The **MC44BC373C** is a high-performance Phase-Locked Loop (PLL) tuned UHF/VHF audio/video modulator. It is primarily designed for use in VCRs, set-top boxes, and video game consoles to convert baseband audio and video signals into a Radio Frequency (RF) signal compatible with a TV tuner. --- ### ## Core Specifications and Features | Feature | Specification / Description | | :--- | :--- | | **Supply Voltage** | 5.0V (Typical) | | **Operating Frequency** | UHF (Channels 21–69) and VHF (Channels 2–12, S1–S41) | | **Interface** | I2C Bus Controlled | | **Sound Carrier** | Programmable (4.5, 5.5, 6.0, or 6.5 MHz) | | **Video Modulation** | Negative Modulation (PAL/NTSC compatible) | | **Package Type** | SOIC 16 or TSSOP 16 | --- ### ### Key Internal Components The MC44BC373C integrates several complex electronic sub-systems into a single chip: 1. **PLL Synthesizer**: Uses an external 4.0 MHz crystal reference to maintain precise frequency stability. It eliminates the need for manual tuning or "drifting" associated with older LC-tank modulators. 2. **Video Modulator**: Includes a "white clip" circuit to prevent over-modulation and an internal video clamp to ensure consistent DC levels for the video signal. 3. **Sound Subcarrier Oscillator**: A programmable oscillator that generates the audio carrier. It can be set to different standards (System M, B/G, I, D/K) via the I2C interface. 4. **RF Amplifier**: An integrated output stage that provides a clean RF signal with low harmonic distortion, typically outputting around 75 $dB\mu V$. --- ### ### Typical Application Circuit (Block Diagram Logic) To function, the MC44BC373C requires minimal external components: * **Vcc/GND**: Power supply pins with decoupling capacitors ($0.1\mu F$). * **Video In**: AC-coupled input for the composite video signal. * **Audio In**: AC-coupled input for the mono audio signal. * **I2C Pins (SDA/SCL)**: Connected to a microcontroller to set the channel frequency and sound standard. * **XTAL**: A 4.0 MHz crystal connected to the OSC pins. * **RF Out**: Connected to a $75 \Omega$ female connector for the TV cable. ```mermaid graph LR A[Composite Video] --> B(MC44BC373C) C[Audio Mono] --> B D[MCU via I2C] --> B E[4MHz Crystal] --> B B --> F[RF Output / TV Antenna] ``` --- ### ### Functional Advantages * **Integrated Power Down**: It features a low-power standby mode controlled via software. * **Multi-Standard Support**: Because the sound carrier and frequencies are software-programmable, one hardware design can be used globally (switching between NTSC and PAL). * **High Linearity**: The internal circuitry is optimized to reduce "snow" or interference in the picture.