? What are their advantages and disadvantages?

Each frame of an image is scanned by an electron beam row after row. This scan method is called row-by-row scan. To obtain a stable row-by-row scan image, each frame of the image must scan an integer row. For example, a frame of image is composed of 625 consecutive lines, scanning 50 frames per second, that is, the frame Scan Frequency is 50 frames per second, or written as 50Hz, the row Scan Frequency is 31.25 kHz.

The line-by-line scanning method enables high requirements on the signal spectrum and the channel bandwidth used to transmit the signal. TV experts have come up with a clever way to divide a 625-line image into two scans. The first one is called an odd number, scan only the odd number of rows (1, 3, 5 ,... Rows), while the second field (even fields) only scans the even rows of the 625 rows (scanning 2, 4, 6 ,... The number of rows scanned by the original frame through two scans. For An interlace scan of 625 rows for each frame of image, each frame of image is scanned in two places, each of which only scans 312.5 rows, and only needs to scan 25 frames of image per second, therefore, a total of 50 fields are scanned per second (25 for each odd and even fields). That is, the frame rate is 25Hz, the field frequency is 50Hz, and the row Scan Frequency is 15.625 kHz.

The row-based Scan Frequency of the line-by-line scan is half that of the line-by-line scan. Therefore, the spectrum of the TV signal and the channel bandwidth used to transmit the signal are also half that of the line-by-line scan. In this way, the channel utilization rate is doubled when the image quality is not much reduced after the interlace scan is adopted. Due to the decrease in channel bandwidth, the complexity and cost of systems and equipment are also reduced, which is why the early television systems in the world were all scanned by the line.

However, the interlace scan will also bring many disadvantages, such as the flickering effect between lines, parallel phenomenon and vertical edge aliasing. Since the development of digital TV, in order to obtain high-quality image quality, line-by-line scanning has become the preferred solution for digital TV scanning.