Amazon (AMZN) launched a new service called Amazon Video on Demand, which allows users, for a fee, to watch any of 40,000 TV episodes or movies, in real time, on their Windows and Macintosh computers, and on specially equipped Sony Bravia TV sets. This service replaces an earlier Amazon video offering called Unbox.

I have been testing Amazon Video on Demand and I found it worked well, although it has some limitations. The user interface is clean and smart, the quality is good if you have a fast Internet connection, and there are some clever features. On the downside, it works poorly with the slow connections typical in places like hotels. And there are some studio-imposed limits on what content is offered and how you can view it.

To date, there have been three major models for legally getting TV shows and movies from the Web.

In one, best represented by Apple’s (AAPL) widely used iTunes store, the TV shows and movies are ad-free, but you pay for each. All of this content is downloaded to your Windows or Macintosh computer or your Apple TV set-top box for later viewing, even when you’re not connected to the Internet — though on Apple TV you can start watching while the material is being downloaded.

A second model, best represented by the studio-owned Hulu.com, presents movies and TV shows on a Windows or Macintosh personal computer free of charge, but requires you to watch commercials that can’t be skipped. The TV shows and movies you view on Hulu are “streamed” rather than downloaded, meaning they are meant to be viewed immediately, in real time, rather than stored for viewing when you’re not connected to the Internet.

The third online model is best represented by Netflix (NFLX), the popular DVD distributor. It offers a “Watch Instantly” streaming option on Windows PCs or on TVs equipped with a special set-top box, for a small portion of its large catalog of TV shows and movies. These videos are ad-free and don’t carry an individual charge, but require a monthly subscription fee.

The new Amazon Video on Demand service is a hybrid of these others. As on iTunes, the TV shows and movies it offers are ad-free and purchased individually, don’t require a subscription, and work on both Windows and the Mac, plus on one type of set-top box. In Amazon’s case, that’s Sony’s Bravia Internet Link, a $299 device that works only with Sony (SNE) TVs.

But, like Hulu and Netflix and unlike Amazon’s older Unbox service, the new Amazon Video on Demand service offers videos via real-time streaming. In many cases, it also allows downloading, iTunes-style, to Windows PCs (but not Macs) and to TiVo devices attached to a TV. The videos can’t be streamed in real time using a TiVo (TIVO).

Amazon’s streaming videos are viewed in any of the major Web browsers and don’t require any special software. I tested the new service on both Windows PCs and Macs, and on a Sony Bravia TV equipped with the $299 adapter box. In my tests, Amazon’s videos looked quite good over a fast Internet connection. However, on a typically lousy hotel Internet connection, the movies were often grainy and kept stuttering.

The new service doesn’t yet offer videos in high definition, something Apple just announced this week it is adding for some TV shows. Amazon says it is working on HD.

Amazon’s user interface for the new service is very nice. All of your purchased videos are available in a library stored on the company’s servers, so you can easily watch them again and again if they are purchased, or resume a partly watched rental. And the service remembers where you stopped watching a video and resumes it at that point, even if you started it on, say, a Dell (DELL), and resumed it on a Mac.

On the downside, the new Amazon service isn’t always simple or consistent. For instance, you can watch only two videos at a time, and not all titles can be either streamed and downloaded, or be either purchased or rented. Most rentals last 24 hours, but some differ.

Selection was OK, but not great. Because of studio policies, many current and recent movie hits aren’t available. There are gaps as well in the TV selection. For example, while iTunes offers the current second season of the excellent “Mad Men” series, Amazon has only the first season.

Prices also can be confusing. Amazon rents most movies for $3.99 and sells them for between $9.99 and $14.99. TV shows generally cost $1.99. But some titles carry different prices, albeit these are often lower.

All in all, Amazon Video on Demand is a good service for people who prefer paying for ad-free TV shows and movies, and is another strong step in the Internet’s rising competition with traditional TV.

Find all of Walt Mossberg’s columns and videos online, free, at the All Things Digital Web site, walt.allthingsd.com. Email him at mossberg@wsj.com.

