[BC] IBOC "secrets" and my opinions.

[Johnson, Richard]

On Tue, 27 Mar 2007 WFIFeng at aol.com wrote:

>
> In a message dated 03/27/2007 11:21:54 AM Eastern Daylight Time,
> rjohnson at analogic.com writes:
>
>> So, I think that instead of complaining that the new
>>  technology is not compatible with 85 year-old radio
>>  design, some entrepreneur(s) should take the bull by the
>>  horns and develop a decent radio.
>
> It is obvious you have a considerable amount of knowledge in these issues, so
> I want to say Thank You for sharing your information with us. You've
> certainly raised some excellent ideas. The problem with what you propose, above
> , is
> that Ibiquity is not sharing the technology... with *anyone*. They have it
> locked-up, and at last check (just a few months ago) the chipsets are $40 a pop
> .
> Their "Licensing Infomation" pages on their WEBsite said "Add your content here
> "
> for the longest time, and may still.
>
> The other issue that it would appear you may have overlooked is the
> *same-channel* interference isses of the so-called "IBOC" (which is really IBAC
> )
> system. For example, if a station on 1000 "lights-up" the HD system, they will
> be
> transmitting digital "garbage" on top of the stations on 990 and 1010. It isn't
>
> "adcacent" to them, it is right on top of their analog signal. To get rid of
> *that* with DSP would leave a *lot* of artifacts, resulting in a still
> unlistenable signal.

I can't comment on the legal issues except that if a certain company
got a federal agency (read FCC) to approve a system, using technology
that is not available to anybody except somebody who pays for a license
from that certain company, then multiple federal laws have been violated.
Start with "restraint of trade" then go on to downright fraud.

For the technical issues, the problem with digital and AM within the
same spectrum space is that the digital bins (samples) need to fit in 
between the AM bins. In the case of digital, the difference between a 
wanted bin and an interfering (unwanted) bin needs to only be that the 
wanted bin be greater. Certainly a ratio of 2:1 (6 dB) would result in a
completely clean digital signal. However, for the AM of conventional
design (peak detector), the ratio ends up being the same as the
S/N, i.e., 100:1 for 40 dB, 400:1 for 52 dB, etc., when you
use envelope detection. Digital detection with filtering works by
completely suppressing __anything__ that is not in the passband.
This means that anything removed from the carrier by more than the
audio passband of 10 kHz is gone. It is removed __before__ any of the 
nonlinear effects of analog circuitry can occur. The nonlinearity is
the nonlinearity of the A/D converter at the head-end.

Of course a practical receiver would have some kind of AGC amplifier
before the A/D converter to prevent outright overload and to try
to exercise as many bits as possible so that you truly have 96 dB
of dynamic range. Nevertheless, the technology is readily available.

Anybody who has "brought up" a ham SSB signal out of the noise
has first-hand experience how synchronous detection works. Digital
synchronous detection works even better because with two sidebands
containing the same information, you get an automatic 6 dB improvement
in S/N. Then, anything that is not the same on the lower sideband
as is on the upper sideband, inverted, will not be recovered. This
means that only the original audio, which was identical on both
sidebands, will be recovered. Any of the so-called "splatter" that
is not synchronous with the audio, and therefore is not identical
on both sidebands, will not be present in the output. Needless to
say, you can't use AM stereo in a detector designed to suppress
anything that is not the same in both sidebands.

Selective fading alters both the phase and amplitude of upper
and lower sidebands. However, this is a relatively slow affect
that can be handled in software. It's the high-frequency differences
that are used to suppress that high-frequency interference, which
if not suppressed will result in what sounds like cross-modulation.

It's really quite simple to implement. The lower sideband information
is subtracted from the inverted upper sideband information. The result
is all the stuff that didn't exist during transmission. You subtract
this (digitally) from the digitally demodulated signal after high-
pass filtering to remove the effects of selective fading. Since
the time-delays through a 20 MHz digital system are practically
nothing compared to the 10 kHz bandwidth, one doesn't even have
to add any time-delays for filter compensation.

All this "processing" is necessary only to compensate for
imperfect propagation and imperfect transmitters. If the
IBOC specifications are correctly followed, these two information
sources can coexist. It's just that envelope detectors will
detect everything that affects the amplitude, including the
digital signal and that's not what you want.

>
> I look forward to your response! :)
>
> Willie...
>
>
>
>
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Richard B. Johnson
Project Engineer
Analogic Corporation


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