Nearly 35 years ago, producers Thomas Miller and Edward Milkis put together a 20-minute presentation to convince ABC that two guest stars on “Happy Days” could be spun off into their own series.

They shot just a few new scenes of Penny Marshall and Cindy Williams as “Laverne & Shirley” but, recalls composer Charles Fox, they insisted on a 75-second main-title sequence with a fully produced song, “so, right from the beginning, people would know what it was about.”

Fox and lyricist Norman Gimbel came up with “Making Our Dreams Come True.” It became a top-25 hit — one of many TV theme hits for Fox, who won an Emmy for “Love, American Style” and wrote themes for “Happy Days,” “The Love Boat,” “Angie” and others.

Today a composer is happy to get 10 seconds on a broadcast skein, and hit TV themes are rare.

Over the past 15 years, the broadcast networks have demanded shorter main-title sequences, preferring to jump into the action faster and thus reduce the chance that viewers will flip to another channel. Emmy’s Main Title Theme Music category, however, disallows themes under 15 seconds, so many network shows are ineligible.

Last year’s theme-music Emmy winner, Russ Landau (“Pirate Master”), says, “It’s getting tougher and tougher to convince (decisionmakers) to spend the time that they would normally be making on advertising dollars.

But some producers like music — Mark Burnett (“Survivor”) likes a good, long-line theme. It sets the tone for the show.”

Jeff Beal, who scores ABC’s “Ugly Betty,” gets 12 seconds — so short a time that the music can’t really be called a theme.

“It’s a little sonic signature that says a lot about the style of the show and who the character is,” he explains, its prominent marimba suggesting Betty’s Mexican heritage.

For USA’s “Monk,” Beal got 45 seconds (and won one of his three Emmys), and for HBO’s “Rome” he got a minute and a half. Longer openings offer a chance “to tell more of a musical story,” he says.

“Heroes,” by contrast, gave composers Lisa Coleman and Wendy Melvoin just 10 seconds. “You have to think in terms of ‘stings,'” says Coleman, referring to the film-music tradition of brief but impactful musical statements. “We knew it had to be big and somewhat supernatural sounding. It didn’t need to be terribly melodic, just atmospheric.”

This year, PBS producer David Horn wanted to reinvigorate the opening sequence of “Great Performances” for the high-def era and called five-time Oscar winner John Williams to compose new theme music. Williams’ piece debuted March 25 and is only his third primetime series signature in 25 years.

“It is elegant, and it sneaks up on you,” says Horn, who previously had commissioned Oscar winners John Corigliano and Maurice Jarre to write “Great Performances” themes.

“I wanted to use a full symphonic orchestra, to invite the viewer to come in to a series that we like to think is classy,” says Horn — “one week the Metropolitan Opera, next week Carnegie Hall, then a musical theater piece, Shakespearean drama, a lot of different things.”

In contrast, say many observers, the commercial networks are missing a bet by ignoring the power of a good theme.

“Quickening the pace, getting into storylines faster, all conspire against the theme,” says Cleveland Plain Dealer TV critic Mark Dawidziak. “But on cable, the name of the game is people knowing who you are — FX viewers, USA viewers, HBO viewers — and this is where that old-fashioned network thinking comes into play. They hear that music and they remember that opening.”

“Eighty years from now,” he adds, “today’s kids, sitting in their wheelchairs in the nursing home, will be humming the ‘SpongeBob SquarePants’ theme in much the same way that we know the theme songs of our youth. It’s more than just a TV theme. It becomes a communal thing, a shared cultural point among your friends, your community.”