Is NTSC 24 fps? A Close Look at Frame Rate Standards

As someone who analyzes video footage and encodes digital videos on a regular basis, frame rates are a topic I spend a lot of time thinking about. In particular, there‘s often confusion around NTSC frame rate standards and how they relate to the 24 fps rate used for cinematic film. In this post, I‘ll give a thorough overview explaining NTSC and 24p formats and why they use different frame rates historically.

What is NTSC?

First, to understand NTSC frame rates, it helps to start with some background on the NTSC standards. NTSC stands for National Television System Committee and refers to the analog TV system that was adopted in North America and some other regions starting in the 1950s.

Some key technical notes about the NTSC standard:

• Frame Resolution – NTSC has a total of 525 scan lines per frame, 480 of which are visible. Combined with the 4:3 aspect ratio, this provided a frame resolution of ~720×480 pixels.

• Interlaced Scanning – NTSC uses interlaced video where alternate lines are drawn in each pass. This reduces flicker compared to progressive scanning.

• Color Encoding – The NTSC color space uses YIQ encoding which is converted from RGB video signals.

• Frame Rate – The NTSC frame rate is approximately 29.97 frames per second. Read on for more details on why this specific rate was chosen.

So in summary, NTSC established analog color TV standards in North America with a frame rate near 30 fps and a 525-line interlaced format.

Why 29.97 FPS for NTSC?

You may be wondering why NTSC didn‘t just settle on 30 frames per second? Initially, it did use 30 fps when the black and white NTSC standard was introduced in the 1940s. However, when color was added to the specification in 1953, the committee selected a frame rate of 29.97 fps rather than 30 fps.

There were a few key reasons behind this change:

• Preventing interference between the audio and color subcarrier signals in the broadcast signal. The color subcarrier frequency is 3.579545 MHz.

• Ensuring the frame rate is accurately derived from the AC power line frequency of 60 Hz. By dividing down to fractions of 60 Hz, the NTSC color subcarrier frequency becomes a simple multiple of the frame rate.

• Enabling TV sets to use less expensive components without sacrificing image quality.

So by selecting 29.97 fps, the NTSC standard could incorporate color while minimizing interference effects and keeping TV manufacturing costs down. This fractional frame rate has persisted into the digital era for consistency.

NTSC Frame Rates in the Digital Age

While 30 fps and 29.97 fps were the only options during the analog TV days, newer digital video standards have loosened the restrictions on frame rates. For example:

• ATSC standards for digital NTSC allow fixed frame rates of 23.976, 29.97 as well as 59.94 fps for high definition. No more fractional rates.

• SMPTE 259M – Can represent NTSC signals with any frame rate between 23.976 and 30 fps.

• SMPTE 292M – Supports 29.97, 59.94 and 60 fps for high definition.

So in the modern digital realm, NTSC-compatible signals don‘t have to adhere strictly to a 29.97 fps rate. Cool! That means no more worries about fractional frame rates.

Interlaced vs. Progressive Scanning

It‘s also worth comparing interlaced scanning used in NTSC with the newer progressive scanning technique.

Remember NTSC‘s 525 lines are divided into alternating odd and even lines that make up two separate fields.

• The advantages of interlacing include:

  • Smoother motion, less flicker
  • Allowed for higher resolutions on early TVs
  • Can display at full resolution with reduced bandwidth

• Drawbacks of interlacing:

  • Can produce combing artifacts in areas of movement
  • Not as sharp for static images
  • Requires deinterlacing for conversion to progressive formats

Progressive scanning became popular in the 90s and paints each frame sequentially line-by-line.

• Benefits of progressive scanning:

  • Eliminates interlace artifacts
  • Allows for simpler image processing
  • Works better for computer graphics
  • Easier to convert between resolutions

Modern TV standards like 1080p HD are purely progressive. However, interlacing is still used in many broadcast formats.

Film Frame Rates vs. NTSC

Now that we‘ve covered NTSC‘s specifications, how does its frame rate compare to actual film content? The standard frame rate for shooting theatrical films has been 24 fps since the late 1920s. But NTSC chose 29.97 fps for television. Why the discrepancy?

Here are some key differences between 24 fps film and 29.97 fps NTSC:

Motion Portrayal

• 24 fps provides a more cinematic look with natural motion blur
• 29.97 fps is smoother but can look less realistic for filmed content

Flicker Reduction

• 24 fps flicker less noticeable in a dark theater environment
• Interlaced NTSC helps reduce flicker on CRT televisions

Broadcast Bandwidth

• NTSC‘s faster frame rate requires more broadcast bandwidth
• 24 fps film makes for smaller file sizes and stream bandwidth

Conversion Process

• Telecine "3:2 pulldown" is needed to convert 24 fps film to 29.97 fps interlaced video
• Involves blending additional fields between film frames

So while 24 fps film and 29.97 fps NTSC both deliver adequate motion portrayal, the formats evolved separately to meet different technical requirements.

