HD Frame Rate Confusion

HD Frame Rate Confusion

One of the main areas of confusion for HD production and post production is the lack of consistency that's used when describing these particular formats, as well as standard definition formats. As discussed earlier, NTSC is 29.97 frames/second, or 59.94 fields/second. Yet most people refer to this as 30 frames for simplicity, or 60i (referring to 60 interlace fields). This is inaccurate, but common. Its more dangerous in the HD world as there are standards for 23.98 and 24, 29.97 and 30, 59.94 and 60. Yet most people again will "round-off", referring to 24, 30 and 60. But its important to know specifically what is being discussed, as it can have serious implications if the wrong format is used because of this inaccuracy in communication.

The Short Answer

When working in the U.S., whenever you hear a whole number, like 30, you can assume this refers to a fractional number, in this case, 29.97. As seen in the last section, when discussing 24p, especially in NTSC countries, generally this refers to 23.98.

The Details

Let's look at a typical HD format expression:

 
It seems fairly specific, but unfortunately the frame rate is misleading. This notation can refer to 23.98 or 24. It usually refers to 23.98.

In this example from Final Cut Pro, notice that audio/video setting presets list this same format (specifically for the DVCPRO HD videotape format).

 
Here it definitely refers to 23.98. In fact, Easy Setups that ship with Final Cut are for "native editing" which, in a nutshell, means a handful of formats where there is a comparable computer file format in the editing system similar to the tape format itself. These native formats and the settings in Final Cut for them are always NTSC or PAL compatible. Therefore, wherever we see a native Easy Setup:

• 24 refers to 23.98
• 30 refers to 29.97
• 60 refers to 59.94 (fields or frames)

On the other hand, if you install a dedicated I/O board, in other words a capture card, like the AJA Kona card, then you will see additional options for frame rates.

 
As you can see, AJA lists the formats precisely, since it allows you to bring in all of these various frame rates, including both 23.98 and 24 frame variations, as well as 29.97 and 30 variations.

Which Format Is Being Described?

To try and simplify things: if your video interface (deck, camera, editing system) lists only one possibility, like 24p, then we can usually assume that this refers to the fractional frame rate of 23.98, in other words, an NTSC compatible frame rate. On the other hand, if the interface lists all variations (like 23.98 and 24), then we need to choose carefully. Generally this means choosing the fractional frame rate for U.S. compatibility, i.e. 23.98, 29.97 and 59.94.

Interlaced Frame Rate Terminology

Let's look at another typical HD format expression:

There are a few misleading things here. For one, when describing an interlaced format, the last numbers could represent the field rate or the frame rate (remember- the field rate is double the frame rate, 2 fields for every complete interlaced frame). In this case, the number is 60, so we can assume it’s a field rate here, and represents 30 fps.

Another misleading aspect is that the number 60 is usually an abbreviation for 59.94 (the NTSC field rate). This format is often referred to simply as 60i. Again, this can be misleading as 60i is not NTSC compatible, yet 59.94i is compatible, and is usually what is being implied. In other words, 1080i60 usually refers to 29.97 fps!

There are a few different ways this one format may be written out:

• 1080i60
• 1080i30
• 1080i59.94
• 1080i29.97

If the implication is 29.97 fps, then only the last two notations below are technically correct (although you should be prepared to see the frame rate described using any of the previous examples):

• 1080i59.94
• 1080i29.97

Why The Confusion With Interlaced Terminology Exists

One of the things we kept pointing out in the previous information about interlaced video is that a format is often described by the field rate, not the frame rate. This actually makes a lot of sense when you understand a bit more about decks and cameras.

Think of it this way- instead of thinking about frame rates, think about the "speed" of the deck or camera. When recording 29.97 fps interlaced, or when recording 59.94 fps progressive, there is one thing in common- the speed or frequency of the recording device. Hertz refers to frequency, which in this case refers to cycles per second, and with both of the previous mentioned formats, both recording devices are running at a speed of 59.94 Hz (Hertz). It’s the same amount of cycles per second with either format. The only difference is that with interlaced you are recording 59.94 fields a second (which in turn becomes 29.97fps). When you record 59.94 fps progressive, you are still recording at 59.94 Hz a second, just this time you're recording frames instead of fields. You will see this more clearly on a deck like an HD D5, where you set the Hz speed, and then separately set the video format like 720p or 1080i. Whether its progressive or interlaced determines whether you are recording 59.94 fields or 59.94 frames.