Device Overview
Figure 2 shows a simplified block diagram of the MC44BC373C/4C device. The MC44BC373C/4C device has three main sections:
1. A high speed I2C-compatible bus section
2. A PLL section to synthesize the UHF/VHF output channel frequency (from an integrated UHF oscillator, divided for VHF output)
3. A modulator section, which accepts audio and video inputs, then uses them to modulate the UHF/VHF carrier

An on-chip video test pattern generator with an audio test signal is included.

FEATUREs
MC44BC373C: Multi-Standard PAL/SECAM/NTSC Modulator
MC44BC374C: PAL/NTSC only Modulator
The channel is set by an on-chip high-speed I2C compatible bus receiver. A Phase-Locked Loop (PLL) tunes the modulator over the full UHF range.
The modulator incorporates a sound subcarrier oscillator and uses a second PLL to derive 4.5, 5.5, 6.0, and 6.5MHz subcarrier frequencies. These frequencies are selectable using the bus.
The modulation standard can be chosen using a control bit that selects between System L or System B/G (MC44BC373C only).
The picture-to-sound ratio may be adjusted using the bus. In addition, an on-chip video test pattern generator can be switched ON with a 1KHz audio test signal.
The MC44BC373C/4C also has the following features:
    • No external varicaps diodes/inductor or tuned components
    • Channel 21-69 UHF operation
    • VHF range possible by internal dividers (30MHz–450MHz)
    • Integrated on-chip programmable UHF oscillator
    • Extremely low external components count
    • High speed read and write I2C-bus compatible (800kHz)
    • Fixed video modulation depth (80% in PAL and 90% in SECAM)
    • Peak White Clip disabled via the bus
    • Programmable picture/sound carrier ratio (12dB and 16dB)
    • Integrated on-chip programmable sound subcarrier oscillator (4.5MHz to 6.5MHz)—No external varicaps
    • On-chip video test pattern generator with sound test signal (1kHz)
    • Low-power programmable modulator standby mode
    • Transient output inhibit during PLL Lock-up at power-ON
    • Logical Output Port controlled by bus
    • Custom masked versions with unique start-up settings possible (no I2C bus programming required)
    • Extremely robust ESD protection, minimum 4kV, typical 6kV