[BC] AM Stereo On A Mono Radio-FM Loudness

[RADIO DOCTOR]

On Fri, 5 Oct 2007, Cowboy wrote:

>> And he built a box that could measure according what to the rules 
>> were and he has a letter (I have a copy) from the FCC approving what 
>> he built the ModMinder to do.  The FCC has never required that no 
>> instaneous peaks exceed 100%.  They defined a peak as 10 cycles of a 
>> 10 kHz sinewave right in the rules.  One millsecond.  It is 'better' 
>> to ignore peaks that they don't recoginize as peaks if one wants to 
>> be as loud as possible, yet legal.  And such weighting is essential 
>> when transmitting high-speed digital SCAs.
>
> Here's my problem with this approach. The rules always had the 
> qualifier "peaks of frequent recurrence" which always seemed 
> reasonable, and more than adequate to me. Now, we need to know how 
> many picoseconds a peak is, for it to be a peak, or not a peak, and if 
> a peak overlaps the window by a picosecond, does it still count/not 
> count ? And, it sorta depends on what the meaning of "is" is.
>
> It's a bad thing ( IMHO ) to try and codify good judgment.

Seems that 'peaks of frequent recurrence' which sounds reasonable is 
exactly a codification of judgment. :)

As referenced above, the FCC said a peak was 1 millisecond; less than 
that, it doesn't count.  The monitors that have peak-weighting ability 
use 900 microseconds for a 10% safty margin.

So we really have an absolute -- peaks that we know don't have 
to be counted.  And the judgment call beyond that as to 'peaks of 
frequent recurrence'.

I fail to see why you would want a tighter absolute than was ever in the 
FCC rules.

The calibration of a scope, while used in the past by FCC roving trucks, 
is certainly not a precision way of determining compliance with their 
own definition of a peak, especially when a station is running more than 
mono programming.  A ModMinder with it's traceable calibration is a 
better way of dealing with the rule's definition.  Then apply the 
judgmental part after that.

No one seems to know what 'peaks of frequent recurrence' means, but I 
read somewhere a long time ago (and I wish I could find it) that up to 5 
weighted peaks within 5 milliseconds would be OK.  That is the reason 
that some peak weighted monitors like the ModMinder and The Wizard offer 
up to 45 cycle (4.5 millisecond) weighting while still providing a 10% 
safety margin.

The reason I talk about peak weighting so much is that I saw how 
important an issue it is while installing the MSN Direct subcarriers for 
Microsoft at dozens of stations in 2003-4.  For years many stations went 
by the conventional (but incorrect) wisdom that if they added a 10% SCA, 
they had to turn down the main channel to 95%.  Two SCAs at 10% each or 
a 20% SCA, and they turned down the main channel to 90%.  They were 
cheating themselves.

In reality, even analog SCAs did not require that much reduction, and 
high-speed digital SCAs required little, if any, reduction, IF ONE 
USED A PEAK-WEIGHTED MONITOR.  All for the simple reason that the more 
frequencies present in the composite signal, the narrower the peaks 
became and one could enjoy the FCC definition of what a peak really is. 
Many of these narrow peaks are narrower than 1 millisecond, so they 
don't count.

With a peak-weighted monitor, there is essentially no benefit if running 
mono.  Add pilot and L-R and there's a small benefit.  Add analog SCAs 
and the benefit increases.  Finally, with high-speed digital 
subcarriers, the effect of weighting is most noticeable.

There is an important assumption in this discussion, that the main 
channel audio is tightly peak-controlled in the first place.  A 
peak-weighted monitor is not a proper or effective solution for 
composite STL or exciter overshoot.

Again, I offer the paper I recently wrote regarding this issue.  It is 
specific to FMeXtra, but is broadly relevant.


...Lyle, FMeXtra evangelist, in Los Angeles, CA     Cell: 213-880-4690
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Lyle Henry, CPBE  THE RADIO DOCTOR  K9DKW/K7OO  Silver Lake/Los Angeles
  SCA Consultant, Contract Engr: Brazil, China, Mexico, Taiwan, SE Asia
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