Question

I basically want to press a button, that starts timecode at 30fps. (called every 1/30th of a second). I want the timecode to be referenced to the clock built in to the computer. I could easily get the current time in HH:mm:ss using NSDate, but I need the counter to start from zero and implement frames- formatted like HH:mm:ss:ff

Thoughts?

Answer 1

Use a CVDisplayLink to generate a pulse with the video card’s accuracy, this will be much more accurate than an NSTimer or a dispatch queue. CoreMedia/CoreVideo also talks SMPTE natively.

CVReturn MyDisplayCallback(CVDisplayLinkRef displayLink,
  const CVTimeStamp *inNow,
  const CVTimeStamp *inOutputTime,
  CVOptionFlags flagsIn,
  CVOptionFlags *flagsOut,
  void *displayLinkContext) {

    CVSMPTETime timecodeNow = inNow->smpteTime; // it's that easy!

    DoStuffWith(timecodeNow); // you might have to modulo this run a bit if the display framerate is greater than 30fps.

    return kCVReturnSuccess;
}

CVDisplayLinkRef _link;
CVDisplayLinkCreateWithCGDisplay(CGMainDisplayID(),&_link);
CVDisplayLinkSetOutputCallback(_link, MyDisplayCallback, NULL);
CVDisplayLinkStart(_link);

EDIT: After playing with this a bit, I’ve noticed that the SMPTE fields from the displaylink aren’t getting filled out, but OTOH the host time is accurate. Just use:

inNow->videoTime / inNow->videoTimeScale;

to obtain the number of seconds uptime, and

inNow->videTime % inNow->videoTimeScale

to get the remainder.

Here’s as far as I got:

@implementation JHDLTAppDelegate

CVReturn MYCGCallback(CVDisplayLinkRef displayLink,
                      const CVTimeStamp *inNow,
                      const CVTimeStamp *inOutputTime,
                      CVOptionFlags flagsIn,
                      CVOptionFlags *flagsOut,
                      void *displayLinkContext) {


    dispatch_async(dispatch_get_main_queue(), ^{

        JHDLTAppDelegate *obj = (__bridge JHDLTAppDelegate *)displayLinkContext;

        uint64_t seconds = inNow->videoTime / inNow->videoTimeScale;
           [obj.outputView setStringValue:[NSString stringWithFormat:@"days: %llu/hours: %llu/seconds: %llu (%llu:%u)",
                                          seconds / (3600 * 24),
                                           seconds / 3600,
                                           seconds,
                                          inNow->videoTime, inNow->videoTimeScale]];


    });

    return kCVReturnSuccess;
}

- (void)applicationDidFinishLaunching:(NSNotification *)aNotification
{
    CVDisplayLinkCreateWithCGDisplay(CGMainDisplayID(), &_ref);
    CVDisplayLinkSetOutputCallback(_ref, MYCGCallback, (__bridge void *)self);
    CVDisplayLinkStart(_ref);
}

- (void)dealloc
{
    CVDisplayLinkStop(_ref);
    CVDisplayLinkRelease(_ref);
}

@end