quality you might lose this way isn't used by the Apple Compact
Video compressor anyway, so this won't reduce the quality of your final movie very much. A number
of video cards now support hardware JPEG compression, which makes this even easier.
TIPS ON COMPRESSING
The MovieShop utility on this issue's CD is indispensable for compressing QuickTime movies. After
an admonition to edit before compressing, I'll take you through the steps involved in using
MovieShop to create CD-playable movies. My emphasis here is more on what to do than why you're doing it.
If you're curious about the reasons you go through the steps you do, refer to the MovieShop
documentation on the CD.
EDIT BEFORE COMPRESSING
To achieve the best possible playback performance with the smallest amount of memory, you should
completely edit your movie in raw form before compressing with MovieShop. By the same token, it's
not a good idea to compress the movie in pieces and then cut and paste the pieces together to form
the final movie, as this will require extra input/output buffers and may cause QuickTime to run out
of memory. If this happens, movie playback will slow down considerably.
Here's why: Movies are composed of tracks that typically contain video and sound data. When you
capture a movie, the track data is often stored sequentially in the file, resulting in a file layout like the
first one shown in Figure 3. To play this movie, QuickTime must allocate four large buffers -- one
for each track -- and seek between tracks during playback. This can cause miserable playback
performance off CD-ROM drives, which typically have very slow seek times (we're talking hundreds
Figure 3 Ways of Storing Video and Sound Track Data in a Movie
To solve this problem, QuickTime enables you to interleave the video and sound tracks, resulting in
a file layout like the second one shown in Figure 3. Because the sound and video track data are now
close to each other in the file, seeking is minimized and only two buffers are needed for data transfer.
This is often the file layout you get when you paste two movies together.
The most efficient layout is to append the second video/sound sequence onto the first, as illustrated
by the third file layout in Figure 3, so that only one buffer is needed and playback is optimized.
When you compress with MovieShop, it automatically merges all the video and sound tracks of a
movie into a single video and sound track, thus giving you the most efficient layout.
SET THE DATA RATE
After importing a movie to compress, the first thing you do in MovieShop is to set the data rate. (See
Figure 4.) To play off CD, a movie must be compressed to deliver a consistent data rate of 100
kilobytes/second or less, the effective data transfer rate of the first generation of CD-ROM drives.
Some of the newer CD-ROM drives can now deliver twice this data rate, but you'll probably want to
make your movie at 100 kilobytes/second anyway, for backward compatibility. So a data rate of 100 isusually best for CD playback, while a range from 90 to 105 kilobytes/second will usually produce
Figure 4 Setting the Data Rate in MovieShop
SET THE VIDEO SETTINGS
Next you indicate your preferences relating to compression method, colors, and key frame spacing.
To do so, choose Video from the Preferences menu. The dialog box shown in Figure 5 will be
displayed. This is where you choose Apple Compact Video as your compression method. The Apple
Compact Video compressor has been optimized for CD playback and has a built-in data rate
constraining algorithm to give consistent playback from CD. Computationally, it's a highly
asymmetric algorithm, taking about an hour to compress a minute of video. The results are worth it,
MovieShop tries to limit the data rate of a movie by adjusting the amount of frame differencing
(motion quality) and lowering the compression quality (spatial quality). However, since the Apple
Compact Video compressor determines motion and spatial quality internally, you should turn these
settings off by entering the numbers shown in Figure 5. (For details about what these magic numbers
mean, see the MovieShop documentation.) That way, Apple Compact Video will always make the
right choices for the data rate you've chosen.
If your source material is in color, choose "Millions of colors"; if it's in black and white, choose "256
grays." "Use previous compressed video" should be checked for most video sources; however, if
you're compressing raw animations or composite movies with a constant background, uncheck this
setting to get more benefit from frame differencing. MovieShop will then use the last uncompressed
frame instead of the last compressed frame as the basis for frame differencing.
The key frame setting should be related to the frame rate of your video. As I mentioned earlier, you
should normally have one key frame per second, although in some cases you might want to have
fewer than one per second to lower the data rate (which will, however, decrease quality as well).
SET THE SOUND SETTINGS
To change sound settings, choose Sound from the Preferences menu. Selecting
22 kHz will ensure that your movie uses the standard Macintosh rate of 22.254 kHz. As discussed
earlier, if you're not concerned about audio quality you should probably resample to 11 kHz as a
tradeoff for sharper video. The Video to Sound setting lets you set how far ahead in seconds the
audio is interleaved on the file from the video on the disc. This setting should normally be at 1.90,
but if you find that CD playback is choppy or the audio portion breaks up, try lowering this number
to 1.5. "Interleaved sound" should always be checked so that the audio and video are interleaved as
explained earlier for smooth CD playback.
SCALE AND CROP THE MOVIE
Now you need to crop the movie, since there's often tape noise and jitter on the edges of the video
frames. At the same time you can scale the movie to ensure that the frame size is a multiple of 4
pixels in each dimension. (Recall that due to its algorithm, the Apple Compact Video compressor
functions optimally if this is so.) To change cropping settings and scale the movie, choose Cropping
from the Preferences menu. In Figure 6, the movie is being cropped by 2 pixels on all sides to
eliminate noisy edges and make the output dimension values multiples of 4.
