[BC] Absolute Polarity (was XLR pinout debate)

[Mark Humphrey]

On 8/12/06, Cowboy <curt at spam-o-matic.net> wrote:

>  Oh, sure, we all know ( or we all should know ) that positive pressure on the
>  microphone diaphragm is supposed to produce positive going voltage at the
>  output pin labled + but how do we know what the polarity was on the
>  record ( that's CD for you kids ) when it was recorded ?

This is a very good point -- and for that matter, how many receivers
maintain "absolute polarity" from the detector through to the speaker?
 Unless the entire system adheres to a standard protocol, I guess we
always end up with a 50/50 chance of getting it right regardless of
the way the connectors are wired.

About 15 years ago, I had a similar discussion with an assistant
engineer I had recently hired.  Turns out he had some strong opinions
concerning polarity, and one day he quietly slipped me a little book
entitled "The Wood Effect" (as if it were a religious tract) and said
"Read this."

Yes, it was interesting and I agreed with Wood's findings (for much
more info, Google "wood effect polarity") -- but I told my assistant
that, given the haphazard phasing of consumer audio gear, his time
would probably be better spent keeping all of the tape heads clean.

But times have changed and maybe we need to start thinking about
polarity as it relates to smooth IBOC "blending".

While driving around, listening to my JVC "HD Radio" receiver
transition from analog to digital, I've noticed the audio on some
stations nearly cancels out during the overlap (apparently due to a
polarity inversion), while on others, blending is hardly noticable.
It's easy to understand how this could happen, especially if the
station uses separate processing chains for the two components.  Some
processor manufacturers (such as Omnia) have provided an AES polarity
inversion switch, which makes it easy to fix  - as long as the
monitoring system is accurate.

So this raises a few more questions:

1)  Does iBiquity require receiver manfacturers to adhere to an
absolute polarity standard on both the digital and analog sections?

2)  Is the HD codec phase linear?

3)  While we're on this subject, what about other lossy codec schemes,
like MP3, AAC, etc.? Have any absolute polarity standards been set for
them, or does anyone even care?

Mark