Imagine All the Problems With Interlaced Versus Progressive Video

As if there weren’t already enough technical considerations for distributing video to multiple screens, Imagine Communications has directed attention to yet another thing to consider. It has to do with the difference between interlaced (i) video and progressive (p) video.

The terms interlaced and progressive refer to the scanning process used to create an image on a video display, says Ron Hranac, CT’s senior technology editor. "A conventional analog NTSC TV set reproduces the video using interlaced scanning, because the picture comprises a bunch of horizontal lines," says Hranac. "If you look closely at a TV screen on an older TV set, you can see those lines. In the world of analog NTSC, the total picture is made up of 525 lines, but the TV set displays perhaps 480 or so. When the TV set creates a video image, it first ‘draws’ 262-1/2 lines on the inside of the picture tube (or whatever the display is) – lines 1, 3, 5, 7, etc. Then it draws the second 262-1/2 lines in between the first 262-1/2 lines that it just finished drawing (lines 2, 4, 6, 8, etc.). Each group of 262-1/2 lines is called a field, and two fields are called a frame."

For progressive scanning, Hranac says, "Instead of drawing half of each frame’s lines at a time, progressive scanning draws them all at once starting at the top of the picture (lines 1, 2, 3, 4, 5…). If NTSC were progressive instead of interlaced, the TV set would draw all 525 lines sequentially rather than the odd lines first, followed by the even lines."

The vast majority of HD programming delivered to service providers is in 1080i format, and lots of people still have older TVs that only support interlaced.

However, PCs, laptops, tablets and smartphones all use progressive formats, without support for interlaced video display, according to Imagine. Further, the majority of TVs sold today natively support 1080p. These devices must deinterlace the video signal to achieve optimal resolution.

"The high-end TVs will have dedicated hardware that will take care of deinterlacing and upconvert to 1080p/60," said Imagine’s CTO and founder Ron Gutman. "PCs and other devices don’t have the silicon to do it all. They support only progressive. They can play interlaced, but it won’t look as good; you’ll see lines throughout the picture, blurriness. The quality, even in high bitrate, looks pretty bad."

Deinterlacing can be done with free or purchased off-the-shelf software. But Gutman said it requires a lot of pixel processing and often the results aren’t that great.

At the CableLabs Winter Conference last week, Imagine demonstrated video on an iPad with a traditional software-only multi-screen transcoder and compared it to an iPad with Imagine’s ICE Video Platform, a hybrid hardware/software solution that deinterlaces video. Both iPads displayed content from Starz’ Spartacus program. The demonstration was intended to show how much higher the quality of video is on an iPad when deinterlaced well.

"With the iPad, video quality is important; the distance of viewing is even bigger than a TV in your living room," said Gutman. "With the amount of view it takes from your eyes when you watch an iPad, you will see the deficiencies very quickly."

-Linda Hardesty