Normally the downlink transmitter was PM, to allow for coherent Doppler tracking. This also supported commands, telemetry and two-way voice. Video signals required more bandwidth than was available on this system. Other wideband signals such as scientific data or engineering data also required more bandwidth. A wideband frequency modulation system provided improved signal-to-noise ratio owing to the capture effect. This improves the signal-to-noise ratio for RF signals with more than 8-10 dB of signal-to-noise ratio (SNR). However, below this threshold the wideband signal has a worse SNR. Reception is "all or nothing". If the receiving antenna is too small to capture the wideband video, the narrowband signals such as voice cannot be received either.
