The Effect of Different Types of Video Filters on PDM-FM and PCM-FM Radio Telemetry

The effects of two limiting types of video filters (namely the ideal low-pass vertical cutoff herein called the ILPF and the Gaussian) on the performance of PDM-FM are compared. It is found that minimum allowable pulse height controls the minimum allowed bandwidth of the Gaussian filter and if crosstalk suppression of 50 to 60 db is required, crosstalk controls the minimum allowed bandwidth of the ILPF. In PCM-FM using the sampling video detector, the critical effect is the overlapping of pulses at sampling time; using the integrating detector, the critical effect is the difference in area under the pulse with adjacent pulses not present and the area with no pulse present but the two adjacent pulses present. In general, the results of the theory agree well with experimental results of Aeronutronic [8] and aid in their interpretation.