Devices as disclosed in Japanese Laid-open Patents S61-151591, H1-191895, and H4-114589 are well-known waveform generators for applying dynamic focusing voltage to CRTs.
FIG. 6 is a block diagram of the major parts of a waveform generator (dynamic focusing circuit) as disclosed in Japanese Laid-open Patent H1-191895 as an example of the waveform generator of the prior art. The prior art comprises a VCO (voltage control oscillator) 61, a binary counter 62, D/A (digital to analog) converter 63, multiplication unit 64, and amplifier 65.
Operation of the prior art is briefly explained with reference to FIG. 6. The conventional waveform generator employs the clock pulse from the VCO 61 to make the binary counter 62 count synchronizing to the scanning frequency. The D/A converter 63 receives the output of the binary counter 62 for producing a saw tooth wave.
Then, the multiplication unit 64 multiplies the signal, after removing the direct current component from the saw tooth wave, by its inverted signal to produce a parabolic waveform. The signal comprising a parabolic waveform produced by the waveform generator is output as the dynamic focusing voltage.
As described above, the waveform generator of the prior art, in general, employs a multiplication unit for converting a saw tooth wave synchronized to the scanning frequency to a signal comprising a parabolic waveform. The parabolic waveform signal which has its minimum value at the center is then amplified to generate a dynamic focusing voltage waveform.
With increasing flatness of a CRT display screen, the optimal dynamic focusing voltage waveform is tending towards being in proportion to the distance from the screen center raised for example to the 2.8th power, whereas a parabolic waveform, which has its minimum value at the center, is proportional to the square of the distance from the screen center.
Therefore, the waveform generator of the prior art which generally produces a parabolic waveform is becoming unsuitable for producing the optimal dynamic focusing voltage waveform for more recent, flat-screen CRTs. The prior art may fail to achieve the optimal focus characteristics over the entire screen. This is a first disadvantage of the prior art.
The output waveform produced by a waveform generator is usually several volts, but CRTs require several hundreds of volts as the dynamic focusing voltage waveform.
Accordingly, the signal produced by the waveform generator needs to be amplified for use by CRTs. To amplify the voltage at low cost, the focusing circuit of the prior art employs a transformer to increase the voltage and supply the increased dynamic focusing voltage waveform to the CRT.
The use of transformers, however, narrows the range of optimal frequency and phase characteristics of the focusing circuit. For example, if the focusing circuit employs a transformer which is satisfactory around the horizontal frequency of 100 kHz for amplifying the horizontal frequency around 30 kHz, the actual dynamic focusing voltage may become asymmetric even though the waveform generator outputs a symmetric waveform. Comparing the left (L) and right (R) from the center (C), as shown in FIG. 5, the actual dynamic focusing voltage has distorted asymmetric waveform.
Therefore the waveform generator of the prior art may produce a distorted dynamic focusing voltage waveform for some horizontal frequencies when it is required to process a broad range of horizontal frequencies such as the case with the latest CRT display monitors for computers.
The prior art may have difficulty in assuring the optimal dynamic focusing characteristics for the entire range of horizontal frequencies. This is the second disadvantage of the prior art.