HD Radio is a bit different than television on the technical side of things but your description regarding the subchannels is correct. An analog television station is on the lower VHF frequencies and television broadcasters were given an additional UHF frequency (channels 14-83). Within that UHF channel they can choose to divide the bandwidth between different subchannels as they see fit. They will be forced to give their VHF channel back to the government once all of the stations are digital but that date keeps getting pushed back.
HD Radio, on the other hand, is done through IBOC (in-band on channel). The digital information is carried on the same channel as the regular broadcast frequency in a way that a ‘normal’ radio would ignore it. The radios are also capable of switching from analog to digital and back if you are in a marginal reception area.
http://en.wikipedia.org/wiki/In-band_on-channel
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