Why Are 1080i@29.97Hz Video Displayed at Double the Frequency as 1080p@59.94 Hz?

If you’re delving into the world of digital video, you may have noticed that some videos encoded in 1080i at 29.97Hz (or fps, frames per second) are often rendered or converted to 1080p at 59.94Hz on modern displays. At first glance, this might seem puzzling, but it’s grounded in well-established technical principles related to interlacing, format compatibility, and how screens function. In this article, we’ll break it down step by step to explain why this happens.

Step 1: Understanding the Difference Between 1080i and 1080p

Let’s start by clarifying what these terms mean:

• 1080i: The "i" stands for "interlaced." A 1080i video has 1080 vertical lines of resolution, but each frame is split into two "fields." Each field contains half the lines (540 lines, alternating between odd and even lines). These two fields are displayed sequentially to form a complete image.
• 1080p: The "p" stands for "progressive." A 1080p video displays all 1080 lines in a single pass, without splitting them into fields, for each frame.

In the case of 1080i@29.97 Hz, this means 29.97 interlaced frames are produced per second, but each frame is built from two fields, delivered at a rate of 59.94 fields per second (29.97 × 2 = 59.94).

Step 2: The Legacy of Interlacing

Interlacing, as used in 1080i, is a technique inherited from the early days of analog television (e.g., the NTSC standard in the United States). Back then, cathode ray tube (CRT) TVs couldn’t display a full frame at a high frequency without noticeable flicker. By splitting the image into two fields (odd and even lines) and displaying them rapidly one after the other, engineers achieved an effective refresh rate of 59.94 Hz in the U.S., while saving bandwidth.

So, for a 1080i@29.97Hz video:

• Each second contains 29.97 frames.
• Each frame consists of two fields.
• These fields are delivered at 59.94Hz (59.94 fields per second).

Step 3: Modern Displays and Progressive Scanning

Today, most screens (LCD, LED, OLED) operate in progressive mode and don’t natively support interlaced display. When a TV or player receives a 1080i signal, it must convert it to a progressive format to display it properly. This process is called *deinterlacing*.

Deinterlacing recombines the two fields (odd and even lines) of an interlaced frame to create a full progressive 1080p image. Since the fields arrive at a rate of 59.94Hz, the deinterlacer can output 59.94 progressive frames per second (1080p@59.94Hz), effectively doubling the frame rate from the original 29.97Hz.

Step 4: Why Double the Frequency?

You might wonder: why not stick to 29.97Hz in 1080p after deinterlacing? The answer lies in fluidity and compatibility:

• Fluidity: The fields in 1080i are captured at slightly different moments (1/59.94th of a second apart). By treating them as separate frames and displaying them at 59.94Hz, this temporal information is preserved, making motion smoother—especially in fast-paced scenes like sports or action movies.
• Compatibility: Modern standards (like HDMI or display refresh rates) are often aligned with 59.94Hz or 60Hz to match the legacy NTSC standards. Displaying at 59.94Hz is thus a natural fit.

Step 5: The Process in Practice

Here’s what happens in practice:

1. A 1080i@29.97Hz video is received, with 59.94 fields per second.
2. The deinterlacer analyzes each pair of fields (one odd, one even) and combines them into a full 1080p frame.
3. Since the fields arrive at 59.94Hz, the result is a 1080p@59.94Hz video.

In some cases, if the video lacks fast motion, an algorithm might "merge" the fields to recreate a static image at 29.97Hz, but for maximum smoothness, the conversion to 59.94Hz is preferred.

Conclusion

In summary, videos encoded in 1080i@29.97Hz are displayed as 1080p@59.94Hz because modern screens convert interlaced fields into progressive frames, and the 59.94 Hz frequency matches the natural rhythm of the fields (two per frame). This process enhances smoothness and aligns with current standards. The next time you watch a TV broadcast or video encoded in 1080i, you’ll know exactly why your screen is showing 1080p at double the frequency!