Christmas on a small patch of New York City pavement. RINGING IN THE CHANGE FOR CHARITY

It is the season for giving. At noon on a cold December day, a young man with a microphone stands on the sidewalk at Rockefeller Center, trying to catch the attention of holiday shoppers. ``Show your true affluence,'' he calls out, pointing to a large glass jug half full of dollar bills. ``Give to the Action Committee to Help the Homeless Now.''

A few steps away a man dressed in a Salvation Army uniform plays Christmas carols on a cornet. ``Sharing Is Caring'' reads a poster on the kettle next to him. Just beyond him a man in a Santa suit rings a bell for the Volunteers of America. A sign on the chimney that serves as his collection pot urges passersby to ``Put A Little Green in Someone's Christmas.''

The presence of these three on this small patch of pavement hints at a growing urgency underlying charitable appeals this year. A report issued last week by the United States Conference of Mayors confirms what officials already knew: the ranks of the needy continue to swell.

The demand for emergency food assistance is up 18 percent over last year. The need for emergency shelter has grown an average of 21 percent nationwide.

As retailers keep a nervous eye on erratic holiday sales volumes, groups like the Salvation Army and the Volunteers of America are logging long hours on the street to keep contributions flowing. The results so far are not conclusive. Regional responses are as varied as the national economy.

Salvation Army officials in cities such as Phoenix, Indianapolis, Buffalo, Seattle, and Nashville report declining contributions. But here in New York, kettle donations are up 5 percent over last year. Mail contributions have increased 17 percent. Yet gifts of stocks and securities, which last year totaled $250,000 by now, stand at only $7,000. With 400 kettles in greater New York, the group hopes to raise $1 million.

Similarly, the sidewalk Santa campaign for the Volunteers of America in Greater New York is running 7 percent ahead of last year, when it raised $83,000.

As the need rises, Christmas appeals become a form of mild competition.

``When I was a kid 20 years ago playing kettles in Manhattan, there was very little competition for kettle dollars,'' says Craig Evans, director of public information for the Salvation Army's Greater New York division. ``The Salvation Army was just about the only group.''

The latest entrant in street-corner appeals, the Action Committee to Help the Homeless Now, has solicited Christmas donations for three or four years on a modest scale. ``But this is the most flamboyant we've ever done it,'' says Jerry Mackey, secretary-treasurer, noting that the committee staffs eight sidewalk collection sites in Manhattan.

Mr. Mackey stops short of specifying a monetary goal. ``Our goal is to do the best we can every day,'' he says.

``There are so many variables - the wind, the rain, the people asking for donations,'' he explains. ``There are certain workers who for one reason or another just have a knack of getting people to donate. Other volunteers can say exactly the same thing and very few people will donate.''

One of Mackey's volunteers, Tyrone Townsend, a tall, friendly young man stationed in front of Tiffany's, outlines his approach.

``You have to have a positive feeling about what you're doing,'' Mr. Townsend says. ``You can't have negativism in your voice. If you challenge people and tell them, `It's your duty to give, you have to give,' they'll say, `I don't have to.' If they want preaching, they can go to church.''

Still, the lot of a sidewalk Santa is not a ho-ho matter.

``It's a grueling job,'' explains John Hartman, director of development for the Volunteers of America of Greater New York. Noting that the organization's 25 to 35 Santas are all recovering alcoholics from the men's rehabilitation centers maintained by the Volunteers, he says, ``Some of them just don't have the stamina.'' Volunteers begin their sidewalk duties before 9 a.m. and stay until after 8 p.m., with four 20-minute breaks.

``Most establish a fairly good rapport with store managers,'' Mr. Hartman continues. ``If it gets really cold, they can step inside the store for a few minutes to get warm. But they also wear thermal underwear, thermal socks, and thermal gloves.''

However big the business of Christmas giving becomes, it is still clearly a case of one-on-one. Beyond the new styles of bell-ringing salesmanship, beyond the marginal ups and downs by region, there remains an old-fashioned constant - Christmas generosity.

``Traditionally, when there's been a major financial crisis in the country, the public seems to have picked up the slack,'' Mr. Evans says. ``Maybe people feel subconsciously they have to support to a greater extent those agencies that will help the public in time of need.''

Cadet Homer Williams, a Salvation Army volunteer at Rockefeller Center, agrees. ``Wherever you go around the United States, people will give. Some people know what it's like not to have enough. They want to give. That's what it's all about.''