Editing NTSC (section)

=== Modulation for analog satellite transmission ===
Because satellite power is severely limited, analog video transmission through satellites differs from terrestrial TV transmission. [[Amplitude modulation|AM]] is a linear modulation method, so a given demodulated [[signal-to-noise ratio]] (SNR) requires an equally high received RF SNR. The SNR of studio quality video is over 50&nbsp;dB, so AM would require prohibitively high powers and/or large antennas.

Wideband [[Frequency modulation|FM]] is used instead to trade RF bandwidth for reduced power. Increasing the channel bandwidth from 6 to 36&nbsp;MHz allows a RF SNR of only 10&nbsp;dB or less. The wider noise bandwidth reduces this 40&nbsp;dB power saving by 36&nbsp;MHz / 6&nbsp;MHz = 8&nbsp;dB for a substantial net reduction of 32&nbsp;dB.

Sound is on an FM subcarrier as in terrestrial transmission, but frequencies above 4.5&nbsp;MHz are used to reduce aural/visual interference. 6.8, 5.8 and 6.2&nbsp;MHz are commonly used. Stereo can be multiplex, discrete, or matrix and unrelated audio and data signals may be placed on additional subcarriers.

A triangular 60&nbsp;Hz energy dispersal waveform is added to the composite baseband signal (video plus audio and data subcarriers) before modulation. This limits the satellite downlink [[Spectral density|power spectral density]] in case the video signal is lost. Otherwise the satellite might transmit all of its power on a single frequency, interfering with terrestrial microwave links in the same frequency band.

In half transponder mode, the frequency deviation of the composite baseband signal is reduced to 18&nbsp;MHz to allow another signal in the other half of the 36&nbsp;MHz transponder. This reduces the FM benefit somewhat, and the recovered SNRs are further reduced because the combined signal power must be "backed off" to avoid intermodulation distortion in the satellite transponder. A single FM signal is constant amplitude, so it can saturate a transponder without distortion.