Three decades ago Arturo Toscanini was rehearsing for a performance of the Ninth Symphony of Beethoven. With the brilliant Nbc Symphony Orchestra at his disposal, he reached the finale and heard the chorus which had been trained for him by Robert Shaw. When it was over, the great Italian conductor said, "In Robert Shaw I have at last found the maestro I have been looking for."

Tomorrow night on the West Lawn of the Capitol, Shaw, now music director of the Atlanta Symphony, will conduct the National Symphony Orchestra, a quartet of soloists and a special chorus, and the free program will close with that same Beethoven finale.

How did Shaw get into choral conducting? "The real thing was that Mother was a famous gospel singer on the West Coast, really a great voice," he recalled recently. "Daddy was a minister and she was the daughter of a minister. She sang the Dvorak Biblical Songs, studied spiritual singing with Roland Hayes. So we had it all around. She was conducting church choirs, my dad had conducted a church choir; in fact, five of us kids conducted the same church choir at $10 a Sunday to help pay our way through Pomona College. Pomona had won the last national glee club competition. They had a distinguished a cappella tradition, not in the renaissance style so much, but the Marshall Bartholomew thing; they sang as well as the Whiffenpoofs ever did.

"It happened that Fred Waring made a picture at Pomona College and I happened to be conducting the glee club -- they had said, 'Let one of the students take it.' And Waring said to me, 'Will you come to New York?' and I said 'No.' I was going into the church somewhere. But then a year later he offered me a specific job to come to New York and form the first separate Fred Waring Glee Club."

The rest truly is history. Suddenly, in those distant, golden days of radio, the singing of the Fred Waring Glee Club on the popular band's regular broadcasts took on a new polish. Every word could be understood, the intonation and balance were flawless, and singing teachers around the country began to tell their students to listen to Fred Waring's programs for examples of excellent singing.

How did Shaw achieve this kind of perfection, a standard he raised still higher when he formed the world-famous Robert Shaw Chorale, a ensemble of professional singers that performed on a level of excellence that has never been matched? How did he get that balance and live sound? Did he talk to his singers about the technique of singing?

"No. I found that what we used to call 'blend' in those old glee club days was the result of four things: singing the same pitch, at the same time, with the same vowel, and at a similar dynamic level or crescendo or diminuendo. And that it was useless -- a lack of knowledge -- not to be able to separate which one of those factors was now separating the voices. You couldn't say, 'Blend, you bastards!' But you could say, 'It sounds to me that one of you is making a diminuendo instead of a crescendo or that one of you over there is a little sharp and one of you over here is a little flat. Or how Italian do we want our Latin? Or are we all singing the same 'u'?"

What did Shaw do with his singers about breathing?

"Nothing with the professionals.With the amateurs you talk about posture, the inhalation through mouth and nose simultaneously so that we don't dry out the throat; and then pracice emitting the smallest possible stream of sibilant sound -- s-s-s-s-s. The first surprising thing after you've been doing that 15 seconds -- now let out the breath -- is that they had no idea they could keep this thing going this long. They never heard of singing for 15 seconds."

Does Shaw have a warm-up period before a concert?"

"I do. I find the 'myayayayaya,' particularly if it is carelessly done, will absolutely destroy a sense of ensemble and pitch. Some people feel that waking the voice up is 'aha, ha' and that sort of thing. I try to make them alive to intonation first. We begin very quietly with a hummed consonant and then usually an 'oo' or an 'ee' vowel. I find that the large chorus is sort of amorphous, vague and dark and woofy. So most of the time we're working with 'ee.' And I do this every time before a performance, even if people have been with me for 10 years.

"By spending five minutes on these and other devices instead of 'moaoaoaoaoa,' all of a sudden you've licked most of your problems."

What is the hardest thing for an amateur chorus?

"I think the most difficult thing, and the thing I have to work with most constantly when I'm guest conducting, is the lack of real rhythmic perception. It seems to me that there are almost as many inaccuracies of tempo realization or perception as there are of pitch perception. People can tell when it's a little flat or a little sharp, but they really don't know when it's a little ahead or a little late. I don't know why this is. We have menstrual periods and day and night and tides -- the whole thing, and watches which create a kind of artificial time. But the perception of rhythm in its metrics, in its very minute things, is not a part of our natural system. This has to be educated. The great quartets -- and we have two or three in our time, the Guarneri, the Cleveland, the Juilliard -- spend hours and hours to get absolutely articulate at the same instant."

What do you do when you come to a new chorus, one someone else has prepared?

"I begin with two cardinal rules about choral rehearsing. The first is that I don't try to do everything at once. I divide, separate text from metrics, from pitches. And we go through everything we do in what we call 'count' singing, which is 1 and 2 and 3 and 4 and 1 and 2 and 3-4, and we may not get to the text for two-thirds of the rehearsal time, until this thing becomes absolutely possible. The second thing I do, I think the voice is a delicate, dangerously perishable instrument. Therefore, I do all of my rehearsing either down an octave or at almost no voice at all, or even occasionally whistling. I save the gold. I find that if you try to accomplish pitch and dynamics at the same time, you can strip the voice in 30 seconds. I tell the people, 'Look. You're sitting on the bus and you're trying to learn a song and you don't want the person next to you to know that you're studying your song. Sing that loudly.' First, it educates the inner ear and the mind. And the second thing, it saves the voice so that when you need it, it's there."

After two decades with his chorale, Shaw turned his attention more exclusively to orchestral conducting, a field in which he has risen to a position of eminence, spending years as associate to George Szell and the Cleveland Orchestra. More recently the Atlanta Symphony has, under Shaw's direction, attained a technical excellence and tonal luster of distinction.

Tomorrow night at 8, Shaw will combine all of his orchestral and choral techniques as he leads the National Symphony, soloists Jean Herzberg, Jacqueline Pierce, Gene Tucker, and John Cheek, and the Westminister College Summer Workshop Choir. They will perform excerpts from "Die Meistersinger" by Wagner, the Prologue to "Mefistofele" by Boito, some patriotic American songs, and the Beethoven finale.