TV Broadcast Formats

High Definition (HD) & 50Hz vs. 59.94Hz

As discussed in the Resolution, Scaling, & Progressive vs. Interlaced article, two of the most important characteristics of a video are its resolution and temporal rate. Additionally, as was mentioned in the Aspect Ratio article, HD video is typically 1280x720, 1440x1080, or 1920x1080. But what about HD video's temporal rate, and what resolution and temporal rate is non-HD video? Take a look at the modified map below from Paul Schlyter’s Analog TV Broadcast Systems article [1]:

TV is typically transmitted at either 50Hz or roughly 59.94Hz [1]. Note that I used the term "Hz" rather than "i/p" since some countries use analog broadcasting, others use analog and digital, while others use purely digital [2]. Analog broadcasting today is limited to interlaced while digital is able to transmit both interlaced and progressive video.

In fact, digital's ability to carry both interlaced and progressive explains why HD broadcasting is typically either 1280x720 or 1920x1080, although 1440x1080 is sometimes used as well. In order to make the 1080-row video stream smaller, 1080-row video is typically broadcast as interlaced, i.e. 50i or 59.94i depending on location. By contrast, 1280x720 is typically broadcast as 50p or 59.94p. That being said, even though BROADCAST 1080-row video is typically limited to 59.94i, video output from devices such as computers is typically able to output 1920x1080 at up to 60p.

The HD resolutions are the same worldwide but, as already explained, the 59.94Hz countries have a higher temporal rate. This gives the 59.94Hz countries, on average, better quality HD compared to the 50Hz countries. However, as we will soon find out, 59.94Hz TV was not always the best.

525-line vs. 625-line Analog

Back in the pre-HD days of purely analog broadcasting, 59.94Hz video had a disadvantage compared to 50Hz video. Though 59.94Hz non-HD had a higher temporal rate, it had a lower resolution compared to 50Hz non-HD. More specifically, the 59.94Hz countries used the 525-line system while the 50Hz countries used the 625-line system.

If you’ve read the Resolution, Scaling, & Progressive vs. Interlaced article, you might reasonably assume that the 525-line video system has a resolution of something-by-525 and that the 625-line video system has a resolution of something-by-625. However, this is NOT the case. The 525-line system actually has a resolution of 710.85x486, while the 625-line system is 702x576. Digital video, however, must have whole number dimensions, meaning that 710.85x486 video would have to be digitized as 711x486 for all the information to be recorded [3].

When broadcast, these resolutions have an AAR of 4:3 [3]. When recording in these formats on your own, such as with an analog camcorder, you may also have the option to record these formats with an AAR of 16:9. That being said, here’s the RAR, AAR, and PAR of the formats we’ve covered so far, along with resolutions that can create a 1:1 PAR:

Analog Color Encoding

Along with having a difference in resolution and temporal rate, analog video transmission also has differences in color encoding. Take a look at the map below from Paul Schlyter’s Analog TV Broadcast Systems article that shows the National Television Systems Committee (NTSC), Phase Alternation Line (PAL), and the French-originated SEQuential A Memoire/Memory Sequential (SECAM) color systems [1]:

If you compare the temporal rate map and the analog color map, you'll notice that most of the 50Hz countries use PAL or SECAM while most of the 59.94Hz countries use NTSC. There are, however, a few exceptions.

Finally, it should be noted that digital video does not actually use these color encodings. So, although the designations PAL and NTSC are sometimes used to distinguish between 50Hz and 59.94/60Hz digital video, this is technically incorrect.

Non-HD Digital Video Resolutions

In the 59.94Hz countries, two commonly used digital video resolutions are 704x480 and 720x480. Meanwhile, in the 50Hz countries, two commonly used resolutions are 704x576 and 720x576. The reasons for these resolution choices are presented below.

For recording video in the earlier digital video file formats of MPEG-1 and MPEG-2, it made things a lot easier to have the number of columns and the number of rows divisible by 16 [3]. As we will soon see, this played a crucial role in determining what resolution digital video should be.

First, let’s look at the resolutions of 704x480 and 704x576. As already mentioned, the 525-line system has a picture area of roughly 711x486. However, some basic math tells us that neither 711 nor 486 are evenly divisible by 16. So, in the 525-line countries, it was decided that digital video should only have 480 rows. As for the number of columns, this would also have to be reduced if the picture was to still have an AAR of 4:3. If you take 480 and multiply it by 711/486, you get 702.2. Even if you want to be more precise about things and multiply 480 by 710.85/486, you get 702.07407. This means that, either way, the central 480-row portion of a 525-line broadcast is about 702x480. Unfortunately, 702 is not divisible by 16. 704, however, is. So, one common approach for digitizing the 525-line system is to record it as 704x480, which ends up having an AAR of roughly 4:3. The divisibly-by-16 rule also explains why 625-line video may be digitized as 704x576 and not 702x576.

