Digital Gain Control System

Remote control is a popular feature in television receivers. One of the functions often the subject of remote control is the audio output of the receiver. In conventional all-electronic remote systems the audio volume is varied by varying the gain of a DC gain-controlled audio amplifier. In digital gain control systems this can be accomplished by transmitting an acoustic signal to a remote receiver. Depending on the frequency of the acoustic signal, a clock oscillator is enabled and provides a pulse train to an UP/DOWN counter. The counter can be made to count either up or down, again depending on the frequency of the acoustic signal. The counter output is coupled to a digital-to-analog (D/A) converter which develops an analog DC signal used to establish the gain of the amplifier.

Conventional DC gain-controlled circuits tend to have an S-shaped characteristic. That is, in order to cause a given change in gain, greater increments in control signal are required at the extremities of the gain-control range than are required over the central portion. As a result, the operator of a remote transmitter tends to feel a loss of control or an excessive time lag between the activation of the transmitter and a noticeable change in audio volume, at both the minimum and maximum levels of volume.

Accordingly, it is the object of this invention to implement a digital gain control system that provides the operator with a sense of continuous control over the entire range of operation.

It is a further object of this invention to provide a digital gain control system that assures a muted condition of the audio amplifier in the initial state of the counter and at least a minimum discernible amount of audio in the state immediately subsequent to the initial state.

These and other objects, advantages, and capabilities are achieved in a digital gain control system having an N-stage counter and a D/A converter coupled to the counter for developing an analog signal in response to the output of the counter and a control signal developing means coupled to the D/A converter for developing a control signal whereby the gain of a gain-controllable amplifier is varied in response to the output of the D/A converter in one aspect of the invention by a pre-adjusting means coupled to the D/A converter and the control signal developing means for effecting an initial increment in the control signal that is greater than the subsequent increments.

FIG. 1 is a schematic diagram of a particular embodiment of the subject invention. Conventional co-operating circuitry is shown in block form.

FIG. 2 is a graphical representation of the gain-control characteristics of a typical DC gain-controlled amplifier.

FIG. 3 is a schematic diagram of an alternate embodiment of the instant invention.

For a better understanding of the present invention, together with other and further objects, advantages and capabilities thereof, reference is made to the following disclosure and appended claims in connection with the above description of some of the aspects of the invention.

As shown in FIG. 1, a remote receiver 10 receives a signal from a remote transmitter (not shown). In practice, this signal may be an acoustical signal within the frequency range of aporoximately 34 KHz to 44 KHz. Upon reception of signals having sufficient amplitude and the appropriate frequency, a clock oscillator 20 is enabled, thereby providing a series of pulses to an UP/DOWN Counter 30. Depending on the frequency of the input signal, the counter will count either up or down from its original state and, in so doing, cause an increase or decrease in audio volume. The output of the counter consists of four binary outputs, B.sub.1, B.sub.2, B.sub.3, and B.sub.4, increasing in order from least-significant digit to a most-significant digit.

The output of an N-bit counter can assume any one of 2.sup.N output states. For a 4-bit counter as shown here, the sixteen output states are designated, in order, 0000, 0001, 0010, . . . , 1110, 1111.

The output of the counter is coupled to a digital to analog (D/A) converter 40 comprising diodes D1, D2, D3, and D4, each having an anode connected to one of the counter's binary outputs and a cathode respectively connected to one end of a current-weighting resistor, either R1, R2, R3, or R4. The other ends of the current weighting resistors are connected to a summing junction at the anode of a diode, D5. D5 has a cathode coupled through a series-connected fixed resistor R5 and a variable resistor R6 to circuit ground.

The counter and D/A converter operate to develop a voltage at the summing junction in proportion to the counter's output. For simplicity, assume that each of the counters binary digits has a logic level ONE output of V.sub.o volts and a logic level ZERO output of zero volts. The current-weighting resistors are chosen so that R1=2.times.R2,=4.times.R3,=8.times.R4. Accordingly, when B.sub.1 assumes a logic level ONE, it will contribute a current I.sub.1, into the summing junction. If V.sub.o =13.8 volts and R1=1.2 Megohms, it can be shown that I.sub.1 will be equal to approximately 10 microamperes (.mu.A). Similarly, when B2, B3, and B4 assume logical ONE levels, they will contribute a current I.sub.2 =20 .mu.A, I.sub.3 =40 .mu.A, and I.sub.4 =80 .mu.A, respectively, Therefore, for a counter output state designated 0101, that is, B.sub.4 =ZERO, B.sub.3 =ONE, B.sub.2 =ZERO, and B.sub.1 =ONE, a total current, I.sub.T , provided by the counter through the D/A converter, into the summing junction will be equal to 50 .mu.A. It will develop a voltage, V.sub.T, across the series-connected combination of D5, R5, and R6.