Partisans of these platforms can argue all day about which is better. But the competitors have some things in common. Windows computers and Apple Macintoshes can both display the same photos and office documents, and play the same MP3 songs and YouTube videos. BlackBerrys and Treos can receive the same emails and call the same phone numbers.

It isn’t perfect. There are some files that don’t play well across platforms. But most common content does.

Yet there’s another technology competition that doesn’t share content well and offers few, if any, benefits to consumers. It’s the fight among two groups of technology companies and movie studios to sell a successor to the DVD. Each has developed a new type of disc that has the ability to show movies in high definition. To my eye, the pictures they deliver are identical.

Neither of these competing new discs, called Blu-ray and HD-DVD, works in current DVD players. They require very expensive new players, and the new players can’t handle both new disc formats, only one or the other.

Adding to the annoyance: Some movie studios release movies only in one of the two formats. Paramount and Warner Brothers support both formats, but Universal supports only HD-DVD, and Disney, Fox and Sony only Blu-ray.

So, in order to be sure you can play any movie released as a high-definition disc, you would have to replace your old DVD player with two new, much costlier players. It would be like having to buy separate TV sets to watch different networks’ programs.

Now, however, one gutsy company, LG Electronics, of Korea, a longtime member of the Blu-ray camp, has broken ranks and introduced a new combo player that can handle three formats: Blu-ray, HD-DVD, and regular old DVDs. It’s called the BH100 Super Multi Blue.

I’ve tested this combo player and found that it plays both new formats, as well as regular DVDs, just fine. But it’s more expensive than most single-format players and has some serious limitations when navigating through the menus on HD-DVD titles. For now, I can only recommend it for serious videophiles with deep pockets, but I’m hoping it’s the start of a trend that will end the foolish war.

The BH100 costs $1,200. That’s vastly more expensive than the newest DVD players, which, for less than $100, can take a regular DVD and “upscale” it so it looks better on a high-definition set. But that $1,200 isn’t so outrageous if you compare it with the price of buying two separate Blu-ray and HD-DVD players, which can reach or exceed $1,000 total. And the new LG takes up only one input on your TV, occupies less space on your component shelf and requires just one remote control.

I tested the LG combo player on my high-definition TV with this year’s Oscar-winning best picture, “The Departed,” and with “Superman Returns,” each of which is available in both of the new formats, as well as on DVD.

All played perfectly. The picture looked great in both formats and was noticeably better than an upscaled DVD image, which the LG unit also can produce. The LG outputs both new formats in a high, but grossly overhyped, resolution called “1080p.”

But the BH100 did a much better job with the Blu-ray discs than with the HD-DVD titles. That’s because while the combo player can play HD-DVD movies perfectly, it can’t display the HD-DVD discs’ menus for selecting scenes or accessing special features.

These menus usually offer the title and a photo to identify a scene, and the title and/or a description of the special feature. But on the LG BH100, the HD-DVD menus have no pictures, titles, or descriptions and look nothing like the original. They only identify scenes by number and duration. That makes it hard to find, say, the deleted scenes from “The Departed,” or the documentaries on the Superman disc.

The BH100 was based on a Blu-ray-only player and lacks the special chips HD-DVD players use to display the menus properly. LG had to concoct its own rudimentary replacements for those menus. The company says a future combo model could include the chips and thus display the HD-DVD menus as well as it does the Blu-ray menus, but it hasn’t decided whether to make such a product.

One reason for that decision may be the competing approach to solving the stupid disc war. Warner Brothers is working on a combo disc, instead of a combo player. This disc would hold both the Blu-ray and HD-DVD versions of a movie, so you could pop it into whichever type of player you own.

Until the electronics and movie companies support universal high-definition players and/or universal high-definition discs, I don’t recommend that most people invest in either technology. Why prolong a war that’s bad for consumers?

Email me at mossberg@wsj.com. See video versions of my reviews at wsj.com/mossbergvideo.

The trouble is, it’s hard to find a DVR that can record in high definition, so most people wind up simply going with the bare-bones high-definition DVR capability built into the set-top box supplied by their cable or satellite service.

