I see this for instance at press conferences where there is two mics. I basically understand that they are used to reject extraneous noise, but how exactly does it work? What is the signal chain (devices connected to the mic) and how does the noise rejection work while still keeping the clear voice?

I've never used 2 podium mics for any sort of noise reduction, nor heard of anything like that - someone else might know something about that. They're usually for redundancy, and often I'll use a cardioid and a hypercardioid to get the best gain before feedback. Usually a podium means no rehearsal and people who aren't often in front of a mic, so the more options I have during an event the happier I am. Some people keep both channels open, but I think this sounds horrible. I've also seen people keep the backup 6-9db below the main mic, which is usually enough to reduce the combfiltering, but still have a hot backup.

On a related note - I seem to remember a Countryman podium mic that had 3 seperate outputs - card, hyper, and omni for different channels / trucks. I can't find any info on it now - am I smoking something? This may have been 10 years ago or so. Just curious.

-Dave

If you’re talking about a presidential press conference the two mics are used separately.
One goes to the press pool feed and the other goes to the Secret Service archive recorders.

In the past they had three mics; SM57s to be exact.
The third mic from what I understand was a backup. As time past they omitted the third backup mic.

You may find a lot more than two mics in your everyday news conferences.
Each and every News Gathering Organization usually provides their own individual mic. Sometimes they may share a pair of mics off of an audio DA, mic splitter or press box of some sort.

When it comes to presentations I usually use two mics on the podium.

I have never used them for any noise canceling technique, but you never know what someone else has worked out.

I too use the mics as a redundant pair. I never tried the cardioid and hypercardioid technique – I have to try that someday. I usually have both mics up with one a bit lower in level for the same reasons that have already been addressed.

Shure had a triple output microphone that was the answer to the Governments triple mic technique. I don’t think they liked the idea because it wasn’t a true redundant set of mics.
I’m not sure if Shure still manufactures that 3way mic.

VIP Dual Microphone Kit

OK so I can assume that the concept of using two mics to block out noise came from some kind of misconception, thanks

That's not necessarily, the Dead had a dual mic setup in the 60s or 70s that did just that...

So. you never know what someone has figured out. I never done it that way, but that doesn't mean it hasn't been done or in use today.

Mark, what you're talking about is exactly what the Grateful Dead were doing in the 1960s when they started their "Wall of Sound" PA according to the late, great Don Pearson (We miss you, Doc!), in the developmental days of rock concert audio. The Dead would mount two RCA BK-5's together and wire them out of phase so that anything from the PA (upstage, or behind) the musicians, would cancel out in the console's bussing. Singers would sing into ONE (the top) of the two mics, making sure they were off-axis to the second mic, and were able to get a clean vocal reproduction with rejection of the PA behind them.

So. You're not off the mark. But for the last 30 years, most ENG have used the mics as either redundant mics or split feeds.

Personally, I always have at LEAST two mics on a podium but try to only use one at a time. I have had great results tagging the two elements together to get one low and one high mic (in the Schoeps tradition) but when they are on opposite sides of the podium or lectern, it's very convenient when you have two people who speak together which will 'accidentally' happen too often at award shows.

Hope this helps!

jim