Comparing ibiquity HD to analog FM
The ibiquity signal is an on-channel qam that carries the an encoded program identical to the analog program, though it seems about 50 ms behind. (Close enough for switchovers due to fading to take place with little notice.)
The digital audio sounds pretty good - similar to MP3.
Have to give 3 answers to the comparison question.
1) local 60 km line of sight signal - sounds like the digital program may have a slightly lower noise floor plus the high audio frequencies sound sharper and cleaner. I guess they push up little higher than the 15 kHz analog limit. Low audio sounds about equal - this could be due to the progam - with similar results on the subwoofer. I suspect most of the stations are using their existing studio equipment and media to produce the program and this probably runs above 25 Hz.
2) 190 km non-line of sight signal - the distant digital audio is virtually as good as a local (outstanding !!) vs. the distant analog normally sounding mushy and containing an audible noise floor due to multipath impairment. (Normally means always, except during some rare big-signal tropospheric events.)
3) old vs new receiver - comparing the famous old Pioneer TX9500 to the new Sangean HDT-1X I was in for a bonus surprize. In analog mode, audio impairment due to multipath is noticably improved on the HDT-1X.
This last bonus may be thanks to 30 years of development - perhaps a more capable detector, e.g. a digital dsp compared to an analog discriminator; and or use of a lower noise local oscillator and, for analog stereo, digital pilot recovery system. However better, distant analog mode still not as good as digital mode (which is dead quiet).