Abstract: In an echo canceller system of the type comprising a circuit for measuring the characteristic of an echo path in accordance with input/output signals of the echo path and cancelling from an actual echo signal an approximate echo path signal synthesized in accordance with the echo characteristic, a level measuring circuit for measuring the level of the residual signal, a residual echo level estimating means for estimating the residual echo level based on the output of the level measuring circuit, and a comparator comparing the output of the residual echo level estimating means with the output of the residual echo level measuring circuit, the output of the comparator controlling the measuring operation of the echo path, there are provided a residual signal level measuring circuit including the residual signal level measuring circuit, a transmitted signal input level measuring circuit, and a detector for detecting an opened state of the echo path and producing an output which controls the residual signal level m

Abstract: A tone disabler is designed for use in a full duplex data transmission system adapted to send a tone before each transmission of data and to reverse the phase of such tone during a later half of a full duplex data transmission. Both the tone and the phase reversal are detected. Responsive to a coincidence of such detections, a control signal is given to disable an echo canceller. The control signal is fed back to latch the control signal as long as the tone is present.

Abstract: An objective system for a video disc comprising four lenses grouped into four lens components. The first lens component is a positive meniscus lens, the second lens component is a positive meniscus lens, the third lens component is a positive meniscus lens having a relatively long focal length or a negative meniscus lens, and the fourth lens component is a positive meniscus lens. The objective system satisfies the following conditions:(1) 1.7F>F.sub.1,2 >1.4F(2) 1.35>F.sub.1 /F.sub.2 >0.75(3) 1.0>F.sub.1,2 /F.sub.1,2,3 >0.8(4) 1.1F>d.sub.5 >0.4F(5) d.sub.6 >0.5F(6) N.sub.4 >1.7where F is the resultant focal length of the objective lens assembly, F.sub.1 is the focal length of the first lens component, F.sub.2 is the focal length of the second lens component, F.sub.1,2 is the resultant focal length of the first and second lens components, F.sub.1,2,3 is the resultant focal length of the first, second and third lens components, r.sub.

Abstract: An objective system consists of three lens components, the first lens component including a positive meniscus lens convex to the light source, the second lens component including a positive meniscus lens convex to the light source, and the third lens component including a positive meniscus lens concave to the light source. The objective system for a video disc satisfying the following conditions:F/0.55<F.sub.1 <F/0.25 (1)F/1.0<F.sub.2 <F/0.5 (2)F.sub.3 >F/0.15 (3)F<.vertline.r.sub.5 .vertline.<3F, r.sub.5 <0 (4)d.sub.2 <0.2F (5)d.sub.4 <0.3F (6)where F is the resultant focal length of the overall lens system, F.sub.i is the focal length of the i-th lens component, r.sub.i is the radius of curvature of the i-th surface, d.sub.i is the thickness or air gap of the i-th lens, and N.sub.i is the refractive index of the i-th lens.