[mythtv-users] CALM Act in US

[Brian Wood]

On Wednesday 14 October 2009 00:02:37 David Brodbeck wrote:

>
> I've noticed that another popular trick in commercials these days is to
> send a lot of audio to the back channels on Dolby Pro Logic systems,
> which effectively increases the loudness at the same volume level
> compared to a program that is mainly using the front and center channels.

That certainly sounds intentional, and could cause problems with some "mono 
guard" systems, designed to prevent reverse phasing in the mono audio 
channel. We had problems with that sort of thing back when the Arsenio Hall 
show was airing.
>
> > Much research was done in the 70s by the Orban Company, they made
> > something called an "Optimod", initially for AM radio, then FM and
> > finally TV. Back then the idea was to be the loudest station on the AM
> > dial of course.
>
> Is this basically like the 'speech compressors' that SSB communications
> radios sometimes use to increase the average power level?

Similar, but a lot more spohisticated, the Optimod split the signal into 
different frequency bands and processed each seperately, using different 
weighting filters for each, and had a lot of what was then proprietary 
algorithms.

Then there was the Aphex Aural Enhancer, used in the music biz, the unit was 
only rented, never sold (like a Fisher boom), and came with its own security 
guard, who took it back to his hotel room after each session. More marketing 
than real security I suspect.
>
> > AM Radio actually has an FCC-mandated spec of no more than 100% downward
> > modulation, now there's a great law: you're not allowed to reduce the
> > power below zero.
>
> Well, sure, then they'd be stealing power from everyone else. ;) ;)

But in a "Green" way, right?

>
> I used to live pretty near a big AM station in a rural area, and you
> could tell they ran their modulation level as high as they possibly
> could get away with.  You could hear brief bits of 'buckshot' splatter
> all up and down the dial on modulation peaks.

That was caused by excessive downward modulation, since you can't reduce the 
amplitude below zero the result is the waveform clips at the zero point, and 
the resultant square wave component has harmonics all up the spectrum.

-- 
Brian Wood
beww at beww.org