All about High Defintion TVs
Menu
1. Introduction
2. What is HD Ready?
3. Resolutions Explained
4. The importance of a good processor
5. Is 1080 worth it?
6. Summing Up
Introduction
This guide describes various aspects of high defintion screen technology, and explains why for most people we shouldn't get hung up on resolution. For most people a suitable HDTV will have (i) an HD Ready logo, (ii) a good processor (iii) The right interfaces (eg scart, hdmi) and (iv) Will look good in its environment. Scrimping on quality, especially to chase high resolution and large screen size could be a very expensive mistake (eg juddery or streaky motion).
There has been a lot of partnering and outsourcing in manufacturing and development, so its difficult to know which TVs are really made by who. Take care when buying cheepy unknown brands or cheap models from large brands (unless you have been given a very knowledgable recommendation). Some of the big brands made their reputation with Tube TVs and have been selling some pretty shoddy flatpanels.
What is "HD Ready"?
To wear the HD Ready logo a TV must be able to support input, processing and display of widescreen HD signals. Specifically, it must support;
- a minimum native resolution of 720 lines in the 16:9 ratio
- HD input via either analogue component video and DVI or HDMI connectors
- HD capable inputs that accept 1280x720 at 50 and 60Hz progressive (720p) and 1920x1080 at 50 and 60Hz interlaced (1080i)
- DVI or HDMI input that supports HDCP.
HD Compatible means that a device can accept HD signals (eg via HDMI, DVI, VGA or component inputs), but doesnt meet the display criteria (eg it has a standard defintion screen).
The HDTV logo is meant for devices and receivers such as DVD players and specify that the device can input, process and output to levels meeting HD Ready criteria, with the exception of the display requirements, as it is intended for devices without a display.
But as described in the rest of the guide, not all HD Ready devices are equal, and not all will display an 'amazing' picture as promised.
Resolutions
Pixels are the small dots on the screen that make up the picture. They can be counted horizontally and vertically to give us the screens resolution (eg 1366 horizontal pixels by 768 vertical pixels).
There are two main HD signal resolutions, 1280x720 (HD Ready) and 1920x1080 (Full HD Ready) signals.
This diagram demonstrates the difference in the amount of information in Standard, HD Ready and Full HD signals on a screen of the same size (eg all need to be dispalyed on a 32 inch screen). You can see how much more detail and granularity the HD pictures would have.
Various common HD screen formats include 1280 x 720/768/800 (found in WXGA PC monitors) 1024x768 (XGA, found in PCs), 1366x768 (most common in medium sized flatpanel TVs) and the full 1920x1080 in larger flatpanel TVs.
The importance of good processing power
As you can see, there is a mismatch between broadcast signal resolutions and a screens resolution (eg 720 signal on 768 pixels). The TV's processor will have to work that out, amongst other things. So although pixel count by definition can create a sharper static image, it is challenged by other adjutments and conversions required to produce a high quality moving image from the signal. Along with the ability to update the pixels rapidly (pixel refresh rate and screen scan rate) having enough picture processing power and accuracy is the most important factor in producing a sharper moving image. The processor not only has to resolve resolution differences, it has to resolve various scanning rate and type issues, not just from the signal, but from the original recording.
Less technical people may want to skip to the next section.
Down Scaling and Upscaling
A 1080 signal being received by the 1366x768 panel will have to be downscaled to convert the image to fit to the panel resolution of 768 lines. By using a very complex algorithms the scaler will systematically crop the image to fit the lower resolution panel. In the same way that "down conversion" happens, up-conversion also occurs. By using clever algorithms (hopefully) lines will need repeating without distrorting the image to be able fit the larger resolution.
Progressive and Interlaced Scanning
The 1080i format contains 1080 lines of image information, which is produced using the interlaced system (hence the 'i'). The pictures on your television are produced in two separate 'sweeps', with the odd lines (1, 3, 5, 7 etc) created during the first sweep and the even lines (2, 4, 6, 8 etc) created during the second. The sweeps are so quick, however, that the human eye sees only one complete image.
The 720p format, while having fewer lines of image information, uses progressive scanning (hence the 'p'), where all the screen image is created in a single sweep. This in theory would provide an exceptionally smooth and stable image, preferable to interlacing, and especially for high speed movements such as sports and action movies.