Pulling Down 24p to NTSC Rates

To convert modern 24p video into NTSC frame rates, the same "telecine 3:2 pulldown" process is used. Here is how it works:

1. The film frame rate is slowed slightly to 23.976 fps to divide evenly from NTSC‘s 59.94 fields per second.

2. A cadence of 3 fields from one film frame, followed by 2 fields from the next frame is established.

3. This alternating 3:2 pattern creates 29.97 interlaced frames per second from the 23.976 progressive film frames.

4. Additional post-processing removes redundant fields to deinterlace back to 23.976p if needed.

Here is an example diagram:

Film Frames:    A   B   C   D   E
NTSC Fields: A1 A2 A3 B1 B2 C1 C2 C3 D1 D2 E1 E2 E3

By using this telecine technique, cinematic 24p footage can be adapted for interlaced NTSC broadcasting and display.

Comparing Common Video Frame Rates

Beyond 24p and NTSC rates, there are a few other frame rates commonly used in video standards worldwide:

Frame Rate	Video Standard	Region
24 fps	Film	Global
25 fps	PAL	Europe, Asia
30 fps	NTSC	North America
50 fps	PAL	Europe, Asia
60 fps	NTSC	North America

Higher frame rates like 100, 120 or 240 fps are often used in video games and VR applications to provide extremely smooth animation.

The choice of frame rate has a significant impact on the viewing experience:

• 24/25 fps – Provides a cinematic look and natural motion blur that our eyes expect from filmed footage.

• 30/60 fps – Minimizes motion blur so motion looks smoother but less "life-like" for film. Better for sports and video games where smoothness is prioritized.

• Higher fps – Eliminates perceptible stutter and blurring entirely for very clear, smooth video. Benefits gaming/VR but overkill for filmed entertainment.

So content creators have to weigh factors like intended medium, desired look, and target viewing devices when selecting a frame rate.

Why 23.976 fps is Used Instead of True 24 fps

You may notice that while 24p formats exist, a rate of 23.976 fps is more commonly used in practice over true 24 fps. The reason behind this has to do with maintaining NTSC color encoding:

• The NTSC color subcarrier frequency is 3.579545 MHz as mentioned earlier.

• This frequency drifts slightly at a true 24 fps rate when converted from film.

• By adjusting the film playback speed to 23.976 fps, the color encoding stays within NTSC tolerances.

• This 0.1% slower rate avoids hue drifts and combing artifacts that could occur from color drift.

So once again, we see adjustments being made to accommodate NTSC technical requirements. The legacy lives on!

Subjective Take – The Quest for Higher Frame Rates

Having worked with all kinds of video footage over the years, I‘ve formed some strong opinions around frame rates and perceived quality:

• For filmed content, 24p or 25p still provides the most cinematic, aesthetically pleasing look in my eyes. The motion blur feels right.

• 30 fps strikes a good balance for general purpose video capture and visual smoothness. 60 fps feels like overkill for sit-down interviews and vlogs.

• For sports, video games, and VR, 60 fps and up is the way to go. The faster the better when it comes to tracking fast motion.

• While some may argue that 48 fps or 60 fps looks more "realistic", I still prefer the aesthetics of 24 fps film. Sometimes perfection lacks personality!

That said, I‘m very interested to see high frame rate and high dynamic range video become more accessible. As display refresh rates improve, buttery smooth 120 fps video may become the new norm across all content!

Summary – Frame Rates Continue to Evolve

To wrap things up, while NTSC settled on an interlaced ~30 fps rate for analog TV broadcast, the 24 fps rate used for cinematic film persisted and is still popular today with digital 24p formats. Conversion between NTSC and film frame rates is possible using telecine techniques, though adaptations had to be made along the way to account for technical limitations.

As standards continue to evolve, we now have far more flexibility with digital video frame rates. While NTSC and 24p frame rates still provide a base reference, new technologies like 4K resolution, HDR color, and high frame rate capture open up many possibilities for both creators and viewers. The frame rate you choose ultimately depends on the creative goal, target audience, and distribution method.

I hope this deep dive into the history and technical details helps explain NTSC frame rates and their relationship with film! Let me know if you have any other topics you would like me to cover in a future post.