Re: [IRCA] recorded the iboc

[Scott Fybush <scott@xxxxxxxxxx>]

CHARLES HUTTON wrote:
> Scott:
> 
> Can you explain a little more about the A-100? Station A's IBOC is designed 
> to not bother an envelope detector receiving station A's analog as pains 
> were taken to have IBOC present a constant envelope.
> 
> Now there's obviously a problem here, so what'sgoing on in addition to 
> normal envelope detection?

I'm right on the edge of my technical understanding here - remember, I'm 
a history major, not a trained engineer - but I believe the issue has to 
do with the quadrature modulation that the IBOC carriers use, and its 
interplay with the AM stereo decoder in the A100 (and the Carver, for 
that matter.) The "Q" in C-QUAM AM stereo stands for "quadrature," too, 
and is sent in a somewhat similar manner to IBOC (albeit without 
extending past the usual analog bandwidth, as IBOC does) and I think 
what's happening here is that the AM stereo chips in the A100 and the 
Carver are trying to decode the IBOC data as stereo audio.

I just ran a little test here, grabbing a few radios that happened to be 
close at hand and using two local IBOC AMs, WLGZ 990 (5 kW days into a 
four-tower directional array; I'm just off the edge of one of its major 
lobes) and WHAM 1180 (50 kW ND).

Using my 2010 in wide IF mode, I hear a very faint hiss behind WHAM's 
analog audio. Switching to narrow mode, there's no sign of WHAM's IBOC. 
In wide mode on the 2010, there's some barely perceptible hiss on WLGZ, 
none at all in narrow mode.

Using the Carver TX11b tuner, WLGZ is unlistenable in wideband mode, and 
in narrowband mode displays the same sort of buzzsaw noise Bill Harms 
reported hearing on WTWP. WHAM is similarly unlistenable in wideband 
mode, and is barely tolerable in narrowband mode.

On to some radios that are far more representative of the typical 
listener's experience today:

The $20 Sony clock radio in the bedroom showed no sign of IBOC noise on 
WHAM or WLGZ.

The Tivoli Audio PAL Radio in the kitchen, which is probably right at 
the high end of AM performance of a typical home radio today, showed 
just the barest trace of an IBOC buzz on WHAM and none on WLGZ.

The Sharp three-piece stereo system in my daughter's bedroom doesn't 
have an AM antenna attached to it, so it couldn't be used for this test. 
(I know - what kind of dad am I, anyway?)

The Sony home theater receiver in the family room had fairly noisy 
signals across the AM dial, even with the AM loop that came with it. 
WLGZ isn't really usable in there, but WHAM was decently listenable, 
with no obvious signs of IBOC buzz.

The factory stereos in my 2003 Jetta and my wife's 1996 Saturn show no 
sign of IBOC buzz on WLGZ or WHAM.

Perhaps I'm just lucky in having fairly clean signals around here, but I 
think these experiences are more representative of what the average, 
non-DXer hears than Marty's A100 audio from San Francisco or Pat's 
skywave audio from Seaside.

s
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