Error: Constructor not defined: [ApexPages.StandardController].<Constructor>()

I got this error when i save my test class:

Error: Compile Error: Constructor not defined: [ApexPages.StandardController].<Constructor>() 

CLASS:

public class ContentVersionAlt {
private List<ContentVersion> ContentVersions;
public string host{get;set;}

    public ContentVersionAlt(ApexPages.StandardController controller) {
           host=URL.getSalesforceBaseUrl().toExternalForm();
    }


    public List<ContentVersion> getContentVersions() {

    ContentVersions= [Select c.id ,c.Software_Download__c,c.title,c.description,c.FileType From ContentVersion c where c.Software_Download__c=:System.currentPagereference().getParameters().get('id')];
        return ContentVersions;
    }

}

TEST CLASS:

@istest
private class ContentVersionAltTest {
 static testMethod void ContentVersionsTest(){

          PageReference pageRef = Page.SoftwareDownloadFiles;
          Test.setCurrentPageReference(pageRef);

        ApexPages.StandardController sc = new ApexPages.standardController();
        //create an instance of the controller
        ContentVersionAlt myPageCon = new ContentVersionAlt(sc);
        myPageCon.getContentVersions();       

         ContentVersion testContentInsert =new ContentVersion(); 
         testContentInsert.ContentURL='http://www.google.com/'; 
         testContentInsert.Title ='Google.com'; 

         insert testContentInsert;   


         ContentVersion testContent = [SELECT ContentDocumentId FROM ContentVersion where Id = :testContentInsert.Id]; 

         ContentWorkspace testWorkspace = [SELECT Id FROM ContentWorkspace WHERE Name='Opportunity Documents ']; 
         ContentWorkspaceDoc newWorkspaceDoc =new ContentWorkspaceDoc(); 
         newWorkspaceDoc.ContentWorkspaceId = testWorkspace.Id; 
         newWorkspaceDoc.ContentDocumentId = testContent.ContentDocumentId; 
         insert newWorkspaceDoc;

         update testContent; 



 }

}

How can i solve this?