Tonight the Amsterdam Avenue loading dock of the Metropolitan Opera House will be crammed with two vehicular behemoths - a Ford 850 and a GM 5000. Inside them will be millions of dollars' worth of electronic equipment and several dozen video technicians with highly specialized skills.

Those skills will be applied to bringing to TV screens the Met's bright new production of Bedrich Smetana's "The Bartered Bride," a work with an all-star cast that is something like a cross between "Fledermaus" and "Meistersinger." It is a work that has lain dormat in the Met's repertory for 35 year while many patently inferior works have been regulars.

It is second of four "Live from the Met" broadcasts this season on PBS - part of a burst of live cultural programming that began on public television early in this decade and has spilled over into the commercial networks. Tonight's "The Bartered Bride' will be transmitted, starting at 8 p.m., to the 60-idd PBS outlets with simulast capability, which means it will be available to about half the TV audience.

The scene in that GM 5000 during the dry run of "Bride" at the Met last Saturday night was about as busy as what was going on in the orchestra pit, and in many ways was not unlike it. Each, for instance, has a conductor. For the orchestra there was the Met's music director, James Levine. For the technical staff, which is about half the size of the orchestra, there was Kirk browning, who has been for almost three decades the preeminent director of televised musical events.

Fortunately, things were considerably more orderly in the pit than in the truck, where Browning, through an earphone with a mike attached, literally called the shots - hundreds of them - to his five cameramen and the rest of the crew. The cameras alone cost $100,000 each.

Browning and his chief assistants were sitting before a console to control the image that was to go across the country. On it were amultitude of buttons and above it were many dials and nine television terminals, one each for the cameras - three of them in boxes and one on each side of the orchestra.

Thus Browning had a choice of three hours and 15 minutes (Counting taped intermissions) of five different camera images to air, each of which he could refine with shouted directions on things like camera angles and cropping.

That was the control room. There was also an audio room. A video transmission area, and a center for making tapes and cassettes.

Just before the start, Browning, 57, with a thick shock of white hair who jogs at 6:30 a.m. daily, rain or shine, warned: "This is the first time we've shot an overture in the pit and it's going to be tricky."

It is 10 seconds before 8 p.m. and Browning begins a spaceshot-like count down: "Ten, nine, eight . . ."

Then comes a taped opening package that goes on for two minutes and one second! "Roll tape," commands Browning, and the evening begins. Then he says into the mike. "Standby, Francis," addressing Met host Francis Robinson, who is in a box at the back of the house, where he greets the viewers and explains on the air: "This is an opera we are doing in English in which nobody dies and nobody gets killed, which is a rare thing in opera. Nonetheless it is a deadly serious subject."

After a taped summary of Act 1, Levine enters the pit, bows, raises his baton and Browning's directions begin to fly. "Head where the action is . . . Pick any violin that looks good to you . . . Next to the conductor and then pan left to the bassoon and then down to the flutes . . ." And so on.

A red light shines above each screen when that particular image is being aired. Short code names are used for the singer. Teresa Stratas is "Terri." Male lead Nicoli Gedda, Jon Vickers and Marti Talvela are "Nicks," "Vic" and "Mick."

The curtain goes up on th ehumble Bohemian village scene and all five cameras go into operation. The cameras follow the action - sometimes sweeping across the stage, sometimes dwelling on a single character or group. When images are similar Browning will say, for instance, "I think two's better, but could you get it in just a little tighter."

Browing makes notes on corrections that are to bem made tonight. Example: "Peter Allen (the announcer) has got to wrap up the end of Act 1 faster."

And when Allen remarks at the end that Vickers has "two fools to pay this year, the stuttering Vasek in this opera and, in the spring, the title role in Wagner's "Parsifal," Michael Bronson, the Met's business manager, slaps his forehead and utters, in exasperation "Did I hear what I think I heard?"

Parsifal is a young innocent who devotes his life to the search for the Holy Grail. "Lord," Bronson says, "if that ever got on the air, the letters would really flow in."

Browning is the most experienced and famous director in the classical music field. "I'm not a trained musician or engineer and I got into this field on a wild fluke," he says. "I was out of the war [he served in the British army], and, of all things, I had an eggs to Samuel Chotzinoff [the producer of the NBC Opera].

"Chotzy hired me in 1949 I think one reason was I was interested in music but didn't play an instrument. I don't think I earned more than $35 a week as a stage manager - my first real job. But the training and experience were priceless in those days of early, live TV. And you could move up so fast because it was new to everybody. I was a producer in a few months." Browning stayed with NBC for 12 years before becoming an independent producer.

Browning had two weeks to choreograph the filming of "The Bartered Bride," and his detailed plans fill a book that resembles a large orchestral score. This is based on a tape made of the production at the final rehearsal. But there is a not always that much time. Earlier in the fall he directed, all within one week, a concert with Rudolf Serkin and the New York Philharmonic under Zubin Mehta for the "Live from Lincoln Center" series on PBS, a concert with Vladimir Horowitz and the Philharmonic under Mehta for NBC, and an "Otello" for "Live from the Met."

Browning believes the TV camera should be as unobtrusive as possible at a live performace. He disapproves of the bright, hot lights that have annoyed audiences at Wolf Trap when performance there are televised. "They just aren't necessary. Some people think you get a better image, but quick frankly I can't tell the difference. And anyway, the performance should always come first I get a perfectly good image simply by adjusting the light levels on stage, and it's always noted in the program book."

Browning rarely deals with the singers and has consulted with none of them in this production. They don't know when they are on and off camera, in line with Browning's policy of non-interfence.

He expressed delight when an observer who attended the televised Horowitz says he could not figure out where the camera for the keyboard shots was placed. Said Browning, "That's great: That's just what I wanted. I was a minicam between the pulled curtains at the window next to the piano."

The burgeoning of televised opera has been a blessing for the deficit-ridden Met. "Since our fist televised opera, the "La Boheme'," ways Bronson, "Opera Guild membership has risen from 60,000 to 110,000 - that's in just a little over two years. And the Opera Fund has gone up from $350,000 to $1 million. As of now we'll continue televising four operas a year, but we hope to go higher."

Tonight's telecast costs "a little over $300,000," said Bronson. "In other words you get three and a half hours for a little less than an hour of "starsky and Hutch.'" It is financed by Texaco, the National Endowment for the Arts and the Charles E. Culpeper Foundation (Locally it will be aired at 8 p.m. on Channel 26 and WETA-FM).

Bronson hopes that in future years the number will go well beyond four, and Browning foresees a real boom in televised classical music with the advent of widespread cable television. But even now 90 percent of Browning's work is in classical music.

Browning's associate, Alan Skog, however, still works intensively in an other medium - soap opera. He directs "As the World Turns."