If you have ever watched The X Factor and wondered how the groups of backing singers can hold a tune while cavorting round stage behind the acts in all manner of bizarre outfits, the chances are they are not.

Instead they will be relying on some unseen – and unsung – musical heroes and heroines who will be handling the tricky harmonies.

People like Birmingham’s a cappella quintet Black Voices.

Founder Carol Pemberton reveals they are often called on by the show to make sure things stay on an even key for the aspiring stars.

“Sometimes on the live shows in The X Factor you will see a choir miming in the background. We would have done the recording of the voices,” she reveals.

“I shouldn’t be giving away these trade secrets but on live TV they can’t trust the backing singers to not get it wrong.

“And when they go on The X Factor tour they have backing tracks made. We’re the kind of singers that will do that so they sound good live, even when they have got colds or can’t sing or whatever.

“When Little Mix went on tour, we did the backing track for their song Cannonball where there is a choir of voices that they sing to.”

Carol got a chance to go front of camera in the BBC TV series The Choir: Sing While You Work, when she was asked to work as a choir director for Birmingham City Council staff, training them in Gareth Malone’s absence. However, she had to turn the offer down because Black Voices was simply too busy.

She knows how far the choir gets in the competition but is staying tight lipped about the result. What she will reveal is how much the staff stand to gain from the experience of finding their voices and raising them.

“When it does come together and it does sound good, there is no better feeling,” she says.

“You can see on those choir members faces, there is a pride in what they have achieved, a good feeling that spreads around everybody. It is a great way to bring people together.”

I chat to Carol fresh from a short but successful tour of Malaysia and Burma, where Black Voices appeared at Urbanscapes (the equivalent of Glastonbury) as part of a British line up with Franz Ferdinand. There’s barely a pause to unpack before starting their festive shows and flying off to Italy for a series of concerts. They return to the UK to appear at The Fort Shopping Centre in Birmingham on December 21 and at Raymond Blanc’s Michelin starred restaurant and hotel, Le Manoir aux Quat’Saisons, in Oxfordshire on Christmas Eve.

And there is no let-up in 2014 as their international engagements already include a trip to Australia. They will also be going to Scotland to appear at events in the run-up to the Commonwealth Games.

With such a punishing schedule, it is no wonder that Carol returned from Burma battling a cold. But she was still up at 6am and “out for my ride and my walk” before tackling it with “100 remedies from my mum” so she was in good voice for a concert that weekend.

In the 27 years since Carol, who is the aunt of this year’s musical darling, the award-winning Laura Mvula, set up the group they have become champions for the city and, for Britain.

They have been invited to perform all over the world for people as venerated as the Pope and Nelson Mandela and icons of music such as Ray Charles and Nina Simone.

“Meeting Ray Charles at the Colosseum in Rome and performing with him was a stand out moment. Similarly with Nina Simone,” recalls Carol.

“I think the biggest concert we have done was outdoors, singing for Pope John Paul in Italy. There were just people everywhere. We had to address them in Italian and sing in front of this immense crowd. That was just ‘Wow!’. We met him afterwards and he gave us all rosary beads, even though we are not Catholic, and said he cried when he heard us, we were so moving.

“We sang for Princess Diana at a private chapel in London after her dad died.

“There have been so many moments. Singing for Mandela and meeting him.

“These people we’re in awe of and then we get to meet them. It is so frightening and humbling, little Black Voices from Birmingham.”

The group’s global success has been unexpected. Back in the mid 80s Carol and her friends were backing singers for pop bands like UB40 and Fine Young Cannibals, and all they had in mind was coming out from behind them so they could be heard.

“We’d be rehearsing our three-part harmonies and always thought we sounded fantastic. Then we got to the band and it was like it didn’t matter. Drums took over or electric guitars took over and we couldn’t be heard.”

It was after hearing an American all-female a cappella group, Sweet Honey in the Rock, while they were touring the UK that they were inspired to start a purely vocal group.

“We couldn’t believe that somebody could deliver a two-hour concert of just voice. The bells went off in our mind from that moment.

“There are fantastic choral groups in our city – Ex Cathedra, the CBSO Chorus, lots of community choirs doing stuff, but I think we felt there was a gap here for an a cappella group.

“We hadn’t got a male backing singer in the middle of us so we thought ‘how powerful would it be to be just women and voices’.

“That’s really how it started. We had no vision of making it or travelling the world. We just wanted to sing and to bring pleasure to others, but to enjoy it ourselves, to not go home with a sore throat because we were trying to sing louder than a drum kit or half deaf because the musicians were so loud all the time.”

A background of singing in churches gave them the confidence to stand up and perform and also to harmonise.

“We can do gospel 24/7, that is where we were born. I just think there are so many great styles of music. Over the years we have tried to challenge ourselves and keep the repertoire moving and fresh, appealing to the widest possible audience.

“One of the things that always amazes me is that people will come up to us after our concerts and say ‘You don’t realise there is no accompaniment. You really don’t miss it because you guys, you accompany yourselves’. That’s a really great compliment.

“I think we have managed to craft it over the years is to make it look effortless but it is much more difficult than singing with an instrument.”

The line up of Black Voices is fluid, changing as singers have married, left to have children or changed careers.

The current quintet of Sandra Francis, Shereece Storrod, Cecelia Wickham-Anderson, Evon Johnson-Elliott and Carol have been together for 13 years and are as close as family.

“None of us are going anywhere because we love what we do and we love each other,” says Carol.

However, the next generation of recruits is already waiting in the youth choir they run at their studio in Handsworth, and the future line-up might not be exclusively black.

“Our manager gave us our name by accident. I think it is more on the outside that a white person feels they probably couldn’t be a Black Voice.

“We work with lots of white musicians and there has never been a conscious decision not to have a white person in the group.

“If somebody felt comfortable to come in and they were able to do the repertoire, we would welcome them.

“I am pretty sure, in terms of all the young people we are working with who tell me they are going to be taking over Black Voices, that will be a reality. They have already told me ‘it will be a much younger repertoire, Carol’.

Black Voices current range is eclectic and broad – from traditional African and Caribbean music to civil rights songs to French nursery rhymes sung to a reggae beat.

“We keep our hands in a lot of different things and know what’s current. Chart music isn’t my forte but the youngest member in our group is a pop line writer and did her masters in songwriting.”

One hope Carol has for Black Voices now and in the years to come is that it will continue to put Birmingham on the musical map.

“People (abroad) only ever mention the CBSO. I love the CBSO, don’t get me wrong but there is so much great music that comes from here.

“Sadly I think a lot of people who make it move down to London where I guess they think there are more opportunities.

“We are very proud of being from Birmingham. We are born here and are proud to say that and we want people to know there is lots of good stuff going on here.”

* For details of Black Voices concert at The Fort look up www.blackvoices.co.uk