Design Article
Deinterlacing with FPGA for HDTVs
Suhel Dhanani<br>Altera Corporation
7/15/2009 12:00 AM EDT
An interlaced video is a succession of 50/60 fields per second, where each field carries only half of the rows that are displayed in each frame of video. In some ways interlaced video was an elementary compression technique when older display technologies were based on cathode ray tubes (i.e. CRTs).
Today deinterlacing video is an important video processing function, as much of the video content is available in the legacy interlaced format and almost all of the newer displays -- LCD or Plasma -- require progressive video input. While deinterlacing is required in many systems, it is by nature complex and no deinterlacing algorithm can produce a perfect progressive image.
This article explores the different deinterlacing techniques and examines how FPGAs are increasingly being used for any sufficiently complex deinterlacing function. The article also examines the hardware tradeoffs when implementing different deinterlacing algorithms.
Deinterlacing Background
In interlaced video one frame of video is broken into two fields -- one field contains the even lines and one contains the odd lines. However, to display any interlaced video on an LCD or plasma display, the display must be deinterlaced. All newer displays are progressive in that each frame is comprised of a set of pixels (i.e. 1920 x 1080).
Figure 1 shows how these two fields contain the pixels in one frame. Also note that each field is recording pixel values that are separated in time.
Figure 1: Two interlaced fields contain one frame data
If it is assumed that there are 30 frames per second or 60 fields per second, then field 0 is at time 't' and field 1 is at time 't+1/60'. Since the fields are recorded at slightly different time intervals, the two fields cannot be combined to create a progressive frame for any video that has motion. The complexity associated with deinterlacing is due to the need to estimate and compensate for the potential motion in that 1/60th of a second.
Next: Basic Deinterlacing Techniques
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K1200LT Rider
7/23/2009 7:59 AM EDT
For the following text from the 5th paragraph:
> Deinterlacing Background
> In interlaced video one frame of video
> is broken into two fields ...
I get confused on how the words "interlacing" and "deinterlacing" are used. When you say you take video data from 2 fields and put it together, you are calling that deinterlacing. I say it is just the opposite. The definition of interlace is "to put together" which is what is being done to the data. I believe that the problem comes in because there are 2 separate contexts in which the term (de)interlace is used. The term "interlaced" in the 1st sentence in the 5th paragraph is referring to the video format, but later in the same paragraph it says that "... the display must be deinterlaced." I think it should say that "... the video data must be interlaced together to create a non-interlaced/deinterlaced format." In other words, the format changes to "deinterlaced" after an "interlacing function" rearranges the video data by *putting it together*. Format vs. verb/action. Am I backwards in my thinking? If so, I have named some of my video manipulation functions exactly opposite of what they should be. Even if I am "wrong," I'm not going to rename them because it would only lead to confusion.
- Brad
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Rick M
10/2/2009 9:48 AM EDT
Brad,
Interlace and De-interlace are common broadcast industry terminology for better or for worse. Analog TV (which no longer exists) broadcast content in interlaced mode - that is, two (2) fields, each at 1/30 sec, to form 1/60 full frames. They needed to do this due to restrictions with regard to RF channel bandwidth allocation. The other thing I think that you are missing is that there is a difference between fields and frames. Fields are only discussed with respected to "i" formats such as 1080i, 480i, etc. "i" or "interlaced" formats always require more than one "field" of data to complete a single "frame" or whole picture. "p" or "progressive" formats such as 1080p (current HD standard), or 720p, or 480p show "frames" at 1/60 sec. This was accomplished solely due to the fact that our new broadcast system (which is really 1080i not 1080p) can accommodate the required bandwidth because of digital compression and transmission techniques. Each "frame" is a whole picture. Therefore, when someone in broadcast or video talks about interlaced, they mean the old "i" format. To de-interlace, you are essentially changing it from the old "i" format to the newer "p" or "progressive" format. I hope that this helps.
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