LCD Shoot-Out

MP3 and Portable Video Players

 

Dr. Raymond M. Soneira

President, DisplayMate Technologies Corp.

Copyright © 1990-2006 by DisplayMate Technologies Corporation. All Rights Reserved.

This article, or any part thereof, may not be copied, reproduced, mirrored, distributed or incorporated

Into any other work without the prior written permission of DisplayMate Technologies Corporation

 

Article Links:  Overview  MP3 and Video Players  Smart Phones   Game Consoles

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Introduction

The original audio-only portable MP3 players have rapidly evolved into multimedia devices that include a color display for navigating on-screen menus with colorful wallpaper backgrounds, looking at slide shows of digital photos, and most recently, watching movies and TV shows. And if a TV is available, most of them will display everything on the big screen with their TV Out capability. But, how good are these tiny screens? Are they toys, pretentious devices, or high quality displays that can produce excellent picture quality comparable to your HDTV or computer monitor? To answer these questions I used the same high-powered analysis methods we use for testing and evaluating high-end HDTVs at DisplayMate Technologies. We’re going to find out how worthy these little players are…

 

The Players

In Part I we will be examining five MP3 and Portable Video Players. The Apple iPod (5^th Generation) and Creative Zen Vision:M are mainstream photo/video MP3 players. The Epson P-4000 is a hefty professional photo viewer that can also play MP3 audio and MPEG-4 videos. The Archos 604 and Cowon A2 are full-featured Portable Video Players that can also record in MPEG-4 format. All of these players have large hard drives and download audio, photo, and video content via USB. The Epson P-4000 can also read content stored on Compact Flash and SD memory cards. Table 1 lists the most important display specifications for the five players, arranged by screen size.

 

Table 1 – Display Specs

 

Don’t be turned off by the relatively low resolutions of these players, because it is the pixel density, or Dots Per Inch, DPI, that determines how sharp their images appear, and these are relatively high DPIs. For reference, a 19 inch LCD monitor has a DPI of 86. The number of Screen Colors that a display can produce is frequently misinterpreted as an indication of its color gamut. It has nothing to do with the gamut, but rather specifies the number of possible intensity levels for each of the red, green and blue primary colors, which can vary from 32 to 256 levels. In principle, displays with more intensity levels will produce a smoother image with less visible contouring, but it turns out that there is very little visual difference between them here because of the high DPI screens.

 

LCD Panel Performance

Before we can evaluate the picture quality of the players we need to measure the performance of their LCD panels for Brightness, Contrast, and Viewing Angles. The results shown in Table 2 were made with a laboratory spectroradiometer. See How We Test for technical details and explanations of the measurements. The results are color coded based on the relative performance of all 11 players, phones and game consoles tested for this 3-part series. Green means excellent or significantly better than the other units; red means poor or significantly worse than the other units; and yellow means mediocre performance. The color coding makes it easy to see trends among all of the units.

 

Table 2 – LCD Brightness, Contrast and Viewing Angles

 

Brightness and Contrast:

Peak Brightness is very important in bright ambient light viewing conditions (the brighter the better) but is not important for low ambient light viewing. Black Level Brightness is the residual dark glow that the screen (and each pixel) gives off when it is supposed to be producing true black. It is distracting and also washes out both the contrast and color saturation of the dark portions of the image. It is very important in low ambient light viewing conditions: the lower the better. The Contrast Ratio is generally considered one of the most important measures of display performance. It is calculated by dividing the Peak Brightness by the Black Level Brightness, and it tells you the range of brightness that the display can produce. The larger the better. However, the Contrast Ratio is only relevant for low ambient light viewing conditions due to reflections off the screen (next).

 

Performance under Bright Ambient Light:

When these players are used in bright ambient light the screen won’t appear as dark because it reflects a certain percentage of the room light. This washes out the images by reducing contrast and color saturation. We measured the Screen Reflectance, which is the percentage of ambient light reflected by each player. The lower the better. The Archos 604 was the clear winner. The Contrast Rating for High Ambient Light is a measure of the contrast you’ll see under high ambient lighting conditions. It is simply the Peak Brightness divided by the Screen Reflectance value in the Table. Again, the larger the better. If you frequently watch a player under bright ambient light conditions then this may be the single most important specification.

 

Viewing Angles:

The brightness, contrast and color saturation that you see on the screen varies significantly with your viewing angle. It has a major impact on image and picture quality. If you’re watching by yourself, the Forward Tilt Viewing angle will vary depending on how you hold the player. If you’re watching in a group, people to the left and right of the screen will be positioned with a Horizontal Side Viewing angle. The industry standard viewing angle that is published by LCD manufacturers is not particularly useful here because it specifies the angle where the Contrast Ratio falls to 10, an incredibly low value that is meaningful for digital watches, but not for imaging displays. Table 2 lists the Contrast Ratio for a 15 degree forward tilt of the screen, which is the largest angle a single viewer is likely to use. Table 2 also lists the Contrast Ratio for Horizontal Side Viewing at ±45 degrees, which is roughly what a person sitting next to the central viewer will see. These Contrast values provide a good measure of how the viewing experience varies with angle.

