Patent GB-1171913-A (GB)
Title: Analogue to Digital Rate Converter
Inventor: ISAACS ANTHONY LEONARD
IPC: H05B37/02
Filed: 1966-10-21
Abstract: 1,171,913. Lighting control systems. THORN ELECTRONICS Ltd. 20 Oct., 1967 [21 Oct., 1966], No. 47339/66. Heading H2H. [Also in Divisions G3 and G4] A voltage-controlled generator 25, which forms part of the lighting control systems disclosed in co-pending Specifications 1,171,915 and 1,171,916, comprises a comparator 27 fed from the output of a 128 state counter 26 driven by the 100 c/s. waveform from the divider 16 of the lighting control system, Fig. 1 (not shown), the counter producing an analogue output voltage having 128 distinct levels each of which is sustained for a complete 400-channel cycle of the divider 16. Voltages appear at comparator inputs 23, 24 according to the position from a centre zero of the fader of the selected channel, these input corresponding respectively to " raise " or " lower " voltages which are compared with the output of the counter 26 and produce a pulse at " raise " output 28 or " lower " output 29 when the input voltage is greater than the counter output. Thus the number of pulses produced for the selected channel during each complete cycle of the counter 26 is determined by the input voltage at 23 or 24, these pulses being stored and used to alter the brightness of the lamps associated with the selected channel to a value determined by the fader setting. To avoid sharp changes in brightness due to bursts of closely-spaced pulses, the 128 levels of the counter are made irregular by transposing the resistor network R1-R7 with respect to the counter stages (as compared with a conventional A/D converter using counter stages and resistors). An example of such a transposition is given for an 8-state counter, Fig. 3 (not shown). The brightness count is changed by one unit per pulse received from output 28 or 29 and at a rate dependent on the counter cycle. Thus the relationship between fader setting and the rate of change of brightness count may be modified by altering the relationship of fader output voltage to dolly position or by altering the analogue-deriving network of the counter to produce as non- linear " staircase " analogue. All fade operations are completed in the same time which may be varied by the modification shown in Fig. 4, wherein an auxiliary voltage-controlled generator 35 controls the rate at which pulses are supplied to the counter 26. To ensure that V.C.G. 25 produces only one output pulse per channel per input pulse from V.O.G. 35, a gate 36 and bi-stable circuit 39 are added to the circuit, the gate 36 controlling the inputs 37, 38 from the master oscillator of the lighting control system and being enabled when the circuit 39 is set at the start of a new cycle of channels by the output of V.C.G. 35. Circuit 39 is reset at the end of this cycle by the output from divider 16. In operation, the time taken for the counter 26 to complete its 128-state cycle is varied by a control voltage input to the V.C.G. 35, which input may be set manually or may be the analogue output of a channel reserved for this use. V.C.G. 35 is generally similar to V.C.G. 25 and its analogue levels may be based on a linear or a logarithmic staircase waveform to produce a hyperbolic or an exponential control law for the fade time and hence a corresponding scale shape of the fade-time control member or indicator. In connection with scale shape, a discussion is given of the relationships between the actual and maximum possible output frequency, the nature of the staircase waveform, the control voltage, the total number of differing analogue levels and the maximum analogue level. Fade time is inversely proportional to the output frequency of V.C.G. 35, the relationship between the levels of the output signals provided by the counter 26 and the resistors R1-R8 being arithmetic in V.C.G. 25 and logarithmic in V.C.G. 35. The V.C.G. 25 may comprise two parts, each having separate connections to the divider 16 and respectively being used to raise or lower the brightness. Two auxiliary V.C.G's are then used to give fast raise, slow dim or slow raise, fast dim operation. Several auxiliary V.C.G's may be used to give fixed and variable face time control. The V.C.G. 25 may be arranged to produce a pulse whenever the counter output is greater than the input voltage, with the result that the number of pulses increases with decreasing input voltage instead of decreasing as in the described embodiment.