From death metal and black metal to gothic metal to metalcore, countless metal bands of the '90s and 2000s have been revolving doors -- and that is certainly true of Zero Hour.

After 2001's The Towers of Avarice, Fred Marshall replaced Erik Rosvold (who had replaced Frank Mendez in 1995) as Zero Hour's lead vocalist -- and on 2006's Specs of Pictures Burnt Beyond, the Bay Area progressive metal outfit unveils yet another new lead singer: Chris Salinas (formerly of Power of Omens).

Salinas has no problems hitting the high notes, offering a piercing wail that draws on influences like Ray Alder (of Fates Warning fame), Queensrÿche's Geoff Tate and Judas Priest's Rob Halford.

It's an approach that is highly appropriate for this 43-minute CD, which clearly favors the ultra-technical side of progressive metal.

The term "technical metal" can be used to describe a variety of metal, ranging from math metal to technical death metal -- and on Specs of Pictures Burnt Beyond, technical metal means progressive metal that is highly melodic yet never goes out of its way to be accessible.

There are many progressive metal bands that, for all their complexity and intricacy, are hooky and groove-minded; like Yes and Pink Floyd back in the '70s, they realize that being complex doesn't mean that you can't groove.

But Specs of Pictures Burnt Beyond thrives on extreme angularity; like Norway's Twisted into Form (who have also recorded for Sensory), Zero Hour loves difficult time signatures.

This is the type of disc that -- like a lot of avant-garde jazz and avant-garde classical -- must be accepted on its own challenging terms.

And for those who are not frightened away by that difficult, ultra-technical approach to prog metal, Specs of Pictures Burnt Beyond is a well executed, decent outing that underscores Zero Hour's ability to keep plugging away despite all their lineup changes.