GB2170071A - Waveform synthesiser - Google Patents

Abstract

A waveform synthesiser for generating a television picture display comprises a plurality of difference amplifiers (A2, A3, A4) which are connected in sequence with the input of an integrator (A8). The difference amplifiers (A2, A3, A4) are supplied with a number of input voltages defining waveform parameters which cause the output of the integrator to rise and fall according to the difference between successive difference amplifier outputs. The time each difference amplifier is connected with the integrator is adjusted by means of variable delay flip-flop triggers (M1, M2, M3, M4) which actuate switches (S1, S2, S3) on the difference amplifier outputs. These delays are ganged to vary in inverse proportion to the variable gain of amplifiers (A5, A6, A7) also located on the output of the difference amplifiers. A first waveform defining voltage level (RV1) is applied to the output of the integrator to set an initial value of the waveform. To suppress glitches during transfer from one difference amplifier to another a low-pass filter is connected with the output of the integrator before the synthesiser is matched to the remaining television circuitry. Each waveform parameter is thus independent and does not require iterative adjustment when synthesising a wave. <IMAGE>

Description

SPECIFICATION Waveform synthesiser This invention relates to the synthesis of waveforms and more particularly to the electrical synthesis of a multi-slope waveform.

The principles of waveform generation are well known. However, the implementaion of such principles, hitherto, has used interdependent controls for adjusting the waveform defining parameters, i.e. adjusting one control setting to alter a corresponding parameter has required compensatory adjustment of the other controls.

This is a time consuming business and does not lend itself to overall accuracy. We have appreciated that what is required is a method of waveform synthesis that allows each waveform defining parameter to be set independently of the others.

Thus, according to the present invention there is provided apparatus for generating electrical waveforms, the apparatus comprising means for generating a series of characteristic waveform voltage levels, difference circuits for forming signals proportional to the differential between successive pairs of the said voltage levels, an integrator, initial value setting means for applying a first one of the waveform voltage levels to the integrator to determine an initial waveform defining output from the integrator, switch means for sequentially connecting the outputs of the difference circuits with the input of the integrator, and timing means for controlling the length of time each difference circuit is connected with the integrator.

An embodiment of the invention will now be described by way of example with reference to the accompanying drawings, in which: Figure 1 illustrates graphically the general case of a multi-slope waveform; and Figure 2 is a circuit diagram of a specific embodiment incorporating the present invention in the production of television pictures.

Referring to Figure 1, a desired waveform is synthesised from an appropriate number of straight lines.

The straight lines are defined by start, intermediate and finish voltages V0, V1, V2 and V and corresponding start, intermediate and finish times to, ti, t2 and t.

The amplitude and polarity of the input current to an integrator, appropriate to the first straight line component, is derived from the start and first intermediate voltages V,, V1; and the duration of this straight line component is determined by its start and first intermediate times to, ti.

The intergrator current obeys the following proportionality conditions:

in general,

and consequently the integrator output,

The circuit of Figure 2 is adapted to generate graded images for television pictures. At an input 11 mixed blanking pulses are received which are converted to transistor-transistor logic (TTL) levels by a TTL level converter 12. The output of the level converter 12 is fed to a field pulse separator 13, the output of which is applied to a chain of one-shot multivibrator circuits four of which are shown referenced M1 to M4. Associated with each multivibrator is a variable resistor RM. Resistors RA are ganged with the corresponding resistor RM of each of the multivibrators except RM1.Variation of resistor RM changes the time as it varies the rate of current flow to a capacitor CM during which the output of the multivibrator M remains in the unstable state.

The upper part of the circuit shows at the left-hand end potentiometers RV1 to RV4 which set the voltages V0, V1 etc. The output of each potentiometer (other than the last) is applied in two ways. The output of the first potentiometer RV is applied through a unity gain buffer amplifier Al to a switch S4. A difference amplifier A2 also receives the output of potentiometer RV1 as well as the output of potentiometer RV2. The output of amplifier A2 is applied through the variable resistor RA2 (ganged with resistor RM2) to a buffer amplifier A5. The output of amplifier A5 is applied to a switch S1.

A similar circuit comprising amplifiers A3 and A6 receives the outputs of potentiometers RV2 and RV3.

This circuit contains resistor RA3 and supplies an output to switch S2. Likewise a further circuit contains amplifiers A4 and A7 and receives the outputs of potentiometers RV3 and RV4. This circuit contains resistor RA4 and supplies an output to switch S3.

An operational amplifier A8 is connected to receive the outputs of switches S1, S2, and S3 through a resistor R and has an integrating capacitor C connected between its output and inverting input through a switch S7. A switch S6 can be closed to connect the inverting input directly to the output, and a switch S5 to connect the inverting input to circuit ground.

The output of the operational amplifier is taken from the junction between switch S7 and capacitor C and is applied to a low-pass filter circuit 15. The filter output is blanked by a switch 16 controlled by circuit 12, and applied to a sync pulse adder 18 and hence to an output amplifier A9.

The operation of the circuit, and in particular of the switches, will now be described.

The conventional mixed blanking pulses are converted to transistor-transistor logic (TTL) levels by the TTL level converter 12. The output of the level converter 12 is fed to the field pulse separator 13 which isolates a field pulse with which to trigger the waveform synthesis. The isolated field pulse triggers the first monostable multivibrator M1.

To set the initial level of the synthesised waveform the triggered multivibrator M1 closes switches S4, S5 and S6 and opens switch S7. The integrating capacitor C is thus allowed to charge to a required initial voltage V0 as determined by the setting of the potentiometer RV1.