 

Image and Picture Quality Ratings

An LCD screen with low ratings in Table 2 can still wind up looking quite good with the right color scheme because color can be used to enhance the visual contrast in the menu and data screens. That’s how many of these high DPI screens can appear gorgeous. But that technique can’t be used with photos and videos.

 

Gray Scale and Picture Contrast

The image and picture quality of any display depends on its gray scale and saturation of its primary colors. The gray scale describes how the brightness of image pixels varies between the black and white extremes that were discussed in Table 2. The details on how it varies between black and white determines the contrast within an image. Too much is just as bad as too little. We carefully measured the gray scale of each player with the laboratory spectroradiometer. Graphs of the gray scales are shown in Figure 1. See How We Test for details and explanations of the measurements. The differences in gray scales are striking and explain much of the differences in picture quality that we witnessed in the viewing tests.

 

    

  Figure 1. Gray Scales          Figure 2. Primary Colors

Click to enlarge                       Click to enlarge

 

The results are summarized in Table 3 by listing the gray scale Compression and Gamma for each player. Compression of the gray scale near its maximum intensity makes the picture look over exposed. Gamma specifies the steepness of the gray scale and the amount of Picture Contrast, which by industry standards should be between 2.2 and 2.5. The Creative and Cowon players both had very high compression and too much contrast, which was easy to see in the viewing tests.

 

Table 3 – Image and Picture Quality Ratings

 

Color Saturation and Gamut

The colors in all color displays are produced through a combination of three primary colors: red, green and blue. These primary colors need to be strong and saturated in order for the displays to be able to reproduce a wide gamut of colors. Too little saturation and the pictures will look flat and pastel-like, too much saturation and the pictures will look gaudy. We measured the primary colors with the spectroradiometer. Details are shown in Figure 2 and summarized in Table 3 by comparing the Color Gamut of each player to that of a standard High Definition TV or sRGB computer monitor. The closer to 100 percent the better. All mobile displays have a reduced color gamut, which can be compensated with proper processing in the player.

 

Reference and Optimized Photos

The players aren’t delivering anywhere near the image and picture quality they are capable of producing. To demonstrate this we’ve produced two sets of photos for each player: a standard Reference Photo that displays correctly on a calibrated HDTV or computer monitor, and an Optimized Photo that has been mathematically processed based on the lab measurements so that it corrects for shortcomings in each unit. Links for each player are included in Table 3. Download both and compare them to each other and to full resolution versions of the Reference Photo for HDTVs (1280x720) and LCD computer monitors (1280x1024). Left click the links to view on your current display, right click to download the image files. Note that the Optimized Photos are modified for the specific players, and will appear to have incorrect gray scales and color on other displays.

 

After downloading the appropriate Reference and Optimized Photos do an A-B comparison by quickly flipping between them. Here is what you should see on a player, computer monitor or HDTV. A thumbnail of the Reference Photo is pictured at left. Good: Be able to clearly make out ALL of the individual moldings on the fireplace mantle. Very Good: See a natural, smooth and rich facial flesh tone without ANY of the highlights appearing white washed, washed out, or over exposed. Excellent: See the left edge of the fireplace mantle where it meets the light green wall.

 

Videos

All videos are digitally compressed and the resulting picture quality depends critically on how well this was done when the material was recorded. Within the player, video images are generated by an extra layer of software that decodes a stored video data stream. After this processing, it is handled in the same way as the photos and other player images, so the video picture behaves exactly as in the measurements above.

 

All videos degrade in picture quality when higher digital compression ratios are used, which is done to decrease the required file size by reducing the video bit rate. In order to maintain the video picture quality as high as possible we used the maximum video bit rate specified by each manufacturer. The performance and efficiency of the codec used in compressing the video also plays an important role in picture quality. We used the Windows Media Video, WMV, and MPEG H.264 codecs wherever possible because they provide the best picture quality for a given bit rate. All of the tested units except the Epson P-4000 support one or the other. The Epson P-4000 only supports MPEG-4 Simple Profile, but at a high bit rate. The Cowon A2 supports WMV only up to 352x288 resolution, which is less than its native screen resolution, so we used a DivX 6.2.2 codec at 720x480. The optimum video encoding parameters used for the picture quality tests are listed in Table 3. The end result of this careful preparation is that compression artifacts for all of the players were small and seldom noticeable on-screen, and therefore were not a factor in the picture quality evaluations. Note that using lower grade source material at lower bit rates, encoded and/or transcoded with other codecs, will likely result in lower picture quality.