The voltage V.sub.T is coupled to a control signal developing means 50 comprising a transistor Q1 having a base connected to the summing junction and an emitter coupled through a resistor R7 to circuit ground. Q1 has a collector coupled through a diode D6 to the DC gain control terminal of a variable-gain audio amplifier 60. With regard to interface between the audio amplifier and the gain control developing means, the amplifiers gain-control circuitry may be represented as a 5 Kilohm (K) resistor connected between the amplifier gain control input and a 5-volt source of potential, B+.

FIG. 2 shows a typical gain control characteristic of DC-controlled amplifiers. Note the nonlinear relationship between the amplifier gain and the control current, I.sub.c, out of the DC gain control terminal. The amplifier gain can be varied a total of approximately 60 decibels (db) by varying the control current through a range of 0.65 milliamperes (ma). Conventionally, a 4-digit counter would divide the total 0.65 ma range into fifteen 0.045 ma increments. A binary count of 0000 would correspond to 0 ma control current, 0001 to 0.045 ma, 0010 to 0.09 ma, . . . , and 1111 to 0.675 ma. However, note that to vary the amplifier's gain from 60 to 50 db gain reduction (i.e., increase the amplifier gain by 10 db), requires approximately 0.18 ma control current. In a conventional digital gain control circuit, this requires the first four counts of a sixteen-step counter. A similar phenomena occurs at the highest 10 db the amplifier' s range. As a practical matter, the operator of the remote transmitter will sense a loss of control or an additional time delay near minimum and maximum levels of volume. That is, even though the UP/DOWN volume control is activated, there is relatively little apparent change in audible volume. Conversely, during the middle range of control, say from 0100 (4) through 1011 (11), the apparent change may be uncomfortably abrupt. By developing an initial increment in the control current, from state 0000 to state 0001, that is larger than the remaining fourteen and by limiting the maximum range through which the amplifier's gain can be varied, the digital gain control system described herein circumvents such effects.

The initial greater increment in control signal is provided by a pre-adjusting means 70 comprising a transistor Q2 which has an emitter coupled through a fixed resistor R8 and a variable resistor R9 to the summing junction of the D/A converter. Q2 also has a base respectively connected through diodes D7, D8, D9, and D10 to the B1, B2, B3, and B4 outputs of the counter and a collector connected to a source of potential, V.sub.DD. The base of Q2, that is, the junction of D7-D10, is coupled through a resistor R10 to circuit ground.

In any state of the counter other than the initial or ZEROth state 0000, a voltage of V.sub.o will be present at one or more of the anodes of D7-D10. As a result, a voltage of V.sub.o -V.sub.BE will appear at the base of Q2. Q2 will be conducting and will provide an additional current to the summing junction of the D/A converter. This results in an additional voltage at the summing junction approximately equal to ##EQU1## By appropriately adjusting R9, the voltage at the summing junction can be made to change from 0 volts in the 0000 (or ZEROth) state, to the desired subsequent value in the 0001 state. In conjunction with automatic volume control circuits, this allows the circuit to guarantee a muted condition in the 0000 state and the minimum audible condition in the 0001 state.

For a particular IC audio amplifier having the gain control characteristic depicted in FIG. 2, R9 was adjusted to establish, in the 0001 state, a voltage of 1.2 volts at the summing junction, thereby providing an initial value of I.sub.T equal to 60 .mu.A, thereby resulting in an initial value of control current, I.sub.c, equal to 0.18 ma. Each additional count of the counter then results in an incremental change in I.sub.T, .DELTA.I.sub.T, equal to approximately ##EQU2## For simplicity, assume that R6 has been adjusted so that (R5+R6)=3 R7. In this instance each incremental change in I.sub.T, .DELTA.I.sub.T, will result in an incremental change in control current, .DELTA.I.sub.c, that is equal to 3.times..DELTA.I.sub.T or 30 .mu.A. It follows that the incremental change in the DC gain control voltage is (5K) (30 .mu.a)=150 mv. Capacitor C1, connected between the collector of Q1 and circuit ground, tends to smooth out the otherwise stair-step waveform of I.sub.c, thereby effecting a more linear change in audio volume and avoiding possible transient effects.

