Is a scaler a scaler regardless of the brand? Experience teaches us otherwise—the quality of a scaler can vary considerably from one manufacturer to the next. But, don't all scalers use the same "scaling chip?" Don't they all use the same scaling techniques? Don't they provide the same video quality? In a word: "no." Scalers can vary greatly in the scaling techniques they use. Scaling is not a one step process, but a sequence of processes, and the final image quality can be considerably affected by the way any one of these processes is done. Surprisingly, the actual scaling part of the process may have the least effect on final image quality.
Video Scalers Versus RGB Scalers
Before getting into the common misconceptions, it should be pointed out that there are two main types of scalers: video scalers and computer scalers. A video scaler will typically accept NTSC/PAL/SECAM and decode it and upscale it to a variety of rates. Extron's DVS 150, the System 7SC, and the new DVS 406 are examples of video scalers. A computer scaler will accept RGB/computer video and scale it up or down to another computer resolution (or HDTV or plasma resolution depending on the scaler). Extron's new DDS 402 and new USP 405 are examples of computer scalers (the USP 405 will also scale video).
A video scaler will first decode and de-interlace a signal before actually scaling the video. Therefore, the way the decoding and de-interlacing is done is extremely important because the scaling section can't improve on the quality of the video it receives. If the decoder or de-interlacing section compromises the video quality, that quality is lost forever. Many of the following misconceptions specifically apply to a video scaler.
A computer scaler doesn't need a decoder or de-interlacer since the incoming video is already in a form the scaling section can process. The quality of a computer scaler is more dependent on the scaling engine itself than is a video scaler, but other factors can affect the output image quality. For instance, mismatches between the input and output video refresh rates may cause glitches with even the best scaling engine. Extron Electronics created its patent pending Accu-Rate Frame Lock (AFL™) technique to eliminate such glitches.
Scaler Topology
As Figure 1 shows, the major steps in a video scaler are decoding, A/D conversion, de-interlacing, scaling and finally, D/A conversion. The first stage of scaling is the decoding stage. This is the most important stage of the process in that any image degradation that occurs here cannot be improved or overcome by later stages.
The decoder will take an incoming composite signal and break it into its components, chroma and luma. Usually the decoding stage includes the analog to digital conversion process. Keep in mind that a manufacturer has a choice in the types of decoder and A/D chips (and methods they use).
Another consideration with a decoder is stability, that is, how well it locks to incoming signals. Stability varies from decoder to decoder. If a decoder with low stability is used in a scaler, the image may break up while using poor quality videotape in a VCR or while viewing a tape during fast forwarding or rewinding. Again, the selection of the right type of decoder in a scaler can make a major difference in the overall operation of the scaler.
De-Interlacing
The second stage in a video scaler is the de-interlacing stage. Like the decoding stage, the de-interlacing stage can have a significant effect on overall image quality. In this stage the interlaced video signal is de-interlaced, that is, converted from two fields into one progressive field. There are a number of ways to do this conversion—and each method has its own advantages and disadvantages. The de-interlacing stage can be used to correct potential image problems by altering the way de-interlacing is done, according to the type of video the scaler detects. Two examples of this are how a scaler handles video that originated from film and how a scaler handles video with motion in it.
The scalers in many Extron products, such as the ISS Series and the ISM Series, System 7SC, DDS 402, USP 405, and DVS 406 all have a feature called 3:2 and 2:2 pulldown detection. When film is converted to video a process is used called 3:2 pulldown (for NTSC) and 2:2 pulldown (for PAL). This process matches the film speed to the video speed. The problem is 3:2 and 2:2 pulldown introduces artifacts into the video such as jagged/uneven diagonal lines. A scaler with 3:2 and 2:2 pulldown detection will recognize that the incoming signal was originally made on film, and will use a special algorithm in the de-interlacing stage to compensate for the artifacts caused by the 3:2 or 2:2 pulldown process.
Motion is another issue: a good scaler will take it into account. If a scaler does not properly recognize and process video with motion, artifacts, such as jaggies, will be introduced into the image. The key is how the video is de-interlaced. For instance, the simplest way to de-interlace is to combine the two video fields into one full frame. This will work fine for still images, but if there is motion in the video there will be jaggies because the position of the lines will shift between the fields, due to the motion. Other de-interlacing techniques work better for motion. The scaler needs to recognize the motion content and still content of each field and apply the correct technique, as needed. Extron has developed Dynamic Motion Interpolation (DMI™), a technique which allows the scaler to compensate in order to avoid motion artifacts.
Now that we have covered the basics of scalers and their operation, the common misconceptions can be discussed:
Misconception #1: A Scaler is the Same as a Scaling Chip
A common misconception is that a scaler and a scaling chip is almost the same thing. As has already been shown, scaling is only a small part of what a common video scaler does. The scaling stage is only one of several stages within a scaler. Currently no manufacturer makes a single chip that accepts composite video and outputs a high resolution PC rate.
Misconception #2: The Scaling Chip is the Most Important Part of a Scaler
You might think that since a scaling chip does the actual scaling in a scaler, the scaling chip is the most important part of a scaler. However, in a video scaler the video is first decoded and then de-interlaced before it before it ever gets to the scaling engine (see figure 1). If video quality is lost in the decoding or de-interlacing stage, the scaling chip can't improve on the quality. The decoding and de-interlacing processes are significantly more complicated than the scaling process. Therefore, the actual scaling engine has very little effect on the overall image quality compared to decoding and de-interlacing.
Misconception #3: There is One Type or Brand of Scaling Chip
Another common misconception is that there is one company that makes a scaling chip that all manufacturers use. In fact, several companies make scaling chips, and they're not all the same. They vary in terms of the algorithm they use (the way they add lines), sampling rates, and so on. The chip that is used will affect final output quality—and the best chip for a scaler may vary depending on the application for which a scaler is used. It is also possible for a scaler manufacturer to create its own scaling hardware. Many times the scaling hardware a manufacturer creates for its own requirements can provide a better solution than the off-the-shelf scaling chip that most manufacturers use.
Misconception # 4: Use an External Scaler to Bypass the Internal Scaler?
Since the actual scaling operation has the least effect on image quality, chances are the scaler in a projector won't compromise the image quality. When you use an external scaler with a projector you can, in some cases, bypass the scaler in the projector. However, it is bypassing the decoding and de-interlacing processing of the projector (going into the projector as RGB) that offers a better chance of improving image quality. Therefore, if you use a scaler with decoding and/or de-interlacing, you can improve the image quality, even though you're not actually bypassing the scaling section of the projector.