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Setting and Changing the Master Clock

This section describes how to set and change the master clock associated with a movie time base.

You can obtain the clock component associated with a given media by calling MediaGetClock.

In this section:

The ChooseMovieClock Function
Associated Components: The Video Output Component Clock
Associated Components: Sound Output Component
Using the Sound Output Component Associated With the Video Output Component
Switching Back to the Default Sound Output Component
Choosing the Clock

The ChooseMovieClock Function

ChooseMovieClock assigns a default clock to a movie. If you have changed a movie’s master clock with SetMovieMasterClock, or would like to reset a movie’s clock, use the ChooseMovieClock function.

Associated Components: The Video Output Component Clock

A Video Output Clock is the clock component associated with a specific video output component. This clock allows the time base used by a QuickTime movie to be driven by a specific output hardware device’s clock, in order to synchronize video and sound when the output device is in use.

An application can ask for the clock component associated with the video output component and use this clock as a movie’s master clock.

When using a Video Output Component, you can get an Instance of the Clock Component associated with the Video Output Component by calling QTVideoOutputGetClock.

Once you have this Clock Instance, it can be associated with a Movie by calling SetMovieMasterClock. Because a change to the display mode could affect a clock component, your application should call QTVideoOutputGetClock only between calls to the QTVideoOutputBegin and QTVideoOutputEnd functions.

When you want to reset the movie master clock back to the default clock, use ChooseMovieClock.

If you were previously using SetMovieMasterClock to reset a movies clock to the default clock, you should change to the new ChooseMovieClock method:

ChooseMovieClock(myMovie, 0);

Associated Components: Sound Output Component

A Sound Output Component is a software module that identifies, controls, and plays audio on a specific hardware device. Video output components, in addition to having a clock component, can have a Sound Output Component associated with them.

Developers can change the Sound Output Component used by a Media Handler by calling MediaSetSoundOutputComponent. This allows choosing between using the audio device associated with a video output device, another sound output device installed on the system, or the default sound output device.

Using the Sound Output Component Associated With the Video Output Component

To find the Sound Output Components associated with a Video Output Component use QTVideoOutputGetIndSoundOutput. Once the component is retrieved, call MediaSetSoundOutputComponent to set the sound output component for a media handler.

Component theSoundOut = 0;

ComponentInstance theVOutClock = NULL;

UnsignedFixed theSupportedAudioRate, myWantedAudioRate = eAudioRate48khz;

    ...

// Does this Video Output Component have a

// Sound Output Component associated with it?

if (ComponentFunctionImplemented(theInstance,

        kQTVideoOutputGetIndSoundOutputSelect)) {

    // Get the first sound output component associated

    // with the video output component

    err = QTVideoOutputGetIndSoundOutput(inVOComponentInstance,

                 1, &theSoundOut);

    if (err || 0 == theSoundOut) goto bail;

    // Not all sound output components support all sample

    // rates, use GetSoundOutputInfo with the siSampleRateAvailable

    // selector and figure it out

    theSupportedAudioRate = MyChooseAudioRate(myWantedAudioRate,

                                                theSoundOut);

    // Set the sample rate for the audio output

    err = SoundComponentSetInfo((ComponentInstance)theSoundOut,

                     NULL, siSampleRate, (void *)theSupportedAudioRate);

    if (err) goto bail;

    // For each audio tracks media set the sound output component

    for (i = 0;i < theNumberAudioTracks; i++) {

        err = MediaSetSoundOutputComponent(inAudioMediaHandlers[i],

                                                        theSoundOut);

        if (err) goto bail;

    }

}

// Use the Video Output Clock as the Master Clock

// Set up the video output clock after sound or it

// gets set back to the default clock

if (ComponentFunctionImplemented(inVOComponentInstance,

                                  kQTVideoOutputGetClockSelect)) {

    err = QTVideoOutputGetClock(inVOComponentInstance, &theVOutClock);

    if (err || NULL == theVOutClock) goto bail;

    SetMovieMasterClock(inMovie, (Component)theVOutClock, NULL);

}

Switching Back to the Default Sound Output Component

To switch back to the Default Sound Output Component, use MediaSetSoundOutputComponent and pass in NULL for the Component parameter.

// Set the Sound Output back to the Default Sound Output

for (i = 0;i < theNumberAudioTracks; i++) {

    err = MediaSetSoundOutputComponent(theAudioMediaHandler[i], NULL);

    if (err) goto bail;

}

// Switch back to the default clock

ChooseMovieClock(myMovie, 0);

Choosing the Clock

As mentioned in the section Associated Components: Sound Output Component, choosing a Sound Output Component will reset the master clock. Therefore, once you choose the Sound Output Component, you should then set up the movie’s master clock. You can either use the Video Output Clock (a logical choice when using a Video Output Component), or you could chose the default clock to provide audio and video sync for a movie.

When using the Video Output Clock, be sure to set the movie’s clock back to the default clock before calling QTVideoOutputEnd.

If you want to use the Video Output Clock, call SetMovieMasterClock and pass in the Video Output Clock Instance.

// Use the Video Output Clock as the Master Clock

// Set up the video output clock after sound or it

// gets set back to the default clock

if (ComponentFunctionImplemented(theVOComponentInstance,

                                  kQTVideoOutputGetClockSelect)) {

    err = QTVideoOutputGetClock(gVOComponentInstance, &theVOutClock);

    if (err || NULL == theVOutClock) goto bail;

    SetMovieMasterClock(myMovie, (Component)theVOutClock, NULL);

}

When choosing or switching to the Default Clock, use ChooseMovieClock.

// Use the default clock

ChooseMovieClock(myMovie, 0);

Remember, not all sound devices have clocks, and not all video output components have clock components associated with them, so be sure to check.

SetMovieMasterClock and ChooseMovieClock will cancel each other out; the last API called is the one that sets the clock.

MediaSetSoundOutputComponent can change the movie master clock.

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