“If running both HD1 & HD2 with music in real stereo, isn’t HD1 going to have audible digital compression artifacts?”
In general, any sideband will interfere with other sidebands around the same carrier. Analog or digital. The sidebands will also interfere with the main band. In analog, it’s called crosstalk, sub to main and main to sub, or in other words how much L+R leaks into L-R and vice versa, and there is a maximum allowable amount.
In digital, or a complete analysis of the analog plus digital waveform, any subchannel will create artifacts in other channels, sub or main. Although “digital” information is modulated around the carrier, the overall waveform is still analog and it can be shown how any subcarrier will interfere with any other sub or main carrier. It’s a mathematical certainty.
Having said all that, adding more subcarriers just makes for more anomalies. Whether or not HD2 really compromises HD1 any more than HD1 compromises the analog signal is not subjective, it is fact and measurable. The more robust the subcarrier (power, bandwidth, rate) the more anomalies (distortion).
Remember, the same argument was made by those folks that wanted FM to stay mono because of the sub to main distortion, multipath and coverage differences between mono and stereo.
Offhand, I appreciate your complaint but it’s really not an issue to consumers. Either the signal is good enough to listen to or not and Lord knows HD is far from a perfect solution. Ibiquity has their method locked in and until that changes there won’t be much improvement.