How does SMPTE work?

The Music Telegraph | Text 2019/07/25 [14:33]

How does SMPTE work?

The Music Telegraph| 입력 : 2019/07/25 [14:33]

© Giphy



 

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.

 

 

SMPTE Word

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

 

 

Bi-phase Encoding

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.

 

 

 

 

 

 

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