But TiVo, the pioneer in digital video recording, has recently entered the high-definition recorder market with a high-end, high-priced product. It’s called the TiVo Series3 HD Digital Media Recorder and it sells for a whopping $800, as much as some HDTVs themselves. And that doesn’t include the $12.95 a month it costs to subscribe to TiVo.

I’ve been testing the new TiVo and I like it a lot, but it’s hard to swallow that huge price, especially since the new Series3 model doesn’t include some nice features available on the much cheaper Series2 version, which doesn’t record in high definition. It also can’t handle certain cable features.

So, why not just stick with the high-definition DVR supplied by the cable company? After all, while it isn’t free, it’s cheaper than the TiVo.

The answer is that, at least in my recent experience with the nation’s biggest cable company, Comcast, the high-definition DVR it supplies is just awful. If cable boxes were sold at retail like consumer-electronics devices, the Comcast DVR I tested, built by Motorola, would get creamed by better competitors.

My Comcast box, a Motorola DCT3412 I, which Comcast rents for about $12 a month, holds a maximum of 15 hours of high-definition programming or 60 hours of standard programming. The TiVo holds up to 35 hours of high-definition programs or up to 300 hours of standard.

Also, the user interface on the Comcast box is crude and confusing — nothing like the elegant interfaces people have become used to on their personal computers and devices like iPods. The TiVo interface, by contrast, is effective and attractive.

The worst problem is that the Comcast box flubs the basic functions of a DVR. It is maddeningly slow at responding to commands sent by the remote control to pause, play, fast-forward and rewind. You press pause and nothing happens. So you press it again. You try to return to normal speed after fast-forwarding through commercials and the unit takes so long to obey your command that you badly overshoot the resumption of the program.

This latency problem didn’t affect just one dud of a Motorola box. In our home, we have four of these units, and three have the problem. All, of course, share the capacity limitations and user-interface problems.

In the program grid, even on a 50-inch, high-definition screen with acres of room, the Comcast box displays just four rows of stations at a time. Until recently, there was a fifth row, but now that has been replaced by an ad. The ad not only sucks up space, but also is aggravating because it gets selected each time you reach the bottom of the grid screen.

Advertising is fine, but in this case, sacrificing 20% of an already paltry information screen for an ad just shows contempt for users.

By contrast, the basic TiVo grid shows eight rows of stations at a time, and offers an alternate view that packs in even more information using two vertical columns: one displaying stations and the other showing a list of shows scheduled in the coming hours.

And, unlike the Comcast box, the TiVo Series3 can be programmed from a Web site, so if somebody at the office tells you about a great show, you can tell the TiVo to record it long before you get home. The new TiVo can also play music and display photos that are stored on Windows and Macintosh PCs on your home network. The Comcast box can’t.

But the TiVo also has some downsides. Unlike older TiVos, it’s intended to replace, not complement, a cable box. So, installing it requires a visit from cable-company technicians to install gadgets called cable cards that plug into the back of the TiVo. In my case, that process took over two hours. Even worse, these cable cards don’t support Comcast’s on-demand feature, which allows you to see certain programs and movies whenever you choose.

And the new Series3 lacks the capability of cheaper TiVos to let you transfer recorded shows to computers and portable devices.

Also, unlike the Comcast box, the TiVo doesn’t have a filtered grid display showing only high-definition shows, which is handy once you become addicted to HD.

Fortunately, it may be possible to get some, but not all, of TiVo’s superior features by just waiting. In 2007, Comcast and TiVo expect to roll out an option for downloading TiVo software to Comcast boxes. This would provide the TiVo interface without sacrificing Comcast features such as on demand. The pricing and details haven’t been announced. Comcast is also working on other new user interfaces and features using non-TiVo technology.

But, for now, the choice is tough. The Comcast high-definition DVR is a cheaper, but flawed product and the TiVo Series3 is an excellent, but overpriced one.

Email me at mossberg@wsj.com.

