Signal formats over satellites For satellite transmission, which device converts an analog baseband signal into a digital bit stream (and vice versa) before uplinking?

Introduction / Context:
Modern satellite links often carry digitally encoded voice, video, and data. The first step for an analog source (e.g., voice) is conversion to a digital representation. Distinguishing among front-end devices—codec, modem, and transponder—prevents conceptual mix-ups.

Given Data / Assumptions:

• Analog source (voice/video) must be digitized for PCM or related schemes.
• Satellite payload may be a bent-pipe (nonregenerative) transponder.
• Ground segment includes baseband processing and RF equipment.

Concept / Approach:
A codec performs analog-to-digital and digital-to-analog conversion along with source coding (e.g., PCM, ADPCM). A modem maps bits to waveforms suitable for the RF channel (e.g., QPSK) but does not perform the actual A/D conversion of the program material. A transponder simply frequency-translates and amplifies the passband; it does not digitize content. A compandor only adjusts dynamic range, typically within analog systems.

Step-by-Step Solution:
Need: analog → digital conversion of baseband.Device that does A/D and D/A: the codec.Modem and transponder roles are downstream (mapping bits to RF and repeating).

Verification / Alternative check:
Any PCM chain starts with a codec sampling at the Nyquist rate (or higher) with defined quantization bits per sample before channel coding and modulation.

Why Other Options Are Wrong:

• Transponder: passband repeater; no source A/D.
• Modem: bit-to-waveform mapping; not baseband A/D.
• Compandor: dynamic range processing only.
• Frequency synthesizer: generates LO frequencies, not A/D.

Common Pitfalls:

• Calling any box that “handles digital” a modem; the codec is the A/D front end.
• Assuming the satellite digitizes content; most GEO payloads are bent-pipe.

Final Answer:
Codec (coder–decoder / A-to-D and D-to-A)