The ultimate would be 1080p, and ideally at a frame rate of interlacing (60 full scans per second). The bandwidth required to deliver this information is huge though.
The problem having two methods is that the input signal will often not be the desired scan system (1080i capable screen will likely recieve mainly 720p signals). Sky wiill by default transmit in 720p, and use 1080i for nature programs so it would change for each program viewed. Blu-ray players capable of outputing 1080p have been known to do a progressive -> interlaced -> progressive conversion between reading the disk, and sending up to the TV. I use this example to demonstrate that there are common and highly technical reasons why scan conversion may occur.
Frame rate adaption
As a interlaced scan freshes twice for any given image (2 field scans for each frame scan), a progressive scan will have half the frame rate of an interlaced scan.
So may see 1080i-30 and 1080p-60 indicating the resolution, scanning system and frame rate for a signal (eg 60 field refreshes of 30 frames). NB The resulting image is the same and the quality difference is almost never noticable.
To further complicate things, the original film may have been recorded with a scan rate of 24 frames per second. The original PAL system used for TV broadcasts used 25 frames per second (without knowing it you watched the film sped up by 4% and with a slightly higher sound pitch!!). The technique of 3:2 Pulldown is used to change the rate from 24fps to 30fps. In simple terms, instead of an interlaced signal performaing two scans of the screen, 3:2 Pulldown alternately places one film frame across two fields, the next across three, the next across two, and so on. For progressive scans (720p) entire frames (rather than fields) are repeated in a 2-3 pattern.
Reverse pulldown is the extraction of that extra field, to get back to the original film data (before subsequent reprocessing such as de-interlacing or compression). Post 3:2 pulldown editing at the studios prevents us assuming we can just follow the 3:2 pattern to identify which fields to remove.
Conversion techniques
Taking advantage of the frame rate differences, the crudest way of converting between interlaced and progressive scans is to use 'line doubler' techniques, such as resending lines twice or holding back alternate lines for each scan. But crude conversions create gremlins in the image to ruin our viewing pleasure such as motion blur, feathering, judder and artifacts.
Advanced processing may utilise the following techniques in order to process the original 'studio' picture into the best moving picture our screen will support
Frame retention buffers - hold a frame while you read the adjacent frames so that you can manipluate the frame contents accordingly
Pulldown reversal - Extracting duplicated frames before processing
Motion adaptive intelligence - blending portions of lines lines in fields (eg in interlaced scans) based on the 'next' frame. Can be an intense pixel by pixel process that influences the other processes (eg whether to bob or weave)
Bob (Interpolation) - A missing line is recreated by blending the two adjacent lines (blend lines 1&3 to recreate 2).
Weave - creating frames from portions of the previous frame and portions of the next (eg odd lines from one and even lines from another).
Diagonal line Intelligence - creating and extracting lines with awareness of diagonally placed pixels instead simple mapping from lines above and below.
Good processors may use a mix of these intelligently. For example, use a frame retention buffer to hold and manipulate frames while analysing adjacent frames. They could then strip out pulldown frames, perform motion and diagonal analysis pixel by pixel, and take this into account while blending, creating and extracting lines (resolution) and frames (frame rate).
Is there any benefit to having a 1920x1080 panel
Yes. Some of the 'Full HD' sets will have large screens where the extra pixels will be a benefit as long as they are high end models with powerful processors and high performance panels. They may also support 'per-pixel-mapping' that allows a 1080 broadcasted signal to be processed by the panel in the original broadcasted format. It should be noted that this benefit will occur on a minority of broadcasts or disk sources (for the foreseeable future)
Summing up
A 720p signal on a 720p screen looks amazing. Even a standard definition PAL broadcast on a good 720p is pretty stunning. On the average family sized 720p panel (assuming an adequate processor and pixel refresh rate) there are benefits for sharp motion pictures and a significant step up in screen quality for everybody. For enthusiasts, prepared to pay for the extra pixels, and the extra horsepower needed to go with it, there will be further benefits in having the full 1080 screen. But chasing purely pixel count or screen size at the expense of processing power and panel quality will be a mistake. In short, for most people, your budget is better spent on a good TV rather than a 'big' one (by size or resolution).
We hope this guide has been helpful
Back to HDTV menu
Back to Learning Zone menu