Your lesson of the day
Why video on a television is 29.97 FPS instead of 30 FPS
Drop Frame Timecode
Drop frame timecode dates to a compromise invented when color NTSC video was invented. Basically, the NTSC re-designers wanted to retain compatibility with existing monochrome TVs. Unfortunately, the 3.58 MHz (actually 315/88 MHz = 3579545.45 Hz) color subcarrier would absorb common-phase noise from the harmonics of the line scan frequency. Rather than adjust the audio or chroma subcarriers, they adjusted everything else, including the frame rate, which was set to 30/1.001 Hz.
This meant that an "hour of timecode" at a nominal frame rate of 30 frame/s was longer than an hour of wall-clock time by 3.59 seconds, leading to an error of almost a minute and a half over a day. This caused people to make unnecessary mistakes in the studio.
To correct this, drop frame SMPTE timecode drops frame numbers 0 and 1 of the first second of every minute, and includes them when the number of minutes is divisible by ten. This almost perfectly compensates for the difference in rate, leaving a residual timing error of roughly 86.4 milliseconds per day, an error of only 1.0 ppm. Note: only timecode frame numbers are dropped. Video frames continue in sequence.
Drop-frame timecode is used only in systems running at a frame rate of 30/1.001 Hz.
Colour framing and timecode
A colour framing bit is often used to indicate field 1 of the colour frame, so that editing equipment can make sure to edit only on appropriate field boundaries in order to prevent picture corruption.
Studio operations and master clocks
In studio operations, longitudinal timecode is generated by the studio master sync generator, and distributed from a central point. Central sync generators usually derive their timing from an atomic clock, either using network time, or GPS. Studios usually maintain two or three clocks, and automatically switch over if one fails.
A recent development is to mount small GPS-synchronized SMPTE timecode generators on each camera, which eliminates the distribution network for portable set-ups and shooting on location.
Longitudinal SMPTE timecode is widely used to synchronise music. The frame rate used for audio sync is arbitrary, and can be chosen to suit the needs of the production. A frame rate of 24 frame/s is popular because it reduces synchronisation problems with 48 kHz digital audio. SMPTE timecodes are often time code not specified by SMPTE.
Drop Frame Timecode
Drop frame timecode dates to a compromise invented when color NTSC video was invented. Basically, the NTSC re-designers wanted to retain compatibility with existing monochrome TVs. Unfortunately, the 3.58 MHz (actually 315/88 MHz = 3579545.45 Hz) color subcarrier would absorb common-phase noise from the harmonics of the line scan frequency. Rather than adjust the audio or chroma subcarriers, they adjusted everything else, including the frame rate, which was set to 30/1.001 Hz.
This meant that an "hour of timecode" at a nominal frame rate of 30 frame/s was longer than an hour of wall-clock time by 3.59 seconds, leading to an error of almost a minute and a half over a day. This caused people to make unnecessary mistakes in the studio.
To correct this, drop frame SMPTE timecode drops frame numbers 0 and 1 of the first second of every minute, and includes them when the number of minutes is divisible by ten. This almost perfectly compensates for the difference in rate, leaving a residual timing error of roughly 86.4 milliseconds per day, an error of only 1.0 ppm. Note: only timecode frame numbers are dropped. Video frames continue in sequence.
Drop-frame timecode is used only in systems running at a frame rate of 30/1.001 Hz.
Colour framing and timecode
A colour framing bit is often used to indicate field 1 of the colour frame, so that editing equipment can make sure to edit only on appropriate field boundaries in order to prevent picture corruption.
Studio operations and master clocks
In studio operations, longitudinal timecode is generated by the studio master sync generator, and distributed from a central point. Central sync generators usually derive their timing from an atomic clock, either using network time, or GPS. Studios usually maintain two or three clocks, and automatically switch over if one fails.
A recent development is to mount small GPS-synchronized SMPTE timecode generators on each camera, which eliminates the distribution network for portable set-ups and shooting on location.
Longitudinal SMPTE timecode is widely used to synchronise music. The frame rate used for audio sync is arbitrary, and can be chosen to suit the needs of the production. A frame rate of 24 frame/s is popular because it reduces synchronisation problems with 48 kHz digital audio. SMPTE timecodes are often time code not specified by SMPTE.