[BC] IBOC "secrets" and my opinions.

[Robert Orban]

At 10:39 AM 3/27/2007, Richard Johnson wrote:

Just a few comments:

>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.

No digital filter can completely suppress out of band power. Theoretical 
"brick wall" filters require infinite time delay, and this is true 
regardless of the technology implementing the filter -- it's a "laws of 
physics" issue. Digital filters have cost/performance tradeoffs like any 
other technology. They can be very effective, and the cost for making very 
selective filters keeps decreasing. But "completely suppressing?" Can't be 
done.

Moreover, DSP technology for consumer radios (including digital IF 
filtering and detection) has been around for some time and available for 
purchase in some currently manufactured radios. Eventually, I expect that 
all radios will be DSP-based because the tech will get progressively 
cheaper as time goes on.


>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.

96 dB corresponds to a very specific word length: 16 bits.

>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.

Assuming that the RF noise floor consists of random noise, the S/N gain 
will be 3 dB, not 6 dB. The noise in the sidebands is uncorrelated and thus 
sums in an r.m.s. manner, resulting in 3 dB greater than the noise in one 
sideband, while the signal sums to 6 dB because the signal in the two 
sidebands is correlated. This gives you a 3 dB improvement in S/N.

If the interference under the sidebands is not random noise, then the 
amount of improvement will depend on the nature of the interference and can 
be quite remarkable if the interference is mainly localized under one sideband.

Bob Orban