A significant aspect of the circuit is that the pre-adjusting means provides an initial value of I.sub.T, which, in this embodiment, three times greater than the incremental current provided by the second-least-significant digit, that is, three times greater than the current corresponding to the counter state 0010, 20 .mu.A. The reason is that an alternate method of providing an initial greater increment in I.sub.T would be to make the incremental current contributed by the least-significant digit greater than 10 .mu.A. However, for values up to the value of current provided by the second-least-significant digit, 20 .mu.A in this embodiment, the subsequent increments in the D/A output become unequal; that is, .DELTA.I.sub.T will be different depending on the counter's original output state prior to the increment. Furthermore, in the event initial increment in I.sub.T is made greater than 20 .mu.A, the output of the D/A ceases to be a monotonic function of the counter's state. That is, the D/A output will be greater in the 0011 state than in the 0100 state, greater in the 0101 state than in the 0110 state, and so on. Clearly, this will result in a control signal that varies in an undesirable manner.

An additional feature of the digital gain control circuitry shown in FIG. 1 is that the maximum audio volume can be adjusted by varying R9 so as to establish and adjust the magnitude of the subsequent increments and thereby limit the maximum voltage appearing at the base of Q1. This limits the maximum amount of control current developed by Q1 and the maximum gain of the audio IC. For an audio IC having the characteristics of FIG. 2, it may be desirable to adjust R9 so that I.sub.c is limited to a value less than or equal to approximately 0.60 ma. This will provide more than adequate audio volume and will additionally assure that the IC is operating over the linear portion of its gain control characteristic for all output states of the counter. Table 1 (below) indicates theoretical values of the control current, I.sub.c, and the control voltage, V.sub.c, as they exist for given states of the counter in a digital gain control circuit as descibed above. Some of the salient features of the circuit inherent in these values are: (1) in the ZEROth state of the counter, I.sub.c is equal to 0 ma, the amplifier gain is 0, and the audio output is muted; (2) in the 0001 state of the counter, R9 has been adjusted so as to develop a control current corresponding to the minimum discernible audio volume; (3) R6 has been adjusted to provide the desired maximum audio volume and; preferably, to confine the control of the audio IC to the linear portion of its characteristics.

FIG. 3 illustrates an alternate embodiment of the preadjusting means 70. The alternate embodiment comprises a transistor Q3 having a base coupled through a fixed resistor R21 and a variable resistor R22 to diodes D7, D8, D9, and D10 and through the diodes to the outputs of the counter. Q3 also has a collector connected to the collector of Q1 and emitter coupled through a resistor R25 to circuit ground. In a manner similar to that described above Q3 will operate to provide an initial increment in the control signal that will be greater than the remaining 2.sup.N -2 increments. As before, whenever the counter is in a state other than the ZEROth state, a voltage approximately equal to V.sub.o -V.sub.BE will be present at the junction of diodes D7-D10. As a result a voltage (V.sub.BQ3 +V.sub.BE) will appear at the base of Q3, ##EQU3## Accordingly Q3 will conduct a collector current equal to V.sub.BQ3 /R25 and this current will be added to the collector current of Q1 to develop the total control current, I.sub.c, at the DC control terminal of the audio amplifier.

Although the digital gain control circuitry has been described in connection with an audio IC whose gain is controlled by a DC current, the invention clearly contemplates the use of circuits whose gain is controlled by a DC voltage. As a matter of fact, the table above indicates the values of DC gain control voltages corresponding to particular values of I.sub.c. In such circuits it may be desireable to buffer the output of Q1 through, say, an emitter-follower thereby providing a low-impedance source at the DC gain control terminal.

Furthermore, although an audio amplifier has been assumed for purposes of description, the invention disclosed herein is clearly not limited to audio circuitry but has utility with other gain-controlled circuitry, such as automatic video or chrominance processors that may also be found in television systems, or circuitry not at all endemic to television receivers.

Accordingly, while there has been shown and described what are at present considered the preferred embodiments of the invention, it will be obvious to those skilled in the art that various changes and modifications may be made therein without departing from the scope of the invvention as defined by the appended claims.