How does SMPTE work?
How does SMPTE work?
Essentially each frame of SMPTE time code is comprised of a string of 80 bits. This string of bits is known as a SMPTE word, but it is more like a sentence because it tells the slave devices all the necessary information about a frame.
Each SMPTE word is comprised of 80 bits:
1. Time Code Address Data (30 bits)
2. User Bits (32 bits)
3. Status Flag Bits (2 bits)
4. Sync Word (16 bits)
1. Time Code Address Data
- The method of numbering time code consists of 8 digits, which are laid out in 4 pairs (00:00:00:00), and which represent 'hours', 'minutes', 'seconds', and 'frames', respectively.
- Just like in a digital clock, the hours are counted from 00 (midnight) through 23; minutes and seconds are counted from 00 through 59; and frames are counted from 00 up to 29, depending on the specific counting method being used
2. User Bits
- These are not standardized
- They usually provide scene and take information, dates, and/or camera and roll information
- The only limitation for the use of these bits comes from the equipment being used to imprint them
3. Status Flag Bits
- Out of six allotted status flag bits, only two have been established: 'The Color Frame Bit', and 'the Drop Frame Bit'
- Color framing is a characteristic particular to NTSC video, where an alternating electrical sequence changes from A to B with every frame of video. Time code can be synchronized to this sequence so that even frames go with "A" frames and odd frames go with "B" frames
- When the drop frame flag is set to 1 the time code reader will adopt a drop-frame counting method
4. Sync Word
- It is a burst of 16 bits that comes at the end of each frame used to derive speed and phase information
- The first bit is always a binary 0 and the last bit is always a binary 1. These are called 'direction sense bits'. They allow the time code reader to read the signal in either direction
- These bits never change
The method used to encode a SMPTE word is called 'Bi-phase Encoding': a time code signal reverses polarity halfway through a bit to represent a 1 and leaves the polarity unchanged to represent a 0.
< Copyright ⓒ The Music Telegraph :: Prohibit reprinting and redistribution >
SMPTE Time Code Related Articles
Audio Engineering (popular articles)