Abstract: The specific embodiment comprises a valve piston mounted on a shaft that is slidably disposed within an aperture in the lower housing of the blowout preventer. The valve piston and shaft may be extended into the central passage of the BOP in an open position through the use of a hydraulic operator assembly. When the valve piston and shaft have been extended into the open position, the wellbore gases enter into the annular space between the shaft and the aperture through the lower housing of the BOP. The wellbore gases are then vented from the aperture through an open vent conduit to the outside of the BOP.The hydraulic operator assembly is spring biased to keep the valve piston and shaft in closed position whenever the hydraulic operator assembly is not activated. The valve piston will therefore automatically close whenever there is a loss of hydraulic control. In normal operation, the hydraulic operator assembly will keep the valve piston and shaft in open position until the wellbore gases have been vented.

Abstract: Video signal lines are added to or omitted from the ends of individual fields of a video signal to produce a modified video signal whose field length varies at a slow rate. The modified video signal can be broadcast and received by conventional television equipment, normal television reception and viewing being not noticeably affected by the variable field length. However, recording of the modified video signal is inhibited because the field length is not constant. The modified video signal is produced by directly modifying the scanning of televising equipment, or by storing the lines of the video signal in a store and reading them in a controlled manner therefrom. A decoder, for producing a constant field length video signal from the modified video signal, is also disclosed.

Abstract: A transversal filter echo canceller includes two stores each for storing a set of coefficients for the transversal filter. The suitability of a set of coefficients stored in one of the stores is checked by supplying a pulse to the receive path and transversally filtering it, and comparing the level of the resultant transmit path signal, after subtraction of the transversally filtered pulse, with a threshold. At the same time the impulse response before the subtraction is stored in the other store. If the threshold is exceeded the functions of the two stores are interchanged and the check is repeated. The checking can be effected only once for a transmission line not subject to change, or, in the case of a transmission line which is subject to change, at the start of each telephone connection established via the line and optionally during conversation pauses during the connection.

Abstract: The horizontal line synchronizing pulses of a video signal are selectively replaced with a video signal blanking level in dependence upon bits of data, whereby the data is transmitted with the video signal. Such data transmission is particularly applicable to a pay TV system in which the video signal is scrambled, because the selective absence of horizontal line synchronizing pulses enhances the video signal scrambling. In this case the data can comprise information relating to the scrambling of the video signal.

Abstract: A scrambler scrambles a video signal by replacing its vertical intervals with dummy video signal lines, and separately providing information relating to the timing of the replaced vertical intervals. This information is encoded using an encryption key and is distributed with the scrambled video signal by modulation of the horizontal sync. pulses of the scrambled video signal. An unscrambler derives the information from the horizontal sync. pulses, which it regenerates, and decodes the information and uses it to generate a vertical interval of correct timing to replace the dummy lines of the scrambled video signal, thereby producing an unscrambled video signal reproducible on a conventional TV receiver. The scrambling is further enhanced by varying the number of dummy lines which are used to replace different vertical intervals, thereby producing a video signal of variable field length, which is not susceptible of recording.

Abstract: A floating seat ring for a gate valve structure has an external pressure source to move the seat ring into engagement with the gate valve. A pocket for the seat ring is formed by a counterbore in the valve body and a tubular insert is inserted within the valve body bore extending within the seat ring. This inner end portion of the tubular insert defines with the counterbore the pocket for the seat ring for floating back and forth movement. An external pressure source is in fluid communication with the pocket behind the seat ring to move the seat ring outwardly.

Abstract: A non-floating seat structure for an expanding gate valve in which a seat ring is pressed within a body recess and a sealing member is inserted in a groove between a rear face of the seat ring and an opposed wall defining the recess to form a fluid-tight barrier. The groove for the sealing member is of a generally rectangular cross-section and is formed by a counterbore in the valve body and a rearwardly extending inner lip on the seat ring which abuts the recess wall when the seat ring is pressed within the body recess.

Abstract: Fans for use with turbine ventilators and apparatus and methods for supporting the same, wherein the fan is supported spaced from the entrance to the turbine ventilator whereby aspiration increases the air flow through the ventilator.