Beyond HD
Nov 1, 2008 12:00 PM, By Craig Birkmaier
Will 1080p improve the quality of HDTV?
This image was taken from John Logie Baird’s mechanical TV system.
Oversampling is highly beneficial in cameras as it helps to eliminate sampling errors and minimize the effect of noise on the sampling process. When we oversample and then resample to a lower resolution, we improve the overall accuracy of the image. This pays significant dividends when the imagery is digitally compressed for emission. Reducing the number of samples that must be encoded, along with improving the accuracy of the samples, makes it possible to deliver higher quality images in bandwidth-constrained channels.
You do not want to push the sampling system and the compression system to the limits. But this is what many broadcasters have chosen to do with HDTV. By selecting the interlaced 1080i format, they are compromising the acquisition system in two ways. First, interlace is a spatial/temporal undersampling system. This adds stress to the MPEG compression system, which is optimized for frame-based images (progressive scan). Second, most current camera designs cannot oversample the 1920 × 1080 format. Many of the less expensive cameras use sensors that only have 960 to 1440 samples per line. This results in less accurate samples that require more bits to compress.
Progressive image acquisition and processing improves the quality of the samples, which reduces stress on the MPEG encoder. And the lower sample rate leaves more room in the 19.3Mb/s ATSC payload to handle peak bit-rate requirements. Oversampling to create 720p further improves the image quality.
Display oversampling
In the decoupled world of DTV, oversampling plays a significant role in display quality as well. One of the major limitations of NTSC and PAL (in addition to the use of interlace) is that the scanning lines become visible as the display size increases, and the artifacts associated with interlace accentuate the perception of the scanning lines in the raster. NTSC was designed to produce a sharp picture on a 19in display viewed at seven picture heights. As larger CRT displays became more common, the perception of both image artifacts and the raster was more apparent. The move to HDTV was driven primarily by the need for more samples to create sharp images on larger displays.
Today we find many display resolutions in the marketplace, some of which are different than any of the ATSC formats. The good news is that virtually all flat-panel and projection displays being sold today offer progressive scanning. But the consumer electronics industry is pushing consumers to purchase displays with 1080p resolution, often in display sizes where the viewer will not be able to resolve this level of detail.
As a rule of thumb, 1280 × 720 resolution is adequate for displays with diagonals up to 50in; 1920 × 1080 is only needed for displays larger than 50in at typical entertainment viewing distances. But that excess resolution is not necessarily a bad thing; it helps to suppress the perception of the display raster, and it may be useful for other applications, such as viewing a Web page on the TV display.
In this, modern TVs are much like computer displays. They can present properly sampled video images with high quality, and they can present computer-generated samples that would cause aliasing in video images. Thus a smaller (e.g., 32in) 1080p display may have more resolution than the viewer can see when sitting at five to seven picture heights, but that resolution may be useful when viewing a Web page — albeit by moving closer to the screen to see the additional details.
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We are also seeing HDTV displays that are oversampling in the temporal domain as well. In Europe, 100Hz displays have been common for several decades to help minimize the perception of 50Hz flicker. Many manufacturers are now selling 120Hz HDTV displays, with image processing engines that can improve the quality of 24p source images.
In the United States, we have lived with improper presentation of 24p since the inception of TV service at 60Hz, which became 59.94Hz when color was added. This required a frame repetition technique known as 3:2 pulldown. In a theater, the projector typically operates at 48Hz or 72Hz, displaying each frame two or three times. To convert the 24p source to 60 video fields, one frame is repeated for three field periods, the next for two field periods. This disrupts the motion adding to the problems that exist with the low 24p acquisition rate.
New 120Hz HDTV displays would allow each film frame to be displayed five times, eliminating the uneven motion rendition of 3:2 pulldown. But these displays go a step further and create in-between frames using sophisticated motion-compensated prediction techniques. The result is smoother motion that one would see in a theater.
Beyond HDTV?
There is a great deal of interest in new imaging systems that offer resolutions well beyond today's HDTV. For applications where the imagery is presented on very large screens, this is both desirable and practical. For a consumer television system, however, there are many things we need more than increased resolution.
First and foremost, we need to deliver high-quality samples, even if this means reducing the resolution slightly to make the content fit the channel's bit budget. The vast majority of digital content offered today is significantly over-compressed. Needless to say, trying to squeeze 1080 at 60p into an existing ATSC channel is only going to worsen the delivered image quality.
Next, we need to get rid of interlace, both as an image acquisition format and as an emission format. And finally, we need to take full advantage of the benefits of oversampling, both during image acquisition and at the display. If we do these things, we will move beyond the limitations of what is rapidly becoming a legacy HDTV system.
Craig Birkmaier is a technology consultant at Pcube Labs.
Send questions and comments to: craig.birkmaier@penton.com
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