Digimarc said Wednesday that it received approval from the U.S. Patent Office on 28 claims related to audio watermarking.

Digital security has become a hot-button issue for film producers, musicians, artists and writers who are increasingly publishing original works online or in other digital formats. So-called watermarking places a unique code into a file that theoretically is difficult to remove without damaging the quality of the sound or image.

Tualatin, Ore.-based Digimarc, which specializes in authentication of digital or analog documents, filed for the patents between 1993 and 1995.

"Digimarc's early patent filings contain a wealth of pioneering watermarking technology relating to audio, video and still imagery," CEO Bruce Davis said in a statement. "As the leader in digital watermarking applications, we have established a large portfolio of landmark patents in still image and video watermarking, and we are now broadening our portfolio with more key patents in audio watermarking."

Last week, Digimarc and major consumer electronics companies unveiled a new video copy-protection coalition. The Video Watermarking Group will give film studios means to distribute content online without fear of copyright pirates.

Once considered a geeky security measure unknown outside of the technology industry, Hollywood has increasingly seized onto the potential security benefits of watermarking. Movie studios want to beef up copy-protection plans for DVDs and other digital formats because hackers have cracked the previous standard with a code known as DeCSS, spawning a series of lawsuits aimed at keeping the DVD circumvention code off the Internet.

Digimarc has been in the forefront of digital watermark development. Late last year, it joined forces with postage giant Pitney Bowes to embed watermarks on envelopes. The result, for example, would be that a telephone company could provide a link to up-to-date account information through a customer's monthly bill.