There are many factors defining a good HDTV image. Among these are brightness, contrast ratio, colorimetry, noise, artifacts, and detail. When all other factors are equal, the ability to reproduce detail is what differentiates HDTV from any other TV. Detail defines the "high" in High Definition. One of the many detail related parameters not usually considered in a pedestrian analysis of HDTV picture quality if something called MTF. MTF stands for Modulation Transfer Function, a rather arcane way to describe the frequency response of an optical system or component. The following may be a little "technical" for some, but please bear with me.

MTF is not a measurement of light frequencies (i.e. color), rather it is a measurement of the ability of optical components to transfer changes in light intensity (or contrast) as a function of the frequency of those changes. Similar to audio or electrical transfer components and circuits (such as amplifiers), in an optical component the higher the frequency of contrast changes, the lower the contrast transfer of those changes, i.e. the lower the MTF. MTF is measured as a percentage of optimum contrast transfer. An MTF of 100%, therefore, means there is no frequency related attenuation of the light. An MTF of 50% means half of the transfer contrast is lost at a specified contrast variance rate (frequency). Got that? If so, please read on.

All imaging components have a measurable MTF. This includes all lenses, prisms, and optical filters. In a front projection TV system there are many lenses, including the projection screen. The projection screen usually consists of a sandwich of two screens, the Lenticular and Fresnel, which are essentially an array of lenses.

Several studies have shown that the human eye, assuming 20/20 visual acuity, can just discern one pixel in a one million pixel 16x9 array at three times the image height, assuming the array is unaffected by any imagine components i.e. it has a 100% MTF. This is similar to being able to see an individual pixel on a 1260 x 720 flat display, such as direct view plasma, LCD or picture tube, at three times the image height. (Note that picture tubes have convergence and focus problems that prevent constant resolution over the entire screen surface.)

However, with projection systems, the lower MTF, due to the lenses and the screen, precludes this 100% degree of acuity. The deleterious performance of projection systems is also manifested in another manner. Since sharp image edges consist of a summation of increasing higher frequencies, a low MTF results in a loss of the highest frequencies comprising the edge. The result is the edge is no longer sharp, and can spread over several pixels.

It is not unusual for an HDTV projection system to have an MTF as low as 45%. Good grief! With that percentage one would need his nose in the screen to see the same detail as a 100% MTF display (assuming the nose is small). In spite of this, HDTV projection performance remains better than SDTV because of the limited electrical frequency response of the latter.

So what does all this mean? It means that any display system with optical components, such as projection systems, cannot produce an image with the degree of detail as a direct view system. Therefore, if a direct view HDTV display system appears sharper than a projection system of the same screen size, that's because it is.

Ed