Our local low wattage 1490 kHz is a repeater for 96.3 FM. Even a couple miles away fom its downtown antenna, the AM signal sounds weak and with few highs. Maybe low modulation?

This afternoon in my car as I approached the antenna, the signsl popped out of AM and my car radio showed KTSN -HD instead of the typical 1490 kHz, i.e.,no call sign. AND the signal was much louder with overemphasized highs. As I drove past downtown it popped in and out of HD.

When I listen on my crystal radio, all I hear is the weak sounding AM with muted highs.

Where is the HD coming from? Is there a carrier I can't hear on my crystal radio? Or is my car radio somehow intelligently switching to the FM HD signal?

Yes, AM-HD is a thing. It's IBOC like FM-HD but unlike FM-HD which can have up to 4/8 sub channels, AM-HD is limited to just one.

1490kHz in Austin does broadcast in HD. https://en.wikipedia.org/wiki/KFIT

Thanks. What frequency is the IBOC signal on? If it's on 1490, is that why the AM signal is so weak and high-hampered?

EDIT: I just looked up IBOC and I see that's exactly what happens. It even says that the analog signal is more suitable for weather and traffic content.

Heres my post from earlier this year about it locally.

viewtopic.php?f=2&t=418507

It's a "thing", but not much of a thing. Here in Chicago, a number of AM stations that had added HD have now removed it. Seems more successful on FM stations, but in general, it's nowhere near as successful as stations had hoped.

You can hear the hash generated by the HD (Hybrid Digital) AM signal on both sides of the carrier. At night the skip carried the noise and interfered with adjacent stations. HD AM was a flop. Many stations turned it off here in NY. They started killing it at night, then turned it off all together.

"HD" actually stands for "Horribly Distorted"

HD AM sounded great, actually. The issue is that no one has the radios, comparitively. When HD Radio came out in 2005 or so the radios were hard to find and too expensive, and by then everyone was on their smartphone, and sales of actual radios began to decline.

We had two HD AM signals here in my metro and they sounded cleaner than the analog AM. But a lot of the first HD exciters were hard drive based, expensive, and stations decided to yank them rather than replace them or spend money to have them fixed when they had breakdowns. And there also was the issue for many stations of reduced coverage because of the IBOC.

Almost all of the local FMs have kept HD. The two AMs that had it got rid of it. There is one DX station that has it (KSL, Salt Lake City), but I've never been able to decode the HD off their signal.

I applaud those few AM stations that are trying the full HD transmissions. At least they're thinking outside the box, and the ones that seem to be trying it have nothing left to lose.