1. Field of the Invention
The present invention relates to a viewfinder brightness control circuit for a camcorder. More specifically, the present invention relates to a circuit which automatically controls the brightness of the image produced by an electronic viewfinder by applying an iris output voltage, which voltage varies in response to the sensed illumination of the environment.
2. Discussion of Related Art
When an object is photographed by a camcorder, the image quality produced by the electronic viewfinder is greatly affected by the illumination, i.e., the brightness of the surrounding. Accordingly, when an object is photographed by the camcorder, the viewfinder needs to be adjusted to compensate for the ambient illumination so that the image quality at the viewfinder is acceptable.
As shown in FIG. 3, a camcorder 100 includes a lens group 102 directing light through an iris 104 to a target 108. It will be appreciated that target 108 can be, e.g., a charge-coupled device (CCD) or similar device. Signals produced by target 108 are provided to a video signal processor 112 via a preamp 110. An indication of the brightness level at the target is provided from processor 112 to an iris motor 106, which is connected to iris 104, via an amplifier 114. Additionally, the video signal produced by video signal processor 112 is provided to an electronic viewfinder (EVF) 118 via control circuitry 116. In an exemplary case, the control circuitry included in block 116 is as shown in FIG. 1.
In general, brightness at the viewfinder is designed to be manually controlled. As shown in FIG. 1, a conventional viewfinder brightness control circuit includes an amplifier 1 and a light controller 3.
Amplifier 1 includes a PNP transistor Q1 whose base is connected to a video signal input terminal via capacitor C1, whose emitter is grounded via a resistor R4, and whose collector is connected to a power voltage (Vcc) input terminal via a resistor R3; a resistor R1 connected between the power voltage input terminal and the base of PNP transistor Q1; a resistor R2 connected between the base of PNP transistor Q1 and ground; and a capacitor C2 having one terminal connected to the collector of PNP transistor Q1 and the other terminal connected to EVF 118 via light controller 3.
Light controller 3 includes a resistor R5 connected between the other terminal of capacitor C2 (hereinafter node A) and the power voltage input terminal, a variable resistance VR1 and a resistor R6 connected to ground via the common node of capacitor C2 and resistor R5, and a capacitor C3 connected between the wiper of variable resistor VR1 and ground. It will be noted that the wiper of variable resistor VR1 is also connected to node A between capacitor C2 and resistor R5.
Voltage at node A varies depending on variable resistor VR1 in light controller 3 and the video signal level output to the EVF 118 is determined in accordance with the variation in node A voltage. Thus, electronic viewfinder brightness is controlled by a video signal level output to EVF 118. In other words, if the viewfinder is located in well illuminated area, the voltage level of the video signal output to the electronic viewfinder will be high. When the electronic viewfinder is located in a poorly illuminated area, the voltage level of a video signal output to a viewfinder will be low. Accordingly, viewfinder brightness can be adjusted by controlling the resistance value of variable resistor VR1. It will be noted that the resistance value has to be selected in response to voltage level corresponding to the brightness level such that the voltage level is controlled after the user confirms the viewfinder brightness.
As described above, in the conventional operating method, the user directly controls variable resistor VR1 attached to the camcorder in order to control electronic viewfinder brightness. When the user operates the camcorder without manually adjusting the variable resistor, e.g., if the user depends on a preset brightness value stored in the camcorder, or when brightness is controlled after a video signal is displayed on the electronic viewfinder, the resolution of a viewfinder image quality can be deteriorated. This is a source of great inconvenience to a camcorder user.