The first computer that office workers encounter is often a dedicated word processor--basically a smart typewriter with memory storage capability, a display screen and special built-in software for cranking out correspondence and reports. Even the world`s worst typist can produce perfect copy on such a terminal, because it lets you catch mistakes on the screen and make all corrections on the keyboard before you print.

On the home front, such time- and trouble-saving word machines have been a long time a-coming--forcing writers either to make do with low-intelligence typewriters or to invest in high-cost computers with lots of extraneous features.

This year, though, two new dedicated word processor systems from Magnavox (North American Philips) and Smith Corona have appeared that promise ease of use and none of the extravagance, each at a reasonable $800 list price.

The Magnavox Videowriter, billed by its makers as ''a revolution in writing,'' is certainly the more interesting of the two word machines. The compact, easy transportable 18-pounder was designed from the ground up for one purpose only.

''Videowriter is the first computer appliance for the home that is not multifunction,'' explains Donald Johnstone, president of Magnavox`s parent company, N.A.P. Consumer Electronics Corp. ''It won`t balance your checkbook, design a building or chart the next mission to Mars. What it will do is let consumers concentrate--not on White Out, retyping or complicated computer use --but on writing.''

Special touches include a logically arranged keyboard, a sophisticated word processing program with split-screen capability, a built-in 3 1/2-inch disc drive (only one is necessary, because the software is on a solid-state chip), an odd-sized 3 1/4-inch-tall by 8-inch-wide amber display screen and a built-in thermal printer with multiple typeface options and motorized single- sheet paper feeding.

Another nice feature: Videowriter automatically saves a user`s pearls of wisdom onto disk whenever the system is left alone for more than a couple of minutes or when it`s asked to perform any special function--making the system virtually idiot-proof.

The Smith Corona Personal Word Processor (or PWP) is a $500 list add-on package that converts a Smith Corona Messenger model or Spell-Right compatible typewriter (about $300) into a word processor. As Smith Corona marketing vice president Fred Feuerhake sees it, ''The PWP system offers the consumer the best of both worlds. It delivers word processor functions that some people have learned in an hour, and it performs as a typewriter when the user just needs to type a single letter or envelope.''

For formal business correspondence, the letter-quality, daisy-wheel printing of the Smith Corona system gives this word processor an edge over the clean, but not quite as crisp, copy produced by Magnavox`s thermal printer on smooth paper (bond paper is a no-no).

In home work applications, the Videowriter`s thermal printer is preferable for its quietness and comparative speed--25 and 50 characters a second in its draft and near-letter-quality modes, versus 12 cps on the PWP daisy wheel printer.

The PWP`s separate 12-inch green screen monitor, displaying 24 lines of text, is likewise superior for office applications to the Videowriter`s small, built-in amber screen, which displays 18 lines of sharp (but tiny) text, plus special help messages on the right quarter of the screen.

Ultimately, though, the Smith Corona package`s multipiece construction and retrofitted word processing programming proves the system`s downfall, seeming cumbersome and antiquated next to the sleek and user-friendly Magnavox Videowriter.

I found it necessary to study two manuals to master the tricks of the PWP, while the Videowriter is so straightforward--with its dedicated operating keys and on-screen user prompts--that I could crank out letter-perfect copy on it without ever flipping open the owner`s book.

It`s also a snap toting the Videowriter from room to room for instant set-up on a table top. The terminal-printer box and cable-attached keyboard require a minimum of 22 inches of depth--a tight fit on some desks but space that`s readily available at a kitchen or dining room table.

To prevent neck strain from looking down at the display screen, the Videowriter box needs a bit of propping up. I`ve made do with inch-thick wood blocks underneath the front corners, and a half-inch lift on the back to tilt the screen up. A $39.95 Moni Base available at computer dealers performs the same function.

The Smith Corona system is much klutzier, filling an entire desk top with components: the typewriter, a separate word processing keypad, the green-screen monitor that sits atop a central processing unit box with built-in software and micro-wafer cassette tape drive, plus a couple of connecting cables. On all but the deepest tables, the SC monitor screen and word processing module and wired keypad must be placed to the left or right of the typewriter. So a user`s head and hands are constantly in motion, keeping tabs on the sprawling operation.

As a marketing strategy, Smith Corona seems wise and kind in offering its word processing system in two steps. Their way, you can invest first in an excellent typewriter such as the full-featured Smith Corona XD 7000 and then get smarter with the computer accessory.

But if the company is serious about dedicated word processors, Smith Corona ought to come out with a typewriter that has cursor keys and other special-function word-processing buttons built in. As the system now sits, a user must juggle controls on two keyboards and memorize lots of two-key codes to access special features--which is just as cumbersome as using a nondedicated computer with a word processing program.

I also think the Personal Word Processor should forgo its cheaper, slowspeed microtape memory storage system in favor of a disc drive, as is installed in the Magnavox Videowriter.

But as it stands today, Videowriter is already a joy, a machine so efficent and logical it becomes an extension of your brain and fingers, practically invisible. For the high school or college student with lots of papers to do, the business person who prefers the peace and quiet of report writing at home or in a hotel room or the freelancer who likes to turn out whistle-clean copy, Videowriter is the ticket.