On 9/12/06 10:04 PM, in article , "Tim Williams"
wrote:

"Don Bowey" wrote in message
...
There is no such thing as a .00000000001 dB signal

Sure there is. It's very close in amplitude to a 0.0dB signal. ;-)

Tim

Yes Master.... I had a momentary lapse of acumen, but it is clear now. dBm
to the people. :-)

"Michael A. Terrell" wrote in message
...
dB without a reference is meaningless. How can you have a ratio
without a reference?

Easy. "dB" in general usually refers to acoustic power, where the reference
is some ungodly small power level (10^-12W/m^2 IIRC?).

I forget if there's a similar radio context used...

Tim

--
Deep Fryer: a very philosophical monk.
Website: http://webpages.charter.net/dawill/tmoranwms

The answer is this:

It would be far more suceptable to interference than the AM equivalent.

The far higher bandwidth gives you a far higher noise bandwidth than the
narrower AM equivalent.

So because of the large bandwidth, AM would beat it hands-down.


Sam

"Radium" wrote in message
oups.com...
This analog signal [which was PCM] is then sent to a
loudspeaker. Just to make things more interesting, the antennae and
receivers are so sensitive that they can pick signals as low as
.00000001 dB. Most likely, what would I hear?


I think you need to be a bit clearer in your thinking. I see several people
have commented on your use of dB and it seems Mike dealing with the digital
side so I'll not pick up on those. I'd like to comment on " the antennae
and receivers are so sensitive that they can pick signals as low as........"
and your other comment about wide bandwidth.

Firstly, a "sensitive antenna" isn't a good concept, better to think in
terms of gain.

However, more importantly, sensitivity isn't just about how "small" a signal
your receiver system can "pick up"- you can (in theory) just add more and
more gain. The issue is the ratio of the signal to the noise- that is the
noise your receiver introduces and that which is "picked up" by the antenna.
Winding up the gain doesn't help much with the latter- the noise in the
available bandwith is amplified as well. Often a good way to get a better
signal to noise ratio is to reduce the bandwidth so, before you get too hung
up on having a wide bandwidth, think about what you need to do the job.


I also notice someone mentioned Galois- there was a thread some time back in
uk.radio.amateur where I explained the maths behind these. I'd sure a search
of Google Groups will turn it up.

--
73
Brian
www.g8osn.org.uk

On 9/13/06 8:43 AM, in article , "Tim Williams"
wrote:

"Michael A. Terrell" wrote in message
...
dB without a reference is meaningless. How can you have a ratio
without a reference?

Easy. "dB" in general usually refers to acoustic power, where the reference
is some ungodly small power level (10^-12W/m^2 IIRC?).

I forget if there's a similar radio context used...

Tim

There is no exception; dB is meaningless without a reference. Decibel is
used in radio also.

Don

Tim Williams wrote:

"Michael A. Terrell" wrote in message
...
dB without a reference is meaningless. How can you have a ratio
without a reference?

Easy. "dB" in general usually refers to acoustic power, where the reference
is some ungodly small power level (10^-12W/m^2 IIRC?).

I forget if there's a similar radio context used...


You need to do a lot of studying on how to use the dB. Without a
reference, it is meaningless. The classic use in audio was 1 mW into
600 Ohms = 0 dBm, and yes, there are a number of RF uses for the dB.
Either in reference to one of several different reference levels, or as
an absolute ratio, such as the input to output level of an amplifier.

--
Service to my country? Been there, Done that, and I've got my DD214 to
prove it.
Member of DAV #85.

Michael A. Terrell
Central Florida

Samuel Hunt wrote:
The answer is this:

It would be far more suceptable to interference than the AM equivalent.

Including heterodynes?

The far higher bandwidth gives you a far higher noise bandwidth than the
narrower AM equivalent.

So because of the large bandwidth, AM would beat it hands-down.


Sam

You got a heterodyne fetish, Radium? There must be some kind of support
group for that.

http://groups.google.co.uk/group/rec...acb45 99d8f13


"Radium" wrote in message
oups.com...
Including heterodynes?

Brian Reay wrote:
"Radium" wrote in message
oups.com...
This analog signal [which was PCM] is then sent to a
loudspeaker. Just to make things more interesting, the antennae and
receivers are so sensitive that they can pick signals as low as
.00000001 dB. Most likely, what would I hear?


I think you need to be a bit clearer in your thinking. I see several people
have commented on your use of dB and it seems Mike dealing with the digital
side so I'll not pick up on those. I'd like to comment on " the antennae
and receivers are so sensitive that they can pick signals as low as........"
and your other comment about wide bandwidth.



Firstly, a "sensitive antenna" isn't a good concept, better to think in
terms of gain.

Okay, in this theoretical experiment of mine, the gain is set at
maximum thats physically-possible


However, more importantly, sensitivity isn't just about how "small" a signal
your receiver system can "pick up"- you can (in theory) just add more and
more gain.

Okay.

The issue is the ratio of the signal to the noise- that is the
noise your receiver introduces and that which is "picked up" by the antenna.

Winding up the gain doesn't help much with the latter- the noise in the
available bandwith is amplified as well.

Hopefully I can get some frightening-yet-enjoyable heterodynes from far
outer space amplified in my hypothetical audio system.

Often a good way to get a better
signal to noise ratio is to reduce the bandwidth so, before you get too hung
up on having a wide bandwidth, think about what you need to do the job.

Okay.


I also notice someone mentioned Galois- there was a thread some time back in
uk.radio.amateur where I explained the maths behind these. I'd sure a search
of Google Groups will turn it up.

--
73
Brian
www.g8osn.org.uk

It would be far more suceptable to interference than the AM equivalent.

Including heterodynes?

Theoretically, with optimal decoding, you require around 3dB C:N to decode
an AM digital signal.

3dB C:N as opposed to the 20dB C:N that you need to get a good AM signal
sounds to be a winner.

But AM would be about 30khz bandwidth, and this PCM signal would be 3mhz.

That means that the bandwidth gives you at least 20dB less sensitivity, so
comparing the signal bandwidth-wise, you only require 0dB C:N across the
same bandwidth to get the AM signal.

So you have a 3dB advantage for conventional AM over PCM.

Next, let us look at the nature of AM and heterodynes.

By the nature of audio AM, you will find that a single heterodyne can
degrade the C:N to as low as 10dB before it becomes perceptible.

So therefore in the same bandwidth with PCM, you then have -10dB C:N, which
is not enough to decode the PCM.

Therefore, PCM is inferior to AM, and you would not only be wasting precious
bandwidth, and face considerable issues with other transmissions and the
physical design of the antenna, transmitters and receivers, you would also
find that it is nowhere near as effective.

Maybe studying something like GSM compression or MP3 compression formats,
FEC and COFDM or similar may be your answer.

COFDM with a good FEC system is one of the most robust methods to transfer
digital data in the presence of heterodynes there is. With the correct
encoding and decoding techniques, you can have easily -80dB C:N because of a
heterodyne some 80dB stronger than your signal, and the data would be still
decoded correctly. Theoretically you could have hetrodynes thousands of dB
stronger than the carrier, but unfortunately the reciever technologies are
nowhere near that advanced yet, but even with cheap decoders, you could aim
for around 80dB as a realistic goal under ideal situations (which is what
you appear to advocate).


Sam
M1FJB