It's possible to hang more than one HD signal on the back of an FM station. Some carry up to three bit the second or third ones if used are of lower sound quality than the primary HD channel.
You might hear Americans referring to HD1, HD2 or HD3. These are the first, second and third HD signals if used.
Although it uses the same bands as FM and AM radio so doesn't waste any spectrum, HD radio has never become especially popular in the US even though most car radio's support it over there.
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