Patent GB-650531-A (GB)
Title: Method and arrangement for measuring the index number of a telegraph modulation
IPC: H04L25/04
Filed: 1947-01-09
Abstract: 650,531. Telegraphy, testing. BAYARD, H. M., and ROQUET, R. J. C. Jan. 9, 1947, No. 799. Convention date, Jan. 9, 1946. [Class 40 (iii)] The index of modulation of a Baudot or like telegraph transmission has been defined by the C.C.I.T. as where a 1  ... a h  represent respectively the values of each of the signal elements from the first to the present or hth. In a double current system each " parameter " a 1  ... a h  has a value of either + or - 1. It is shown that where a h  is the element last transmitted, and i h-1  is the index for the previous element. If only the last k elements are considered it is further shown that The Specification describes means for automatically determining the varying value of the index as successive elements of a telegraph transmission are received by performing the calculations embodied in equation (i) or (ii) above. In one embodiment (Fig. 4) the Baudot or like signals are received on a telegraph relay R1 whose armature K1 charges a condenser C1 to a potential +E (or - E) over a segment 1 of a commutator K3. The commutator arm rotates once per telegraph element period and when it reaches a second segment 2 the charge on condenser C1 is shared with a second condenser C2 whose charge Q is allowed to decay through a resistor R to a value C1+C2/2C2 . Q in a telegraph period t. If the existing charge on C2 is called V h-1  . C2 then by the time brush K3 again contacts segment 2 the charge on C2 has partially leaked away to C1-C2/2C2 . V h-1 . C2. When therefore the incoming telegraph element charges condenser C1 to a value  E . C1 this charge is added to that on C2, when the brush K3 contacts the segment 2, and if the new potential of each condenser is V h  we have the condition Dividing by EC1 we have The values of V h  and V h-1  thus follow the law of equation (i) and are a measure of i h  and i h-1  respectively. A measure of the index i h  is therefore continuously indicated via a valve M on a galvanometer GE. In a modification (Fig. 3) a relay R1 records the element being received and applies a potential of  E to the condenser C1, while a second relay R2 operates with such a delay that it simultaneously records the kth preceding element and applies a potential of # E/2k to the condenser C1. When the brush K3 contacts the segment 2 the new potential of each condenser is given by the equation which is of the form The galvanometer GE thus indicates the current value of the index of modulation derived according to equation (ii) above. The resistor R may be infinite and the condensers equal and the last equation then becomes If 2V h /E =ih then V h-1 /E = <SP>i</SP>h-1/2 and this circuit herefore also measures the value of ih. In a further modification, Fig. 5, the incoming telegraph signals control a relay R1 which applies to a device I a generator G1 for marks, or a generator G2, in anti-phase, for spaces. The received signal is thus recorded on a magnetic tape xy together with a voltage fed back from a pick-up device L. The device L reads off the signal previously received and passes an alternating potential of half this value and the correct phase to the recording- device I. This, therefore, always records the currently-received signal plus half the value recorded for the previous signal, and a galvanometer GE thus indicates succeeding values of the index of modulation extended to an indefinite number of elements. The magnetic tape system can be replaced by any other system, e.g. by a phonographic disc or a photographic film recording system. The Specification as open to inspection under Sect. 91 comprises also an arrangement of relay chains for recording the limited index taken to three terms. The three relevant signal elements may form any one of eight possible combinations and eight final relays are used to mark a recording sheet in the corresponding one of eight columns headed with a predetermined index value. This subject-matter does not appear in the Specification as accepted.