Once the initial voltage V, has been set by such charging of the capacitor C, the multivibrator M1 returns from the unstable state, closing the switch S7 and opening the switches S4, S5 and S6. This con nects the operational amplifier A8 as an integrator incorporating the capacitor C and the input resistor R.

The input to this integrator is common to the plurality of differential amplifiers A2, A3, A4 which are con nected to the integrator by means of the corresponding set of switches Si, S2, S3.

The trailing edge of the pulse from multivibrator M1 also triggers the next monostable multivibrator M2 as it closes switch S7 and opens switches S4, S5, S6. The time the multivibrator M1 is maintained in the unstable state is determined by the setting of the variable resistor RM1, this allows the delay before commencement of waveform generation to be adjusted to coincide with the television picture information timing requirements. The leading edge of the triggered multivibrator M2 closes switch S1 which connects the output of difference amplifier A2 with the input of the integrator by way of the amplifier A5.

The amount of difference between the outputs of potentiometers RV1 and RV2 determines the gradient of the slope of the waveform, between points to and tl, shown on the graph of Figure 1, and a measure of this difference is produced by amplifier A2.

When the time duration between t, and t1 is altered to alter the wave shape, the gain of the amplifier A5 must alter in inverse proportion so that the voltage V1 is reached at time t1. For example, increasing the duration between to and t1 must result in an inversely proportional change in the gain. To achieve this, the variable resistor RM2, which in conjunction with a capacitor CM2 varies the time the multivibrator remains in the unstable state, is ganged with the variable input resistor RA2 which varies the gain of the amplifier A5. Thus, the voltage at time t1 is proportional to the output of the potentiometer RV2.

The trailing edge of the monostable multivibrator M2 re-opens the switch S1, and at the same time triggers the next monostable multivibrator M3 which closes switch S2, allowing the output from the difference amplifier A3 to be connected with the integrator, by way of amplifier A6. The two inputs to the difference amplifier A3 are from the potentiometers RV2 and RV3. At the time t1, the slope of the waveform as output from the integrator is changed to reflect the new input to the integrator for the time between t1 and t2. Again, variable resistors RM3 and RA3 of multivibrator M3 and amplifier A6, which correspond to the variable resistors RM2 and RA2 associated with multivibrator M2 and amplifier A5 re spectively, are ganged to maintain the constant inverse proportionality between the gain of amplifier A5 and the duration for which multivibrator M3 stays in the unstable state. At time t2 the voltage at the output has reached V2 and the multivibrator M3 returns from the unstable state, opening the switch S2 and triggering the final monostable multivibrator M4. Again, the process is repeated up to a time t4 which is not specifically referred to in Figure 1. The end of the field blanking pulse is used to clear down the monostable M4 before the next line of picture waveform information is generated.

The changeover from one pair of potentiometer outputs to the next is prone to causing glitches or other inconsistencies at the output of the integrator. To smooth out this changeover the low pass filter 15 is connected with the output of the integrator. The output of the low pass filter 15 is then blanked by applying the mixed blanking signal from circuit 12 to a switch controller 16 which earths the output of the filter 15 before every commencement of the waveform synthesis, this corresponds to a period of picture information.

Synchronisation signals are added to the blanked waveform signal by adder 18 and the result buffered by amplifier A9 before being matched to the television picture circuitry terminating impedance.

Using a waveform generator as described above it is possible to set up a waveform according to its characteristic voltages at specific moments during the course of the waveform. This does not require any iteration on the part of the user as each characteristic voltage is set independently of the others. By ganging the timing circuit to the adjustable voltage levels the accommodation of one parameter with respect to another is maintained throughout.

The straight line waveform components can be of positive or negative slope depending on the relative values of the start and finish voltages. The maximum permitted values of the points Vo to Vn may all be made equal provided that the integrator input current is set so that the maximum straight line amplitudes equal the maximum start condition amplitude, then the synthesised waveform will never exceed Vo (maximum), or go below OV thus eliminating any need for waveform clipping.

A unit based on the principles described can be used to provide a shaded background for a computerised map display.

While a maximum of three slope changes are possible using the system illustrated in Figure 2, more or less than this may be realised by using an appropriate number of difference circuits connected to the integrator and corresponding monostable multivibrators.

The concept of the invention may be used in many differing applications unrelated to that described in the above description and as such the invention is not limited by this.

Claims (5)

1. Apparatus for generating electrical waveforms, the apparatus comprising: means for generating a series of characteristic waveform voltage levels; difference circuits for forming signals proportional to the differential between successive pairs of the said voltage levels; an integrator; initial value setting means for applying a first one of the waveform voltage levels to the integrator to determine an initial waveform defining output from the integrator; switch means for sequentially connecting the outputs of the difference circuits with the input of the integrator; and timing means for controlling the length of time each difference circuit is connected with the integrator.

2. Apparatus as claimed in Claim 1, including a variable gain amplifier connected between each difference circuit and the switch means.

3. Apparatus as claimed in Claim 2, wherein a gain control of each variable gain amplifier is co-operatively connected with a corresponding variable delay trigger of the timing means, operable to actuate the switch means such that a change in delay before connection of a difference circuit with the integrator is accompanied by an inversely proportional change in the gain of the corresponding variable gain amplifier.

4. Apparatus as claimed in any of the preceding claims, wherein the output of the integrator is low pass filtered.

5. Apparatus for generating electrical waveforms substantially as described with reference to the accompanying drawings.