[mythtv-users] Obama Recommends Delay in Digital TV Switch

Brian Wood beww at beww.org
Mon Jan 12 00:40:55 UTC 2009


Drew Tomlinson wrote:
> Brian Wood wrote:
>> Jeff Walther wrote:
>>
>>   
>>> Do you think they would be amenable to a call or email?  The thing I have
>>> not done, which seems kind of obvious is to contact them and ask, "Hey, is
>>> this problem at your end or my end?"
>>>     
>> They would almost certainly welcome your input and questions. We
>> certainly always did, and in some cases we actually sent an engineer to
>> the home of people with reception problems.
>>
>> Of course you want to immediately make it clear that you are a rational
>> and polite viewer. You wouldn't believe some of the "complaints" we
>> received, the usual ones being "why do you jack up the sound during
>> commercials?" (we didn't) or "why do you send all the signal to the rich
>> neighborhoods?" (obviously silly).
>>   
> 
> OK, I have to ask.  Now while I agree that sending "all the signal to 
> the rich neighborhoods" is silly, there certainly appears to be 
> occasions when commercials are much louder than the programming.  Our 
> local Fox affiliate comes to mind as a broadcast that seems to be this 
> way, especially during prime time.  So if audio levels aren't 
> intentionally manipulated by some, what other things may cause this 
> apparent behavior?

OK, a reasonable and common question, but a complicated one.

The question is "what is loudness?", or "what is volume?"

The audio for commercials has usually been processed heavily, and is
more "dense" than normal program audio. Although it might give the same
reading on a standard VU meter, it sounds "louder" to people. that's why
they process the audio.

The precise technical term is "psychoacoustic perceived loudness", not
volume.

Controlling perceived audio levels is fairly easy, for a human being,
but doing so with automated equipment is not. Some stations, especially
small market or low budget (or both) ones, often do not have a real
person monitoring and controlling things all the time. They depend on
AGC (automatic Gain Control) of some sort to try and keep the audio
levels constant. Because such circuits can't take into consideration the
"density" of the audio, the result is that the commercials sound
"louder" to most viewers.

If you watch the VU meter on normal program audio you will see some high
points, and some low points, and a lot of bouncing up and down. The
audio on commercials, however, will cause the meter to pretty much stay
at a high point, but still "legal". This is the whole point of the
processing.

So the audio is in fact being "intentionally manipulated by some", as
you put it, but it is not the station doing so, rather it is the
commercial producers, by heavily processing the audio.

Prime-time commercials are usually network spots, or otherwise are more
expensive than daytime avails, so they tend to have "better" produced
commercials, resulting in the comment you made about prime-time.

The situation is even stranger with AM radio, where FCC rules prohibit
more than 100% negative modulation. In fact this is a needless
regulation, as more than 100% negative modulation in an AM system means
that you have reduced the carrier level to less than zero, obviously not
possible, so the audio gets processed to go more positive than negative,
thus sounding "louder" while still obeying the regulation. It is
possible to modulate positively more than 100%, since that only means
you have increased the carrier output to 200% of its normal value, not
hard to do. What they really want to prevent is clipping from an
"attempt" to reduce the carrier to less than zero.

beww




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