HD radio is such a spectrum hog.Wipes out reception on adjacent channels on each side.So 1 HD station takes up 3 channels.So an HD station on 96.3 wipes out 96.1 and 96.5.

As for how the asymmetry works, it's actually pretty simple: an FM HD signal consists of mirror-image carriers above and below the center frequency of the analog signal. So when 93.5 added HD, Toronto listeners suddenly lost WBLK because the HD carriers actually sit on the same frequencies used by the 93.3 and 93.7 analog signals.

The FM HD standard says those upper and lower carriers are supposed to be transmitted at the same power level, which can be as little as 1% and as much as 10% of the analog power level.

Radios can usually decode HD from just one set of carriers, so the new rule lets stations use a higher power level on one set of carriers in situations where using higher power on both would interfere with an adjacent-channel station.

So for instance, my 103.9 in Rochester is short-spaced to 104.1 in Buffalo. If I wanted to crank 103.9 up to the maximum 10% digital power, my upper carriers would interfere with WHTT in areas between Rochester and Buffalo. So I can't run that power level on both sides - but in theory I could do it now on just my lower side and expand my HD coverage a few miles.

It's complicated along the border, of course. US signals have no protection from interference on Canadian soil and vice versa. So CKFM's HD is hard to lock in in Buffalo because of the WECK 100.1 translator. Nothing in this rulemaking changes that at all.

I am aware of a couple of instances where assymetrical sideband was used here in Canada.  This may be out of date, but my recollection was that Durham Radio uses(used?) assymetrical on 94.7 Hamilton in order to offer some protection to their own 94.9 in Oshawa.  There were experiments back in the 2014-2015 period with Saga’s 92.9 Bellingham - which was causing interference to 93.1 Vancouver - CKYE-FM.  But Saga stopped “cooperating” on the 2nd day of the “experiment” and resumed symmetrical sideband.  There might be another situation in Canada…perhaps Skywave knows.

Interference or not, it's coming. The FCC has approved this proposal in the U.S. It remains to be seen if it will affect any nearby stations in Canada.

FCC Approves HD Radio Change That Supporters Say Will Improve Signal Coverage

Again, very little. Asymmetrical HD sidebands are already in use by most of the stations that would benefit. The only difference is now they can be permanently licensed this way instead of having to renew an experimental permit every six months.

If you haven't noticed any effect until now, that's unlikely to change.