The project entitled, ‘ Video Game RF modulator’ is a small portion of the gaming circuit. Here this circuit can be divided into two different sections: an RF oscillator and the RF modulator, as per the functions each of them carry out. The oscillator section comprises an external LC network that is tuned to the preferred TV channel which is not used in the TV network, generally Channel 3 or 4. The audio and the composite video signal act as a modulator for the output signal (which act as the carrier) obtained from the RF oscillator. Required color information is also connected to the video input. Thus obtained output signal is a double side-band, video and audio modulated TV signal. To match the vestigial sideband transmission standard of U.S TV channels, a separate, external filter is used to remove one of the sidebands. TV video modulator circuits as described in this project are usually contained in a compact 8-pin DIP package or else in the standard metal TO-5 can. However, due to the involvement of high frequencies, the actual circuit layout can be troublesome and vital at the same time.
The basic key parameters of the circuit are explained hereby:
- Input dynamic range: – It is the amplitude range of the input signal.
Generally, 0 to 1.5 V.
- RF output voltage: – Usually stated at approximately 60 to 70 MHz with a video input signal of 1.0 V. 15 mV RMS is typical.
- Video linearity: – It is the degree of distortion of the video input signal at RF output. Generally, 2%.
- Color linearity: – It is the amount of distortion of the color information at the modulated RF output. Generally, 1%.
- Audio input resistance: – Common value-800 ohms.
- Video input resistance: – 100 k ohms minimum.
- Audio or video input capacitance: – Preferred value, Less than 5.0 pF.
- RF oscillator stability: – At a frequency of 67.25 MHz and over a temperature range from 0 to 70 °C, the RF oscillator is expected to change ± 250 kHz typical.
- Maximum power dissipation: – At 25 °C, typical value is 1.25 watts.
This modulator circuit allows using the TV set as display for video games and personal computers and is used as the output stage on video tape recorders and TV test equipment.
In order to comply with FCC regulations, the modulated RF video output of any equipment such as video games, personal computers, video tape recorders, etc., must be well shielded and must be submitted to the FCC for prototype testing and approval.