 

TV Out

The Archos, Cowon, and Epson players have screens large enough for three (very friendly) people to view at the same time. But the TV Out capability makes all of the players useful for presentations to much larger audiences. All of the tested players can output their images to a TV with 640x480 or 720x480 resolution signals, which is higher than the resolution of all but one of the tested LCD screens. We evaluated the TV Out Quality by looking at test patterns, photos and videos on a calibrated professional television studio monitor. All of the players produced very good TV picture quality, although we had to readjust some of the TV’s calibrated controls to get the best picture. Note that lower performance TVs may not be able to produce as good a picture quality as the player’s own screen. All of the players include a composite video signal output for the TV. The Universal Dock for the Apple iPod has a higher signal quality S-Video output and the DVR Station for the Archos 604 provides the highest quality RGB and Component Video outputs in addition to S-Video.

 

Viewing Tests and Final Grades

After the extensive lab tests it was time to see how well the units would perform visually with real photos and videos. So, for the Shoot-Out style Viewing Tests, we compared all eleven tested units simultaneously, side-by-side, showing identical content and comparing them to each other and to a carefully calibrated digital HDTV connected to a PC. I used a large selection of challenging photos that we have for evaluating HDTVs. For video picture quality I used a number of movie clips including The Matrix (which has lots of dark content and a subtle green caste that is difficult for displays to reproduce accurately) and Seabiscuit (which has lots of colorful outside scenery and great facial close-ups).

 

Table 4 provides the results. For the grading and rankings I carefully arranged all of the units until they were ordered from best to worst picture quality, left to right. The evaluations are based on ideal viewing conditions with no ambient light and perfect face-on (zero degree) viewing angle. Each display was scored based on how close it came to delivering an ideal picture, so its screen size, brightness, and pixel count are not part of the grade. Looking at photos and videos on all eleven screens at once was an amazing experience. The range and differences in picture quality were staggering.

 

Table 4 – Shoot-Out Results and Grades

 

Two players (and two smart phones, Part II) delivered excellent image and picture quality. The top honors go to the Archos 604, which has the best all-around image and picture quality, and to the Epson P-4000. Both earned As and were the top two performers of the eleven units tested. The Creative Zen Vision:M and Cowon A2 were penalized for producing severely over-exposed photos (bright-end compression) and excessive picture contrast. The Apple iPod produced washed out images due to a triple whammy of low LCD Contrast, low Picture Contrast, and low Color Saturation. Coolness and promotional considerations aside, it simply did not measure up to the other units in both the lab tests and the visual comparisons. Clearly Apple hasn’t yet taken picture quality as seriously as audio quality for its iPods.

 

Much of the reduced image and picture quality of the players is due to sub-optimal processing within each unit. The Optimized Photos (available via links in Table 3) demonstrate the picture quality the units are capable of producing when mathematical corrections based on the lab measurements are applied to the image files. You can download them and see for yourself if you have one of the tested units. The bottom row of Table 3 provides the image and picture quality ratings based on the Optimized images. When viewing them, the differences between players become relatively small and all of the players deliver very good to excellent picture quality. Hopefully, the next generation of devices will improve their software/firmware processing and produce this kind of picture quality on their own.

 

What’s Coming Next

In Part II, we’ll examine the image and picture quality of four high-end Smart Phones and compare them to the Players.

 

Article Links

Series Overview

MP3 and Portable Video Players

Smart Phones

Handheld Game Consoles

How We Test Mobile Displays

 

About the Author

Dr. Raymond Soneira is President of DisplayMate Technologies Corporation of Amherst, New Hampshire, which produces video calibration, evaluation, and diagnostic products for consumers, technicians, and manufacturers. See www.displaymate.com. He is a research scientist with a career that spans physics, computer science, and television system design. Dr. Soneira obtained his Ph.D. in Theoretical Physics from Princeton University, spent 5 years as a Long-Term Member of the world famous Institute for Advanced Study in Princeton, another 5 years as a Principal Investigator in the Computer Systems Research Laboratory at AT&T Bell Laboratories, and has also designed, tested, and installed color television broadcast equipment for the CBS Television Network Engineering and Development Department. He has authored over 35 research articles in scientific journals in physics and computer science, including Scientific American. If you have any comments or questions about the article, you can contact him at dtso@displaymate.com.

 

Copyright © 1990-2006 by DisplayMate Technologies Corporation. All Rights Reserved.
This article, or any part thereof, may not be copied, reproduced, mirrored, distributed or incorporated
into any other work without the prior written permission of DisplayMate Technologies Corporation