Interlaced Video

隔行扫描视频 Dividing a scanned raster into two fields: one containing an alternating set of lines, and the other containing the remaining lines. The two fields are acquired and displayed sequentially, increasing the refresh rate without transmitting a complete frame each time; this is a key foundation of analog television and early digital television.

Field order is classified as top-field-first or bottom-field-first, indicating which set of scan lines appears first in time. Misinterpreting the field order can reverse the direction of motion and cause judder. In true interlaced capture, the two fields originate from different points in time and cannot simply be interlaced into a single frame; stationary areas can be merged, while moving areas require interpolation or motion compensation.

Interlaced formats are commonly denoted as 480i, 576i, and 1080i. There are industry-specific variations in how frame rates are denoted in these numbers; sometimes “1080i60” refers to approximately 60 fields, while technical interfaces may display approximately 30 frames. Complete records should explicitly state the field rate or frame rate, particularly distinguishing between 59.94 and 60. Films and progressive-scan programs can also be encoded as interlaced signals using segmented frames or pull-down techniques. Such video streams have an interlaced transmission structure but may be restored to a complete progressive-scan source. Determining this requires analyzing the temporal relationships and repetition patterns between fields, rather than simply reading a single interlaced flag.

Modern flat-panel displays operate in progressive mode and must deinterlace when playing back interlaced sources. The final result depends on the source type, field order, scaling, and algorithms used; the display’s progressive output does not alter the original interlaced structure of the file.