Einstein's light postulate:
"...light is always propagated in empty space with a definite velocity c which is independent of the state of motion of the emitting body." Albert Einstein, On the electrodynamics of moving bodies, 1905
http://www.fourmilab..ch/etexts/einstein/specrel/www/
This independence from the state of motion of the light source is only conceivable if the motion of the source is able to change the wavelength - an ability existing for sound waves but not for light. The following two conditionals are both valid:
(A) If the motion of the source DOES change the wavelength, the frequency shifts the observer (receiver) measures ARE NOT due to changes in the speed of light - Einstein's relativity is saved.
(B) If the motion of the source DOES NOT change the wavelength, the frequency shifts the observer measures ARE due to changes in the speed of light - Einstein's relativity has to be abandoned.
The problem is that "motion of the source changes the wavelength" contradicts the principle of relativity - by measuring the wavelength of the emitted light, someone at rest with respect to the source would know whether the source is stationary or moving. Accordingly, "motion of the source changes the wavelength" has to be rejected. The moving source does not emit changed wavelength - it emits faster or slower light. If the source starts moving towards the observer with speed v, the speed of the light relative to the observer becomes c'=c+v, in violation of Einstein's relativity.
Pentcho Valev