Figure 6 MovieShop's Cropping Dialog
DISABLE EXTRA COMPRESSION METHODS
MovieShop can apply a large number of techniques when compressing a movie to get the data rate
you specify. These techniques are used by the Apple Compact Video compressor, but since the
compressor itself takes care of all data rate limiting, all of these methods should be turned off. To
turn them off, choose Methods from the Preferences menu. Then drag item 18 to the second
position in the list of methods, as shown in Figure 7.
Figure 7MovieShop's Methods Dialog
The version of MovieShop on the CD (v. 1.0c2) has a bug that causes it to use the settings for
methods 2, 3, 4, and 5 even when they're below item 18. If you're working with that version, you
should additionally set those methods to the following values to really disable them:
2. Forced Key frame -- 255
3. Natural Key frame -- 1
4. Natural Key frame -- 200
5. Drop duplicate frame -- 255
Again, for details about what these magic numbers mean, see the MovieShop documentation.
Once you've set up MovieShop this way, click "Make the movie," choose an output file, and sit back
and watch the show. It can be a long wait, but the results will be worth it.
TIPS ON PLAYBACK
If you're developing an application to play QuickTime movies, there are four things you can do to
make movie playback everything you'd hoped it would be:
- Optimize the movie's screen position and depth.
- Avoid clipping any portion of the movie.
- Hide the movie controller.
- Don't call WaitNextEvent as often.
OPTIMIZE SCREEN POSITION AND DEPTH
The position of the movie on the screen can affect playback performance. For greatest efficiency, the
left edge of the movie should be aligned to a long-word boundary in video memory. A new function
in QuickTime 1.5 called AlignWindow moves a window to the optimal screen location for movie
The screen depth also affects playback. If the screen is set to millions of colors, the movie will play
back more slowly than at 256 colors, because there are more bytes to move to the screen every frame.
The Apple Compact Video compressor is optimized for thousands of colors (16 bits/pixel), so you'll
get the best performance and quality at that depth.
If any portion of the movie is clipped, playback performance will be substantially decreased because
QuickTime will have to do a lot more work to draw the frames. Try to avoid overlapping windows
and drawing to multiple screens. Be aware of the menu bar and the rounded corners on the edges of
the Macintosh screen. Set the clipping region correctly for the movie.
HIDE THE MOVIE CONTROLLER
Displaying and updating the movie controller can cause the movie to play back more slowly,
especially on low-end machines and for shorter movies, so you may want to hide it to achieve better
playback. Still, it's desirable to have an option to show/hide the controller in your human interface.
You might consider using the badge option for the movie controller to achieve this.
DON'T CALL WAITNEXTEVENT AS OFTEN
Since QuickTime performs all its drawing operations at main event loop time, the more often you
call MoviesTask the better movie playback you'll get. However, most applications call
WaitNextEvent once every event loop, which can go away for a fairly long time under System 7,
effectively reducing the number of times MoviesTask gets called each second. To improve this,
simply call WaitNextEvent only once a second or so while movies are playing. This will allow
background tasks time to run but won't interfere with foreground event processing.
SEE YOU AT THE MOVIES!
Now you know a little bit more about making and playing movies than you did before you sat down
with this article. You understand what the tradeoffs are in making CD-playable movies: to get
smooth playback, you may need to limit the frame size and rate, minimize the differences between
frames, and trade off audio for video quality. You know that to get the best possible quality when you
capture, you need to start with a clean source, adjust levels, and grab at a larger size than you need.
You know how to use MovieShop with the Apple Compact Video compressor to compress your
movie. And you know that to get the best possible playback, you need to optimize the movie's screen
position and depth, avoid clipping any portion of the movie, hide the movie controller, and not call
WaitNextEvent as often.
In a nutshell, if you want your movie to play well on a Macintosh LC II with an AppleCD SC and
thousands of colors, you should use the following setup:
- Frame size: 240 x 180
- Frame rate: 12 fps
- Sound rate: 11.127 kHz
- Data rate: 100 kilobytes/second
- Compressor: Apple Compact Video
- Key frame: every 12 frames
With this knowledge, you can really put this technology to work and produce a QuickTime movie
with the best of them. While you may not become the next Steven Spielberg, you can at least see
your name in lights (check out theQuickTime 1.5 Developer CDAbout Box movies to see the
QuickTime team in action). See you at the movies!
KIP OLSON lives by the motto "I never met an avalanche I didn't like" and spends his winters skiing the extreme in places
like Chamonix, Jackson Hole, and Hoboken. When not cornice surfing, he's been known to cast a #12 Adams to a rising
brookie and score a perfect 18 at putt-putt golf. He works for Apple in his spare time. *
For more information on QuickTime 1.5, your best source is the QuickTime Developer's Kit v. 1.5, available from APDA.
This kit includes the complete printed documentation for QuickTime 1.5 as well as the QuickTime 1.5 Developer CD , which
is full of example code, movies, and utilities. The new Inside Macintosh: QuickTime and Inside Macintosh: QuickTime
Components together provide a complete description of QuickTime 1.5. *
For more on capturing, see the article "Video Digitizing Under QuickTime" in this issue. *
For more information on AlignWindow and other alignment routines, see the Image Compression Manager chapter of the
QuickTime 1.5 Developer's Kit documentation, and Inside Macintosh: QuickTime .*
THANKS TO OUR TECHNICAL REVIEWERS Dean Blackketter, Bill Guschwan, Peter Hoddie, Eric Hoffert *