Solving digital image artifacts with advanced video processing

If you find the picture from your new HDTV set less than perfect, some new video processing technology might be the solution you are looking for.

Television, Analog vs. Digital
The technology behind what was the centerpiece of American culture is on its last legs. The de-facto standard for the last 40 years, the NTSC-Analog television system, will soon become obsolete with the promise of a sharper image, higher resolution and more vibrant colors thru Digital Television (DTV).

DTV technology is widely considered the most significant step in TV technology since color television, but the process of its introduction extends for over more than a decade. Let us put aside for now the politics and legislative activities concerning its adoption and focus on its technical merits. We’ll also look into how this new standard, for years to come, will affect the end-user.

The Bandwidth Battle
In theory, DTV’s picture quality is undoubtedly superior to traditional Analog TV: no more “ghosting”, “snow”, “judder”, “Never The Same Color”, etc. Nevertheless, analog signals’ arguably most glaring weakness is its blurriness and lack of fine image details due to shortcomings in high-frequency response, or simply put, in bandwidth. The more detailed an image is or the more resolution it has, the more bandwidth it needs.

Traditionally, this bandwidth limitation has always been transparent to the end user. Long ago, it was agreed upon that 6 megahertz (MHz) of bandwidth from the allowed spectrum would be allocated to each channel for broadcasters in America to provide these analog TV signals. This limitation in video bandwidth and its corresponding standard (NTSC) in turn dictated the specifications of the traditional TV set, as well as its picture quality for decades.

With the advent of DTV, broadcasters saw a great opportunity to make much better use of their bandwidth (and to ask for more…). Indeed, from their standpoint, one of the sterling advantages of DTV was that it allowed for multiple channels in the same amount of bandwidth, and would later on allow for High-Definition Programming (HDTV).

Everywhere, HDTV
The underlying principle behind HDTV is to recreate the movie experience by increasing the visual field contained in an image. Basically, it means more resolution within a more immersive aspect-ratio (16:9). It also means a surge in technical requirements.

A conventional NTSC signal has 525 lines scanned at 29.97Hz for a 4.2MHz minimum bandwidth to carry the analog video off of a 6MHz channel [0]. When digitized and compressed, this signal can be recorded on a DVD and its bit rate varies from 2 to 10 Mbits/s (adaptive) with an average of 4 Mbits/s. For comparison, a typical HDTV feed has roughly 5 times the resolution. All things being equal, the transmitted bit rate should be around 5 times more important to deliver similar performance. Imagine everyone swapping their cars for motorhomes and they are all trying to reach their downtown office at the same time. Unless you could do some major urban reconstruction in a blink of an eye, it’s safe to say that you won’t be getting to work anytime soon. The challenge is very similar when integrating and delivering HDTV to the consumer. The roads that were built are now simply not big enough.

Whether it’s the traditional over-the-air (OTA) broadcast, the cable company’s set-top box or the satellite-TV provider, they all have a limited amount of bandwidth to send you all these feeds, to which, they add other bandwidth-intensive services such as interactive broadcasts, subscription channels, TV schedules, etc.

So what is the solution? Compression.

Digital Video Compression
Even if you could magically double your bandwidth, it would still not be enough to meet the needs of the brave new digital world. This is where codecs (Coder/Decoder) come into play. The most commonly used method today to compress digital video data is called MPEG-2.

From current satellite streams and digital cable feeds to off-the-air digital broadcasts, MPEG-2 has now been internationally adopted for a variety of applications.

In a nutshell, MPEG-2 first exploits temporal redundancy through motion estimation and then proceeds to spatially subdivide the image in 8x8 blocks upon which the DCT (Discrete Cosine Transform) is applied to exploit spatial redundancy. Compression is done by quantizing resulting DCT coefficients and re-ordering them to maximize the probability of long runs of zeroes, and then run-length coded. Finally a Huffman encoding scheme is used. The whole process allows for great savings in terms of bit-rate ratio (>10:1).

However, these savings don’t come free, and because the codec discards some of the original video information, there can be serious side-effects; MPEG-2 is what we call a lossy codec. It discards image information believed to be of lesser visual importance. The more you want to compress, the further away you get from the look of the original image. Image quality and fidelity now depends on the chosen (or often imposed) level of compression. And since that is directly tied to the available bandwidth, we must ask ourselves when is the video simply too compressed?