When cruising past some XM channels some of the more obscure sports talk channels like the hockey talk channel there is this really weird quavering, near mechanical timbre sound to the voices. I’ve also noticed this in other (non-XM) broadcasts like weather radio channel voices from regional NOAA offices giving current weather updates. What causes this?
It’s digital audio, and the sampling rate isn’t quite high enough - for the overall system, there will be a budget of however many bits per second that the satellite can carry. The premium content (eg: the “CD-quality” music) will get a full allocation of bandwidth, but they throttle it back for the all-talk channels so they can fit in more of those obscure programs. The trick is to find the balance of shrinking the file or stream size, while maintaining intelligibility and clarity.
The same thing happens in digital satellite or cable TV systems - the weather channels, home shopping channels and similar things will get lower bitrates so they can cram in more channels without hurting the quality of the high-def and premium movie channels.