Various methods for determining baseband frequency response of an FM transmitter are known. One technique relies upon the FM transmitter response alignment as performed in conjunction with a deviation meter or FM demodulator. These latter devices lend a frequency dependent response characteristic which cannot be quantified independently of the FM transmitter. Another previous technique employs a pre-emphasis network at the input to the FM transmitter having a 6 dB per octave amplitude vs. frequency characteristic curve for altering the modulating signal input to the FM transmitter to compensate a 6 dB per octave decrease in the first sideband output of the FM transmitter as input frequency is increased.
While these prior approaches have proven useful for their intended purposes, they are subject to various objections. Both the FM demodulator and the pre-emphasis methods introduce a frequency dependent response characteristic relative to the particular application. This injected, relative response characteristic is not amenable to an absolute, universal standard. For example, using the pre-emphasis method, FM transmitters at distant locations could be checked against the same quantized standard only by checking the first transmitter with a given pre-emphasis network to the second station for testing of its FM transmitter, or by building exactly the same network for testing at the second station. The first alternative is not practicable, and the second alternative is subject to a certain unavoidable margin of inaccuracy due to manufacturing tolerances, etc. Furthermore, in either alternative, the network is volatile and will drift in its response characteristics.
The response of an FM transmitter may be compensated by tuning the response of the corresponding FM receiver or demodulator at the receive end of a transmission system. From a manufacturing and engineering standpoint, there is a need to alleviate this burdensome compensation and instead provide uniformity and interchangability among FM transmitters at the same or different locations, or at different times of manufacture. There is a further need for industry wide compatability of modulated transmission systems. Interchangability is particularly important in standby switching type systems and for customer support in exact replacement parts.