Is it any wonder that America's trade deficit remains catastrophic? To bring our greatest native opera, George Gershwin's "Porgy and Bess," to television we import a production from Great Britain. Quality, of course, and not the country of origin is the concern when "American Playhouse" and "Great Performances" join hands to open their seasons with "Porgy" at 8 p.m. Wednesday on PBS (channels 22 and 67). And in the staging of the opera, first seen at England's Glyndebourne Festival in 1986, they have found a performance that is close to perfect in every detail. Indeed, this is a "Porgy" that -- if it suffers from anything -- is burdened by an excess of excellence. Gershwin, his sympathy for America's minorities sharpened by his own roots in New York's Jewish ghetto, decreed that "Porgy" was to be performed only with blacks in leading roles and in the chorus. In 1935, when the opera was new, there was no lack of vocal talent among African Americans. Few singers, however, had had the conservatory training demanded by Gershwin's sophisticated score. This led to bowdlerizations and truncations of the score. Today, of course, there's hardly an opera house in the world that could survive without the artistry of America's blacks. Indeed, the Glyndebourne "Porgy" documents just how far black singers have come. Jamaican-born and Juilliard-educationed Williard Scott and Cynthia Haymon, a Florida native who graduated from Northwestern, are cast in the title roles. Mr. Scott, already a finite Porgy in the first full recording of the score by the Cleveland Orchestra under Lorin Maazel 15 years ago, is now a ruling Wotan of the Wagnerian world. Ms. Haymon, a magnificent Mimi at the Santa Fe Opera last summer, now includes Pamina, Micaela and Liu in her repertory and appears on stage in London, Paris and San Francisco. And thus there is a gloss of professionalism to their portrayal of the downtrodden inhabitants of Gershwin's Catfish Row that is too good to be totally true to the spirit of "Porgy." "They must sound like people who live on the edge," wrote one critic of the EMI compact-disc release of the Glyndebourne production that provides the soundtrack for the PBS offering. "Even in moments of relaxation, religious fervor, or escape into dreams or drugs, they cannot be allowed to lose consciousness of who they are; every aspect of 'Porgy' depends on this." And that is precisely the point at which Ms. Haymon -- despite all vocal excellence -- shows herself out of touch with the deep roots of Gershwin's operatic South of 1910. The best of the cast from this perspective is undoubtedly Damon Evans, a former Baltimorean who is equally at home in musicals and opera (to say nothing of his work as Lionel in "The Jeffersons" and his portrayal of Alex Haley in "Roots: The Next Generation"). He sings Sportin' Life, the peddler of the "happy dust" that brings Bess to yet another fall, and he is a singer still capable of losing himself totally to his role. In so doing he brings Mephistophelean dimensions to this demonic figure. It is largely Mr. Evans who is responsible for the dramatic drive of the final scene of "Porgy," the one part of this production that really shivers the timbers. Gregg Baker (Crown), Cynthia Clarey (Serena) and Marietta Simpson (Maria) sing the remaining major roles. The chief flaw in the televised performance is a technical one, and this quite simply in the genesis of the production. The Glyndebourne cast went into the recording studio to produce the EMI recording of the opera. The production, directed by Trevor Nunn, was restaged at London's Royal Opera in 1992, from where the cast moved to the Shepperton studios for telefilming. Thus the singers seen on TV lip-synch throughout the 3 1/2 -hour show. At times this is done with such imprecision that the result suggests a film poorly synchronized into another language. A further handicap is that Clara is played by Paula Ingram, while the voice that sings the most gently understated "Summertime" ever heard belongs to Harolyn Blackwell. Despite reservations, however, the greatness of Gershwin and of "Porgy" comes through. Wes Blomster writes on classical music for the Boulder (Colo.) Daily Camera.