imagemagick quality image issue using php

I have convert the PDF to image,I try using the exec,by excute 开发者_如何学Gothe linux command like '/usr/bin/convert -density 300x300 file.pdf[0] -resample 160 -resize 512x700! images/sample-0.jpeg 2>&1'.But i have read some articles that it slow as compare to imagick php module.so try using php with code

  $uploadfile = "file.pdf";
$im = new Imagick ( $uploadfile . "[0]" );
$im->setImageFormat ( "png" );
$im->writeImage ( "p-10.png" );

As you can see that i am not setting the desity and resample,the quality of image is not good at all.In php manual also there no function that set the density,i also try with setResolution,but does not work.

Please help if any body any idea about this. have dream day