The table below looks at some possibilities for the 704x480 and 704x576 formats:

Next, there are the resolutions of 720x480 and 720x576. Although the additional 6 rows in a 486 picture typically do not get recorded, it is still possible to record the additional columns using a 720x480 resolution. This 720x480 resolution may have one of the PARs of 704x480 video. On the other hand, 720x480 video may in fact end up having an AAR of 4:3 or 16:9 [4]. Some 720x480 variations are detailed below:

720x576 is used in 625-line countries to gain additional resolution for digital recordings and typically has an AAR of either 4:3 or 16:9. If it is scaled to a resolution with a 1:1 PAR, then this resolution, as previous tables have shown, is scaled to either 768x576 or 1024x576. 720x576 is further detailed below:

Finally, you may have heard of the terms Standard Definition (SD) and Enhanced Definition (ED). SD refers to non-HD video that is interlaced, while ED refers to non-HD that is progressive.

Movie-to-TV Conversion

As already discussed, TV is either 50Hz or 59.94Hz depending upon the country. Movies, however, are 24fps [5]. Therefore, in order to show movies on TV, there needs to be some way to convert 24fps to 50Hz and 59.94Hz. Luckily, there is. This section goes over the conversion of 24fps to 50Hz and to 59.94Hz.

The most common way to convert 24fps to 50Hz is to show every film frame twice in half the time [5]. This, however, creates a problem: the movie ends up being shown 4% too fast. Why? Remember, if a video is 50Hz, that means that 50 pictures are meant to represent one second. However, if all you do for a movie is to show every frame twice, then that means that one second of a movie ends up being shown in 48/50 of a second.

The most common way to convert 24fps to 59.94Hz is to slow down the movie by 0.000999% (1/1001) [6] to about 23.976Hz and to then use the 3-2 pulldown method. In 3-2 pulldown, the first frame of the movie is shown 3 times, then the next is shown 2 times, then the next shown 3, etc. Unfortunately, this can create jerky motion. Ideally, every frame should be shown for the same amount of time, but with the 3-2 pulldown process that does not happen. The "3-2" in the 3-2 pulldown process can also be referred to as 3:2, 2-3, or 2:3 [5].

Digital TV Transmission Types

Take a look at the map below [7]:

This map shows the major digital television transmission systems that countries are either currently using or plan on using. ATSC stands for Advanced Television Systems Committee, DTMB stands for Digital Terrestrial Multimedia Broadcast, DVB-T stands for Digital Video Broadcast – Terrestrial, and ISDB-T stands for Integrated Services Digital Broadcasting – Terrestrial [8].

ATSC and DVB-T both support a variety of SD, ED, and HD formats. This is because the designers of ATSC and DVB-T wanted their respective systems to be able to be used in as many countries as possible. So, in addition to supporting both 59.94Hz and 60Hz transmission [9 page 9], ATSC also supports 50Hz transmission [10 page 5]. DVB-T also supports both 50Hz and 60Hz [11 page 1].

The Analog-to-Digital Transition

Finally, to wrap things up, here is yet another map [2]:

Legend:

Slowly but surely, countries around the world are ending analog broadcasting and replacing it with digital, and the map above shows this progress. Looking at the U.S. specifically, equipment sold in the U.S. that had a TV tuner was required to have ATSC compatibility starting Mar. 1, 2007 [12], and full-power analog TV broadcasting ended on June 12, 2009 [13]. Low-power analog TV broadcasting in the U.S. is still available, and no date has been set to end it [14].

©2012 AVHelpZone.com

Sources

1. Paul Schlyter. Analog TV Broadcast Systems. Jan. 6, 2001.

2. Denelson83. File:Worldmap digital television transition.svg. Created Jan. 12, 2009. Wikimedia Commons.

3. Jukka Aho. A Quick Guide to Digital Video Resolution and Aspect Ratio Conversions. Jan. 15, 2008.

4. Eric Olson. About Aspect Ratios. July 21, 2009 5:56:36 PM PDT. RenoMath.

5. Don Munsil and Brian Florian. DVD Benchmark – Part 5 – Progressive Scan DVD. Sept. 30, 2000 4:00 PM. ©2012 Secrets of Home Theater and High Fidelity.

6. Bob Myers. Why 59.94 Hz?. Dec. 4, 1993 7:21 PM. ©2012 Google.

7. DVB Worldwide. ©DVB 2003.

8. David Neff. INTEGRATING MOBILE MULTIMEDIA SERVICES INTO A BROADCAST SYSTEM; OPTIONS, CHOICES, AND WHAT YOU NEED TO KNOW. 2009.

9. ATSC Digital Television Standard: Part 4 – MPEG-2 Video System Characteristics. Aug. 7, 2009. Advanced Television Systems Committee, Inc.

10. Standard for Coding 25/50 Hz Video. May 2, 1997. Advanced Television Systems Committee.

11. DVB-T Factsheet. Aug. 2011. Produced by the DVB Project Office.

12. FCC MODIFIES DIGITAL TUNER REQUIREMENTS TO ADVANCE DTV TRANSITION. Nov. 3, 2005. Federal Communications Commission.

13. What is DTV?. Federal Communications Commission.

14. Low Power TV (LPTV) Stations. Federal Communications Commission.