The third filter is sources and the fourth filter is known as flak

—  The third filter is sources 

a     as the media prefers to go government sources or their own PR as they portray as being profession and seen as more relievable. And it saves the commercial mainstream media the cost of their own independent background check. They make their sources seem more credible even though they would be the ones most likely to water down and leave out information. To insure their own interest are kept safe. By controlling the supply of news they control the public interest in the information. An example of this would be coca cola new wildlife concerns for the polar bear. An animal that coca cola has been using as a mascot in the Christmas season adverts for years. All of a sudden are pushing for the protest of their habitat. The series of ads coca cola run are opposites one informs us that the polar bears are in trouble and need your help. But what they plan to do is never stated just buy coca and we’d see what we can do. But within ten minutes of that ad we are always treated to an advert of CGI polar bear having fun in the snow. All’s well and good until you really think about it. really coca cola doesn’t require your help saving the polar bears. If they improve they own business model and practices it would make the campaign almost completely pointless. This scheme really can be sum up as “give us your money so you can feel better about something that you didn’t care about until we told you to start caring about it”.

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—  The fourth filter known as flak is less about how the media is control and more about how the media control is enforces. Creating negative responses to content be it from a TV program, radio show or newspaper can take the form of complain? Corporate would create what's known as flak machines, which seek out news stories that disagree or harm the viewpoint they are trying to convey through mass media. Then process to attack that information through letters, reports, government involvement, counter articles that attack the creditable of the original.