The present invention is directed toward an improved digital-to-analog converter circuit for a speech-synthesizing electronic timepiece.
In the past, various means have been adopted for providing audible indications of time information from a timepiece. In the case of conventional mechanical timepieces, this has been performed for example by striking a bell or other sound emitting device, by a number of times which indicates the current time. Somewhat similar methods have been proposed for use in electronic timepieces, for example whereby an audible tone is emitted a number of times in succession to indicate a time value, or combinations of audible tones of different frequencies are emitted. However, due to recent advances in the technology of integrated circuits and memory devices, the sizes of such devices have become such that construction of a timepiece is possible in which indication of time information is given by means of synthesized speech signals. Hitherto, synthesized speech data output has been achieved for some devices of relatively small size such as desk calculators. But in the case of an electronic timepiece, there are certain limitations which have made it extremely difficult to implement in practical form such synthesized speech data output. These limitations are due to the fact that, because of the small size of electronic timepieces in general, the battery which supplies power to the timepiece circuitry is also of very small size, and hence produces a low supply voltage and cannot tolerate a high level of power consumption. For these reasons, all of the circuitry which is utilized for speech synthesis in an electronic timepiece must be capable of operating at a low level of supply voltage, and must have a very low power consumption. However, although the various digital logic circuits and memory circuits associated with a speech synthesis system can be designed to meet these limitations, the digital-to-analog conversion circuit (whereby digital signals representing speech information in sampled and quantized form are converted into analog signals to be output by an audio transducer) presents serious problems in this respect, if conventional types of such digital-to-analog converters ae utilized. The reasons for this will be described hereinafter with reference to circuit examples. Such a digital-to-analog converter produces output signals which are of analog form, and which must be amplified in this form in order to drive an audio transducer. Such amplification is highly inefficient in relation to the power consumption of the digital circuits of an electronic timepiece. Another feature of a conventional digital-to-analog converter circuit is that it is not capable of operating in a satisfactory manner at very low levels of supply voltage, due to limitations imposed by such factor as the saturation voltages of transistor elements, for example.
For these reasons, the application of speech synthesis for providing time information output from electronic timepieces has been hindered by the inherent characteristics of conventional analog-to-digital converter circuits, and there exists an urgent requirement for an improved type of digital-to-analog converter circuit which will be more suited for the special requirements imposed upon circuitry for use in an electronic timepiece. Such an improved type of digital-to-analog converter circuit is disclosed by the present invention.