Getting started with video collection and processing (bt656 Introduction)

The bt.656 standard is rarely involved in analog signal acquisition. Because common analog video signal acquisition chips support the output of digital signals of bt.656, what is the format of bt.656?
This article mainly introduces Standard 8bit bt656 () YCbCr sdtv (SD) Digital Video Signal FormatIt is mainly used for beginners who are just getting started with simulated video acquisition.

1. Frame Concept)

A video sequence consists of N frames. There are generally two scanning methods for Image Collection, one is progressive scanning ), one is interlaced scanning ). For a line-by-line scan, each frame usually has two fields.
Field), a bottom field ). Assume that a frame of image is 720 rows.IncludeAll the even rows, while the base fieldIncludeAll the odd rows.

2. Field) 

Note that the top and bottom fields mentioned above use the word "include", rather than the latter, because in the bt.656 standard, a field consists of three parts:

Field = first vertical blanking + active video +
Second vertical blanking)

For the top field, the valid data row is all the even rows of a frame image. For the bottom field, the valid data row is all the odd rows of a frame image. The number of blank rows in the top field and the bottom field is also different.Standard 8bit bt656)Sdtv (SD)
For a video, the format of an image is defined as follows:

As you can know, for the PAL system, each frame has 625 rows. Among them, there are 288 rows of valid data in the top field, 288 rows of valid data in the bottom field, and the remaining rows are vertical hidden signals. Why is the number of rows 288? Because the resolution of PAL sdtv or D1 is 720*576, that is, one frame has 576 rows, so one field has 288 rows.

We can also know that the starting behavior of valid data in the top field is 23rd rows, and that of valid data in the bottom field is 335th rows.

In addition, the F field indicates the parity field, and the V field indicates whether the vertical blanking signal is used.

3. Lines)

The following describes the composition of each row. A Row consists of four parts:

Line = end code (EAV) + horizontal vertical blanking) + start code (SAV) + active video)

Shows the typical data structure of a row:

The start code (SAV) and end code (EAV) indicate the beginning and end of a line. They also contain other important information, which will be discussed later.

Why is the horizontal elimination of 280 bytes? I haven't figured out yet. I don't know if it's a standard definition.

Why is the valid data in a row 1440 bytes? Because the resolution of PAL sdtv or D1 is 720*576, that is, a row has 720 valid points. Because a colored image is collected, one line is based on the brightness information (y) and the color difference information (cbcr). Because the format is ycbcr422, there are 720 columns of Y and 720 columns of cbcr in a row, the number of valid bytes for a row is 720x2 = 1440 bytes.

4. EAV and Sav

Both EAV and Sav are 4 bytes (bytes). As you can see from the figure above, Sav is followed by valid video data. What is the format of EAV and sav?

The format of the four bytes of EAV and Sav is as follows (represented in hexadecimal notation below ):

Ff 00 00 XY

Among them, the first three bytes are fixed and must be ff 00 00, while the first 4th bytes (xy) are determined based on the field and blanking information. Their 8 bit meanings are as follows: 1 f v h P3 P2 P1 P0

Where, F: indicates the field information. When the top field is transmitted, It is 0, and when the bottom field is 1.

V: indicates the amount of information that is hidden. The value is 1 when the data is transmitted, and the value is 0 when the valid video data is transmitted.

H: Mark EAV or Sav, sav as 0, and EAV as 1

P0 ~ To protect bits, P3 depends on F, H, and V and performs verification. The calculation method is as follows:

5. Summary

This article only describes some of the most basic concepts in the bt.656 format. Many video capture chips will be involved in this aspect. If you have any questions, please leave a message for discussion. Finally, I would like to thank a brief introduction to digital video for its introduction to bt.656, which is very well written. I basically learned about bt656 and wrote this article based on it, if you are interested, take a look.