Downhole telemetry system

In well drilling, a downhole telemetry system, employing pressure pulsing of mud flow down a drillstring, comprises a valve having a pattern of orifices which variably chokes mudflow to cause dependent variations in back-pressure at the surface. The pattern of orifices is preferably formed by one or more orifices in each of two mutually telescoping sleeves 116, 120 whose relative positions determine the orifice pattern and total mud throughflow area. One sleeve is preferably fixed and the other sleeve slides or turns in dependence on the parameter being telemetered. The telemetry system may be reversed in operation to allow remote operation of downhole equipment from the surface.