… for Codecs & Media

Tip #744: What is Interlacing?

Larry Jordan – LarryJordan.com

Interlacing was needed due to limited bandwidth.

Even in today’s world of 4K and HDR, many HD productions still need to distribute interlaced footage. So, what is interlacing?

Interlacing is the process of time-shifting every other line of video so that the total bandwidth requirements for a video stream are, effectively, cut in half.

For example, in HD, first all the even numbered lines are displayed, then 1/2 the frame rate later, all the odd numbered lines are displayed. Each of these is called a “field.” The field rate is double the frame rate.

NOTE: HD is upper field first, DV (PAL or NTSC) is lower field first.

In the old days of NTSC and PAL this was done because the broadcast infrastructure couldn’t handle complete frames.

As broadcasters converted to HD at the end of the last century, they needed to make a choice; again due to limited bandwidth: They could either choose to broadcast a single 720 progressive frame, or an interlaced 1080 frame.

Some networks chose 720p because they were heavily into sports, which looks best in a progressive frame. Others chose interlaced, because their shows principally originated on film, which minimized interlaced artifact, which is illustrated in the screen shot.

As we move past HD into 4K, the bandwidth limitations fade away, which means that all frames are progressive.

EXTRA CREDIT

It is easy to shoot progressive and convert it to interlaced, with no significant loss in image quality. It is far harder to convert interlaced footage to progressive; and quality always suffers. Also, the web requires progressive media because interlacing looks terrible.

For this reason, it is best to shoot progressive, then convert to interlacing as needed for distribution.