Video signal digitalization

In video digitalization, the selection of Brightness Signal Sampling frequency should be considered from the following four aspects:

1) First, the sampling theorem is satisfied, that is, the sampling frequency should be twice the video bandwidth. If the brightness signal bandwidth by is 6 MHz, FS> = 2b = 12 MHz

2) to ensure that the sampling structure is orthogonal, the sampling frequency fs should be an integer multiple of the row frequency FH, that is, FS = N * FH

3) in order to facilitate international communication, the selection of Brightness Signal Sampling frequency must also take into account different scanning formats internationally. the current Scanning formats mainly include pal and SECAM's 625 rows/50 rows and NTSC's 525 rows/60 rows. their row frequencies are 15625hz and 15734.265Hz, respectively. The minimum common multiples of these two line frequencies are about 2.25 MHz. That is to say, the sampling frequency should be an integer multiple of 2.25 MHz, that is, FS = m * MHz, where M is an integer.

4) the bit rate after encoding RB = FS * n, where N is the quantified bit rate. to reduce the bit rate, the closer the FS is to 2B, the better. in ITU-R601 recommendations, M = 6, the Brightness Signal Sampling Rate FS is 13.5mhz. for brightness signals of 625 rows/50 fields scan format, the number of sampling points in each row is 13.5*10exp6/15625 = 864. For brightness signals in the format of 525 rows/60 fields, the number of sampling points in each line is 13.5*10exp6/15734.265 = 858.

 

As for the origin of the value 15734.265 mentioned above, I found the relevant information as follows:

3-2 drop-down frequency:

The standards for film and television program production are all 24 p/s, while most of the TV playback signals are the same as the local AC frequency. For example, the 60Hz AC country is 30 p/s during the line scan, 50Hz alternating current (AC) is 25 p/s. For the United States, it is a line scan and 30 p.
To meet the needs of TV broadcasting, the 24p standard signal converts the first frame to three full screen interlace signals, and the second frame to two full screen interlace signals, with a loop at every two frames. This satisfies the requirement of 30 p for TV playback or 60 p for interlace broadcasting. This is the 3-2 drop-down frequency.

What is 23.976p?

There are two versions of 24 p for Program Production: 24 P for the film industry and 23.976 P for TV production.
23.976p after 3-2 drop-down transformation, the field frequency is 59.94Hz. However, due to habits, 23.976p is often called "24p ".

Why 23.976 P?

The field or frame rate of a TV is generally the same as that of a local power grid. When a black/white TV is broadcast in the United States, its field frequency is also 60Hz, which is the same as that of a power grid.

However, after NTSC color television broadcasting, the field frequency is changed to 59.94Hz, reducing by 1.

The color sub-carrier frequency of the original NTSC that changes the field frequency adopts the 1/2 line frequency intercept, in order to avoid interference between the color sub-carrier and the original compatible black and white, the brightness signal of the signal and the audio carrier must meet the following requirements for various frequencies:

1: The color sub-carrier is an integer multiple of 1/2 line frequencies (1/2 line frequencies)
2: The audio carrier is an integral multiple of the line frequency.

To meet the compatibility requirements of NTSC color signals and original black and white TV signals, the audio carrier frequency cannot be changed, or 4.5 MHz

Now let's take a look at the NTSC signal frequency selection method:

The line frequency is 4.5 MHz/286 = 15,734Hz, and the black/white TV is 15,750Hz, which is compatible
Frame Rate: 15734/525 = 29.97Hz; black/white TV: 30Hz; compatible
The field frequency is 29.97x2 = 59.94Hz, and the black/white TV is 60Hz, which is compatible
All frequencies decreased by 1‰ year on year (F/1.001)

In countries and regions where NTSC is used, in order to meet the requirements of conversion between standard definition and high definition TV signals, the High Definition TV adopts the same field frequency of 59.94Hz as NTSC, therefore, the correlation frequency is reduced by 1‰ compared with 60Hz,
The sampling frequency is reduced from 74.25 MHz to 74.25 MHz/1.001 = 74.176 MHz
The line frequency is reduced from 67.5 kHz to 67.5 kHz/1.001 = 67.433 kHz
Field frequency decreased from 60Hz to 60Hz/1.001 = 59.94Hz

This is the source of the 59.94hz field frequency signal. To put it simply, 60Hz is reduced by 1/1000.