scheme to monitor the exact instantaneous power consumption of smartcards as they make their computations also provides clues as to what type of computations are being performed. In some cases, fraudulent cloning has been used to assign identical serial numbers to multiple receivers or cards; subscribe (or unsubscribe) one receiver and the same programming changes appear on all of the others. Various techniques have also been used to provide write protection for memory on the smartcards or receivers to make deactivation or sabotage of tampered cards by signal providers more difficult. Systems based on removable smartcards do facilitate the implementation of renewable security, where compromised systems can be repaired by sending new and redesigned cards to legitimate subscribers, but they also make the task of replacing smartcards with tampered cards or inserting devices between card and receiver easier for pirates. In some European systems, the conditional-access module (CAM) which serves as a standardized interface between smartcard and DVB receiver has also been targeted for tampering or replaced by third-party hardware. Improvements in hardware and system design can be used to significantly reduce the risks of any encryption system being compromised, but many systems once thought secure have been proven vulnerable to sufficiently sophisticated and malicious attackers. Two-way communication has also been used by designers of proprietary digital cable TV equipment in order to make tampering more difficult or easier to detect. A scheme involving the use of a high-pass filter on the line to prevent two-way communication has been widely promoted by some unscrupulous businesses as a means of disabling communication of billing information for pay-per-view programming but this device is effectively worthless as a cable operator remains free to unsubscribe a digital set-top box if two-way communication has been lost. As a device intended to pass signals in one direction only, the line filters offer nothing that couldn't be done (with the same results) by an inexpensive signal booster - a simple one-way RF amplifier already widely available cheaply and readily for other purposes. Also, many such boxes will disallow access to pay-per-view content after a set number of programs are watched before the box can transmit this data to the headend, further reducing the usefulness of such a filter. Terminology and Definitions Some of the terminology used to describe various devices, programs and techniques dealing with Pay-TV piracy is named for the particular hacks. The "Season" interface for