POP MUSIC REVIEW : 3-Dimensional Simpsons Are Flesh, Blood : Guitarist-singer brings fullness to his material; his wife, though not the most professional of vocalists, is able to connect emotionally.

They may be far from the most popular Simpsons in America, but Martin and Jessica Simpson proved themselves to be every bit as special as the cartoon family in their performance Friday at Shade Tree Stringed Instruments.

Martin may be one of the best-kept secrets of the guitar world. Like fellow Brits Richard Thompson and Bert Jansch, he has a remarkable command of the acoustic guitar, bringing an orchestral fullness to his material, which ranged Friday from ancient airs to jagged blues. His playing may not have displayed quite the fire of genius that Thompson’s does, nor the fragile personality of Jansch’s, but it was still some spell-weaving stuff, played with a powerful spirit and delineation.

Singer Jessica’s abilities were far from commensurate with her husband’s: Her solo vocals were often hesitant and awkwardly phrased, and her harmonies behind Martin’s more assured vocals weren’t always complementary. But though not the most professional of performers, she was far from a debit to the program. The advantage of an untrained voice is that it doesn’t necessarily fall into set paths, and her earnest delivery did connect emotionally at several instances.

Most of those came during her own compositions--another advantage of the couple’s working together, as Martin isn’t exactly a prolific writer. Her “Spare Change” was an effective lament on the more-than-monetary poverty of the homeless, while her “The Company You Keep” was a bittersweet reflection on bad relationships.

The couple also performed a pair of typically despairing Richard Thompson songs, “Down Where the Drunkards Roll” and “How I Wanted To,” a song of love’s unmade connections that was well prefaced by a similarly themed poem of Jessica’s, titled “Walking by the Shore in My Thin Black Coat.”

While Martin sang persuasively his own “Dreamtime,” a Ry Cooder-like arrangement of “Man Smart, Woman Smarter,” and other songs, his fingers clearly were the stars of the show. On “Forgotten the Blues” there was a languorous tension to his bottleneck playing, like the hypnotically slow motions of a tropical snake that might suddenly strike.

Martin’ remarkable blend of delta slide and British finger-picking styles also was evident during Willie Dixon’s “Spoonful,” as it fleshed out the tune’s bare-bones riff with some elegant arpeggios, an ascending bass line and stinging slide lines.

Martin practically made time stand still with his instrumental versions of traditional airs including “Rosie Anderson,” “Banks of the Bann,” “Pretty Saro,” “Long Steel Rail” and “The Shearing’s Not for You” from his last two instrumental solo albums, “When I Was on Horseback” and “Leaves of Life,” both on Shanachie Records.

While the convoluted runs he exercised just while tuning up would be enough to scare most guitarists away, there was no ego flash to his playing, only a breathtakingly intricate technique, honestly searching for the emotional heart of each tune.

Guitarist Phil Clevenger would have been the opening act, had he not been stuck in the rain for five hours driving from Los Angeles. As it was, he performed a short set sandwiched between the Simpsons’ two sets. Clevenger (who also plays in the brilliantly named Orange County band the Dead Sea Squirrels) has become quite a capable player, particularly given that he once was in DIN, easily the most leaden, unmusical band Orange County ever produced.

In a self-deprecating comment on Simpson’s having opened for him, Clevenger joked: “It’s kind of like Jimi Hendrix opening for the Monkees, isn’t it?” The disparity wasn’t that great, though: As a fan and student of Simpson’s, Clevenger has found a solid, if not especially individual, style. The only debit to his wholly instrumental, four-song set was an unrefined bottleneck technique that undercut some of his slide playing with a scratchy, abrasive tone.