Modulation For TVRO Transmission
576i when it is transmitted for TVRO viewing is transmitted substantially differently from terrestrial transmission.
Full transponder mode (e.g., 72 MHz)
- Luma signal is FM modulated, but with a 50 Hz dithering signal to spread out energy over the transponder
- Chroma is phase modulated
- An FM subcarrier of 4,50, 5.50, 6.0, 6.50 or 6.65 MHz is added for mono sound
- Other FM subcarriers (usually 7.02, 7.20, 7.38, 7.56, 7.74 and 7.92 MHz) are added for a true stereo service and can also carry multi-lingual sound and radio services. These additional subcarriers are normally narrower bandwidth than the main mono subcarrier and are companded using Panda 1 or similar to preserve the signal to noise ratio
- Data subcarriers may also be added
Half transponder mode (e.g., 36 MHz)
- All of the above is done, but signal is bandwidth limited to 18 MHz
- The bandwidth limiting does not affect audio subcarriers
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