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X Factor producers have been accused of enhancing contestants' vocals to make them sound better - but what is "autotune" and how does it work?
If you've ever removed red-eye from a photograph, or used a spell checker to correct an essay, then you know how computer software can... well, "enhance" your raw material.
Autotune performs exactly the same function with music, by smoothing out sharp or flat notes to make singers pitch perfect.
It is actually a brand name - owned by Antares, a company based "a stone's throw" from Silicon Valley in California - and there are plenty of competing products that do a similar job.
All of them require the user to know what key they were (supposed to be) singing in.
The software then scans their vocals, finds any notes that don't adhere to that scale and pulls them back to the "correct" pitch.
However, even the world's best singers slide between notes - and if the autotune settings are too strict, they eliminate those interim notes, creating a robotic, stepping-stone effect.
In some cases, most notably Cher's Believe, the dials are deliberately turned to 11 to create an other-worldly, tin man vocal. But autotune is generally much more subtle.
"It's pretty much used in 99% of recorded music now," says Daniel Griffiths, editor of music recording magazine Future Music.
"I've spoken to engineers who have recorded really big artists - who I won't name - and they just say it's there on the mixing desk all the time.
"If the artist wants to use it, then they just flick the switch.
Most of the time, a casual listener may not even know their favourite artists' vocals have been tampered with.
The best practice for using autotune is to pick out individual wrong notes from a recorded performance and fix them individually - rather than running it all the time in the background.
But if you know what to listen for - a "bubbling" tone, or a jagged step between notes, pitch correction can be quite apparent.
Recording engineer Des McKinney, who writes a blog about home recording, recently pointed out 10 examples of what he called "autotune abuse".
None of the artists involved has openly admitted to altering their vocals, but we have taken four of McKinney's examples for you to judge for yourself.
Listen to each of them using the audio player on the right:
The effect has become ubiquitous now.
Artists including Kanye West, JLS and The Saturdays use it as just another tool in their production armoury - as commonplace as echo, reverb, distortion or compression.
And home users can get in on the act, too, with a consumer version of the software selling for as little as £80.
Jim Anderson, president of the US Audio Engineering Society recently told Time magazine: "People are getting used to hearing things dead on pitch, and it's changed their expectations."
And, according to rock producer Tom Beaujour, autotune is sometimes a practical necessity.
In 2008, he Beaujour told The New Yorker the process was frequently used when "an artist has left the studio and has no opportunity to return just to re-sing one or two off notes".
It also helps to smooth the transition between different vocal takes that have been "cut and pasted" together in modern music software like ProTools.
Daniel Griffiths says he disagrees with ITV's apparent decision to process contestants' vocals on X Factor, but says that, in a professional context, pitch correction is "fantastic".
"I've spoken to people who've said if it had been around when they started recording, they would have used it.
"It could be argued the other way - that when it didn't exist, it made everybody more committed to becoming better singers.
"But if you're not using it now, you're the only one that isn't".