"Telamon" wrote in message
...
In article ,
"David Eduardo" wrote:

The general consensus as to why far less signal is about as effective
has to do with noise. A digital signal can be correctly decoded even
when there is noise just a few db below the signal itself.

The claim here "can be correctly decoded even when there is noise just
a few db below the signal itself" is no more possible than an analog
signal can be heard a few dB over the noise floor. Now both these
claims depend on the probability of the ratio of the instantaneous
noise power over the instantaneous signal modulation power.

The HD signal is digital. The noise is not, it is analog. As long as there
is enough digital data to extract, the analog noise is not the issue.

HD duplicates the same data on both sides of the carrier, so there is
the ability to select the best data, much like diversity reception.
And HD "dithers" in the case of small dropouts.

This is a rational explanation based on the argument that the total
bandwidth utilized by the digital mode may be better utilized over the
analog mode but this depends on wether an analog radio output employing
an envelope detector suffers when one side band degrades. Ask your
engineering buddies.

We are talking about the ability to receive a substantially intact digital
data stream in an analog noise filled environment. This can be done with the
noise floor just a few db below the digital data. In analog, the noise and
the information you wish to recover are both analog and mix. So the signal
has to be significantly, on the order of around 60 db, above the noise
floor. So, tihe a difference of perhaps 57 db between digital and analog
usability, low power on the digital can produce excellent results.

BTW, I have been lead engineer for a group of a dozen stations, including
building the transmitters and studio gear from scratch. I talk to our
engineers often because we use technology to our advantage to create better
radio stations.

Analog requires something over a -57 db noise floor to be useful to
the average listener, and something in the -60's for really nice FM
reception.

Arbitrary numbers that do not account for individual reception
situations.

Wrong. Analong noise and signal combine. Digital can be plucked out of the
analog noise. And for analog, the noise has to be around -57 db or the
average listener finds it noise and unlistenable. This is why below the 64
dbu contour of the average radio station there is essentially no
listening... it is too noisy.

All the engineers (and there are 8 of them for our 5 signals)
believed, in conclusion, that the determining factor on usability on
an analog signal is also noise, which is why in LA we get no listeing
outside the 64 dbu on FM and about the 10 to 12 mv/m daytime and the
15 mv/m night on AM.

OK it is fine to set limits on what is considered good or bad signal to
noise but that does not change the fact that when the signal to noise is
small both HD and analog are not easy to listen too.

The HD digital stream can be picked out of the analog noise. The analog
signal becomes a part of it and is inseparabble.


The reality is that the HD data can be extracted and DACed when the
noise is only a few db below the signal itself.

What do you mean by "DACed." If you mean digital to analog converter I
hope you understand that every time an analog signal goes through the
process of analog to digital conversion at the transmitter and then
digital to analog in the receiver that a set of errors and distortion is
added to the resulting analog signal but that is not the argument you
are trying to make.

Of course I understand that. The fact is, the last step of a digital
transmission system is to do a digital to analog conversion, since the ear
is not digital.

Can you understand that as the signal level approaches the noise floor
that the probability of the 1/0 data stream being correctly detected
decreases? If the data stream becomes corrupted and then converted to
analog it will not represent the original programing now will it. Can
you see the similarity to analog in this regard?

No, I understand that the noise is analog. And the HD stream is digital, and
once detected is separable from the noise. Not so in analog. The data stream
is redundant (to each side of the carrier center, and has dithering as well.
If the digital signal fails, it falls back to analog. However, as stated,
the analog signal gets essentially no listening beyond the 64 dbu curve due
to... you asked for it... NOISE!