1080i

1080i is a term used in high-definition television (HDTV) and video display technology. It means a video mode with 1080 lines of vertical resolution. The "i" stands for interlaced scanning method. This format was once a standard in HDTV. It was particularly used for broadcast television. This is because it can deliver high-resolution images without needing excessive bandwidth. This format is used in the SMPTE 292M standard.

Definition

The number "1080" in 1080i refers to the number of horizontal lines that make up the vertical resolution of the display. Each of these lines contributes to the overall detail and clarity of the image. The letter "i" stands for interlaced. This is a technique where the image is not displayed all at once. Instead, the frame is split into two fields. One field contains the odd-numbered lines, and the other field contains the even-numbered lines. These fields are displayed in rapid succession, giving the appearance of a full image to the human eye. The interlacing technique was developed to improve the motion portrayal of images without doubling the required bandwidth. This is particularly useful in broadcasting, where bandwidth efficiency is crucial. The frame rate is typically 50 or 60 fields per second, depending on the region. This effectively means 25 or 30 frames per second when the fields are combined.^[1]

Comparison with 1080p

A key difference between 1080i and 1080p is how the lines of resolution are displayed. Both offer 1920x1080 pixels, but the display method is different. In 1080p, the "p" stands for progressive scan. Each frame is drawn line by line, from top to bottom, creating a complete image in a single pass. This results in a sharper and more stable picture, especially in fast-moving scenes. On the other hand, 1080i uses an interlaced method. The two fields that make up a frame are captured at slightly different times, leading to a misalignment in fast-moving parts of the image. This can cause artifacts like "combing," where fast-moving objects appear to have a serrated edge. However, 1080i has been preferred in broadcast television due to its lower bandwidth requirements, making it more efficient for over-the-air or cable transmission.^[2]

Historical context

The origins of 1080i can be traced from Multiple sub-Nyquist sampling encoding, a Japanese analog high-definition television system. 1080i emerged as a leading standard for HDTV broadcasts. Many broadcasters worldwide adopted it. The ATSC (Advanced Television Systems Committee) standards and the DVB (Digital Video Broadcasting) standards allowed for the transmission of 1080i video signals. The adoption of 1080i was particularly significant in sports broadcasting. The higher resolution allowed for more detail and clarity, especially in large stadium shots and fast-paced action. The format's efficiency in utilizing available bandwidth made it a practical choice for broadcasters. However, it required more complex processing on the receiving end to deinterlace the image for display on progressive-scan screens. Overall, 1080i played a crucial role in the early days of HDTV. It bridged the gap between standard-definition broadcasts and the high-definition future that would soon become the norm. While its use has diminished with the rise of 1080p and 4K resolutions, 1080i remains an important milestone in the evolution of television technology.^[3]

Technical overview

Resolution

The core of 1080i is its resolution. The resolution is 1920x1080 pixels. This means that each video frame has 1,920 pixels horizontally and 1,080 pixels vertically. This results in over two million individual pixels per frame. This high resolution is why 1080i is called "high-definition". It offers a significant improvement over standard-definition formats, which typically have much lower pixel counts. The 1920x1080 resolution allows for greater detail and clarity in images. This makes it ideal for larger screens where higher pixel density is essential to maintain image quality. The increased resolution is particularly noticeable in fine details such as textures, text, and intricate patterns. These can be rendered with much greater accuracy than in lower-resolution formats.^[4]^[5]

Interlacing

The "i" in 1080i stands for interlaced. This refers to how each video frame is displayed. Instead of showing the entire frame at once, the interlacing technique divides each frame into two separate fields. The first field contains all the odd-numbered lines (1, 3, 5, etc.), and the second field contains all the even-numbered lines (2, 4, 6, etc.). These two fields are displayed one after the other. The odd lines are shown first, followed by the even lines. This happens very quickly, around 50 or 60 fields per second. The human eye sees these two fields as a single, continuous image. The main advantage of interlacing is that it allows for a higher frame rate without needing more bandwidth. This results in smoother motion, especially for content with moderate to fast movement, like sports broadcasts.

An example frame of poorly deinterlaced video. Despite the fact that most TV transmissions are interlaced, plasma and LCD display technologies are progressively scanned. Consequently, flat-panel TVs convert an interlaced source to progressive scan for display, which can have an adverse impact on motion portrayal.

However, interlacing also has some drawbacks. Since the two fields are captured at slightly different times, fast-moving objects can appear misaligned between the two fields. This creates a visual artifact called "combing." This can reduce image quality, especially in scenes with a lot of motion. Modern display devices often use deinterlacing algorithms to combine the two fields into a single progressive image before displaying it.^[5]^[6]^[7]

Frame rate

The frame rate of 1080i is usually 50 or 60 Hz. It depends on the region. In areas using the PAL or SECAM standards, like Europe and parts of Asia, the frame rate is 50 Hz. In regions using NTSC, like North America and Japan, the frame rate is 60 Hz. The frame rate refers to how often a new field is shown per second. At 50 Hz, 50 fields are shown each second. This results in 25 full frames per second when the odd and even fields are combined. At 60 Hz, 60 fields are shown per second. This results in 30 full frames per second. Interlacing affects how motion is perceived in 1080i. Since each field represents a slightly different moment in time, motion can appear smoother compared to lower frame rate progressive scans. However, this also means 1080i can struggle with fast-moving scenes. The interlaced fields might not perfectly align, leading to motion artifacts.^[5]^[8] The European Broadcasting Union (EBU) prefers to use the resolution and frame rate (not field rate) separated by a slash, as in 1080i/30 and 1080i/25, likewise 480i/30 and 576i/25.^[9] Resolutions of 1080i60 or 1080i50 often refers to 1080i/30 or 1080i/25 in EBU notation.

Signal transmission

The 1080i video signals can be carried by four main digital television broadcast systems: ATSC, DVB, ISDB and DTMB. In both ATSC and DVB systems, the 1080i signal is compressed using codecs like MPEG-2 or H.264 to reduce the bandwidth required for transmission.

In the United States, 1080i is the preferred format for most broadcasters, with Warner Bros. Discovery, Paramount Global, and Comcast owned networks broadcasting in the format, along with most smaller broadcasters. Only Fox- and Disney-owned television networks, along with MLB Network and a few other cable networks, use 720p as the preferred format for their networks; A&E Networks channels converted from 720p to 1080i sometime in 2013 due to acquired networks already transmitting in the 1080i format. Many ABC affiliates owned by Hearst Television and former Belo Corporation stations owned by Tegna, along with some individual affiliates of those three networks, air their signals in 1080i and upscale network programming for master control and transmission purposes, as most syndicated programming and advertising is produced and distributed in 1080i/p, removing a downscaling step to 720p. This also allows local newscasts on these ABC affiliates to be produced in the higher resolution (especially for weather forecasting presentation purposes for map clarity) to match the picture quality of their 1080i competitors.