Convert frames to NTSC Drop Frame Timecode

Question

I'm looking for a function that converts an integer value in frames to NTSC Drop Frame Timecode (hh:mm:ss.ff).

I'm using Delphi, but could be in any language.

Thanks

Answer 1

There is a well-known classical solution for that problem...

• the input frame-count nr is relative to the real framerate, which is allways 30000/1001 ~= 29.97 frames per second for NTSC
• the calculated result frame-nr is to the nominal framerate of 30fps (and will exhibit a +2frames jump at each full minute, unless the minute is dividable by 10

(of course you may use a smaller int type if you know your value range is limited)

const uint FRAMES_PER_10min = 10*60 * 30000/1001;
const uint FRAMES_PER_1min  =  1*60 * 30000/1001;
const uint DISCREPANCY      = (1*60 * 30) - FRAMES_PER_1min;


/** reverse the drop-frame calculation
 * @param  frameNr raw frame number in 30000/1001 = 29.97fps
 * @return frame number using NTSC drop-frame encoding, nominally 30fps
 */
int64_t
calculate_drop_frame_number (int64_t frameNr)
{
  // partition into 10 minute segments
  lldiv_t tenMinFrames = lldiv (frameNr, FRAMES_PER_10min);

  // ensure the drop-frame incidents happen at full minutes;
  // at start of each 10-minute segment *no* drop incident happens,
  // thus we need to correct discrepancy between nominal/real framerate once: 
  int64_t remainingMinutes = (tenMinFrames.rem - DISCREPANCY) / FRAMES_PER_1min;

  int64_t dropIncidents = (10-1) * tenMinFrames.quot + remainingMinutes;
  return frameNr + 2*dropIncidents;
}                 // perform "drop"

From the resulting "drop" frameNumber, you can calculate the components as usual, using the nominal 30fps framerate...

frames  =    frameNumber % 30
seconds =   (frameNumber / 30) % 60

and so on...

Answer 2

function FramesToNTSCDropFrameCode(Frames:Integer;FramesPerSecond:Double):string;
var
  iTH, iTM, iTS, iTF : word;
  MinCount, MFrameCount : word;
begin
  DivMod( Frames, Trunc(SecsPerMin * FramesPerSecond), MinCount, MFrameCount );
  DivMod( MinCount, MinsPerHour, iTH, iTM );
  DivMod( MFrameCount, Trunc(FramesPerSecond), ITS, ITF );
  Result := Format('%.2d:%.2d:%.2d.%.2d',[iTH,iTM,iTS,iTF]);
end;

You will need to copy the DivMod routine from the SysUtils unit, and also include the sysUtils unit in whatever implements this function.