time more commonly such files are queued and processed by a computer distinct from the crawler computer instead of performing watermark read operations on each such file a screening technique can be employed to identify those most likely to be conveying watermark data one such technique is to perform a dct operation on an image and look for spectral coefficients associated with certain watermarking techniques e g coefficients associated with an inclined embedded subliminal grid to decode spread spectrum based watermarks the analyzing computer requires access to the noise signal used to spread the data signal in one embodiment interested parties submit their noise key signals to the crawler service so as to enable their marked content to be located the crawler service maintains such information in confidence and uses different noise signals in decoding an image image is used herein as a convenient shorthand for imagery video and audio until watermarked data is found if present this allows the use of web crawlers to locate content with privately coded watermarks instead of just publicly coded watermarks as is presently the case the queuing of content data for analysis provides certain opportunities for computational shortcuts for example like sized images e g 256 u00d7 256 pixels can be tiled into a larger image and examined as a unit for the presence of watermark data if the decoding technique or the optional pre screening technique employs a dct transform or the like the block size of the transform can be tailored to correspond to the tile size or some integral fraction thereof blocks indicated as likely having watermarks can.