Out of the crackle-and-pop noise of the police radio scanner, the usually soft voice so familiar to the family suddenly rang out loud -- and afraid.

"Send us a back-up! Send us a backup!"

The family tensed as their son's partner sounded the 10-33 distress call for a police officer in trouble.

Trapped in the hallway of a Southeast Washington housing project, police officer Dennis Last and his partner, Cynthia Hannibal, were trying to scramble out with a drug pusher they had arrested -- and 18 of his friends in pursuit.

Ruth and Bob Last, Dennis's parents, listened to the radio intently.

"I knew he'd be all right," Bob can say now. But that assurance didn't come until several hours after the distress call, when Dennis Last came home from another night on the Seventh District vice squad.

While other families tune in to "Starsky and Hutch," Ruth Last and her family have gotten their nighttime police entertainment by following their son's career over a police radio scanner.

The scanner is a shortwave radio receiver that "scans" frequencies used by police and fire departments, stopping at a channel when people are speaking. Or a listener can pick a particular channel to monitor, as the Lasts do with the Seventh District channel.

For six years, the family rode with Dennis as he progressed from an officer in scout car 187 to detective in the gambling unit of the police department's morals division.

Through the scratchy world of the police scanner, they traveled with Dennis as he stalked murder suspects, sifted through bombed-out buildings for survivors and eluded dope dealers. But underneath the excitement of the chase was a constant fear -- what if they were listening and Dennis were killed?

"We all hoped and prayed he wouldn't e killed," Ruth said. But if it had happened, she said, the family would have been stoical. "We're just on earth as a loan. When our time comes, no matter how it is, God takes us back."

In 1977, officer Bruce Wilson, one of Dennis' partners, was killed in a drug bust that went awry. Dennis wasn't with him.

In a lively family session, sort of a "confessions of a scanner junkie," the family told some of their most memorable scanner stories.

One icy winter evening Dennis and a rooke policeman were stalking a murder suspect. Ruth was listening anxiously for her son's voice, but was interrupted by her husband.

Ruth said, "The next thing I heard them say was, 'Send an official. One of the officers has hurt his shoulder." I said, 'Damn, Bob! Why did you talk to me then and disturb me?' I knew it was Dennis."

It was. The rookie had spun the police cruiser into the side of a taxi, injuring his partner in the process.

Another evening Dennis was sent to search the site of an explosion in a building in Southeast.

"The whole top of the building was blown off."' Dennis recalled. "We set up a command post and called for fire apparatus and extra police."

But then in the middle of it, the police switched to "citywide one," a channel that the family could not receive on their scanner. Ruth fumed over the interruption well into the next afternoon.

The family refers to Ruth's addiction to the box as "scannitis," a condition that began when Dennis' wife, Donna, turned over the scanner he had given her because she was too nervous to listen to it. But Ruth insists that her condition is nothing more than a reflection of her pride in her son and the rest of the police force.

For several years now, she has written poems of encouragement to the men in blue, who include her son and her brother, an 18-year veteran of the D.C. police. Several of the poems have been published in police newsletters.

In Oil City, Pa., where she used to live, Ruth would toss her sleepy children into the back of her car and chase fire engines to invite the displaced victims to her home for shelter until permanent arrangements could be made.

With Ruth's helpful phone calls pointing out suspicious persons, police have apprehended arsonists, a peeping Tom and several burglars, family members said.

And last year, just before the big snow of February, "she made her first arrest."

She had heard a commotion in the hallway of the apartment complex where the family was living. Opening the door, she found six boys, ranging in age from 6 to 14, who didn't live in the building.

Ruth shooed them out and stood by the window to watch them leave. They didn't.

Shortly afterwards her son David -- a Metro security officer -- came home.Ruth sent him to check the laundry and storage rooms. David found the youngsters sifting through the family's storage bin.

A tall, husky man, David was able to grab four of the boys and return them to Ruth for questioning.

Ruth sat them down and made them give their names, addresses and telephone numbers. Then she ordered them to crowd together and took a "mug shot" with the family camera.

Two of the boys started to cry -- three of the four had been picked up before on burglary charges.

A year ago, when the family moved to a new home in New Carrollton, the resident manager of their old building wailed, "Who's going to watch our apartment complex for thieves and such?"

Now Ruth Last has a new neighborhood to scan. She already has helped Prince George's County police find unfamilar streets by telephoning in directions to police dispatchers. They in turn greet her regularly over the radio at night.

Six new scanner crystals have been added to the radio, enabling the family to tune into wider range of channels. But what Ruth has her heart set on is a big "Bearcat" scanner that woud let her listen to the whole metropolitan area.

For she never knows what will be at the end of a call -- such as the one four years ago when Dennis' scout car was ordered to report to the helicopter pad immediately.

The family waited eagerly for Dennis' return to get the full story.

But he laughed as he told them, "I just took them a pizza."