To find out, my wife, Edie, and I lived for several weeks with a big, beautiful HDTV, the Pioneer Elite PRO-1140HD, lent to us by Pioneer Electronics. It has a 50-inch screen, a long list of features and lists for $5,000, though you can find it for under $4,000 on the Web.

I tested the Pioneer with digital cable service from Comcast, my local cable provider. This service transmits high-definition programs where available and can record them to a digital video recorder (DVR) built into its set-top box. Comcast also has an on-demand feature that allows you to watch certain programs whenever you like.

The test demonstrated why people are so hooked on HDTV. The Pioneer Elite set performed brilliantly and was a joy to watch with HDTV programming. With HDTV, you are not only increasing the size of the picture, but its quality as well. On the Pioneer, colors popped, details I never saw before emerged, and the whole experience was almost cinematic. DVDs looked great, as did content from a computer plugged into the set.

But there is a hitch: Most TV programs aren’t available in HDTV, and these non-HDTV shows can actually look worse on an HDTV set than they do on older, standard TV sets. So do most videotapes. Also, buying a big-screen HDTV carries hidden costs and hassles. You may well need help installing the set. You may also have to switch or upgrade your cable or satellite service, get a new DVD player and buy new furniture.

The Pioneer Elite model I tested happens to be a plasma TV, which is one of the three major types of HDTV sets. It works by stimulating a captive gas with an electrical charge. The other two are LCD, or liquid crystal display, which uses a screen like those on laptop computers; and “microdisplay” sets that project the image onto the screen from the rear of the set, mainly using two technologies: DLP, or digital light processing, and a form of LCD.

Plasma TVs tend to have the blackest blacks and the best ability to be viewed from all angles of a room. Their colors are warm and vivid. And they cost less than LCDs in large sizes. But their screens are more reflective and a bit darker than LCD screens. There is also a slight chance they can suffer “burn-in,” the permanent embedding of an image, like a network logo, if you leave such an image on for a very long time without changing channels.

LCDs are bright, and they are the thinnest and lightest of the HDTVs. But their colors often seem cold and their blacks too gray. Their viewing angles aren’t as good as with plasmas. And in some cases, fast motion can look blurry.

Microdisplay sets typically cost the least, but they are the thickest of the three types. They tend to have limited viewing angles and can display a “rainbow” effect, which causes problems for some people.

Our Comcast service gave us high-definition channels from all the big broadcast networks and some of the major cable ones. We immediately started watching shows like “Grey’s Anatomy,” “Desperate Housewives” and “Heroes” in high definition, and found they were greatly enhanced. Watching the World Series and NFL games was a great experience, with every clump of dirt, bead of sweat and blade of grass looking so much more real.

The Pioneer also did a great job with an Apple Mac Mini computer we plugged into it, displaying family photos and downloaded videos stored on the Mac.

The big downside was that only a small fraction of programming is high definition. At 8 p.m. on Tuesday night this week, there were just 13 high-definition programs available from Comcast, out of more than 230 total. The on-demand service had a smattering of additional high-definition shows and movies.

And standard TV shows on a high-definition set can look awful. They can be fuzzy. They also typically fill only a portion of the wide screen, with big black or gray bands on the sides. You can eliminate the bands using TV features that stretch or zoom the picture, but these modes either cut off too much or distort people so they look unnaturally short and stout.

Also, we ran into plenty of extra costs and hassles. We had to buy new furniture to hold the TV and all the gadgets that attached to it. We had to replace our DirecTV satellite service with Comcast cable, because the trees in our yard blocked the high-definition satellite signal — which is beamed separately from another position in the sky. The Comcast digital service with high-definition costs more than the company’s standard cable service and its DVR holds only 15 hours of high-definition programming versus 60 hours of standard programming.

Despite all these costs and limitations, we were won over by our HDTV test. After returning the test unit, we went out and bought our own HDTV. We decided that in the slow transition to high-definition programming, there’s now enough content to make HDTV worthwhile. And once you get used to high definition, it’s tough to go back to plain old TV.

Email me at mossberg@wsj.com.