Receiving Pulse-Code Modulation on AM radio at 3 Mhz?

[[email protected]]

As a bit of trivia, Reed Solomon encoding was invented without a way to
decode it. That's what you get when you let mathematicians run wild.
For absolutely nothing of any value other than bragging rights, name
the guy who invented the decoding scheme for Reed Solomon. [Hopefully
this isn't wikied someplace. I did one class in grad school on error
detection and correction, and it was a pain in the ass if you get into
the theory. Implementation is quite simple.]

It makes sense. The encoding software had to be ready to put into a probe
before the launch date. Once it was up it could not be changed.

Decoding software was another matter. Since they had years, maybe even decades
to decode the data, and it did not have to be real time, they could continue
to work on it.

All they had to do is not loose the tapes. :-(

Geoff.

I'm not sure what you mean by the "probe". The deal with Reed Solomon
is it is a non-binary code, which was a big deal at the time. The
buzzword is Galois mathematics. There wasn't any hardware that could
handle the code when it was invented.

 

[Don Bowey]

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.

 

[Tim Williams]

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

 

[Samuel Hunt]

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

 

[Brian Reay]

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.

 

[Don Bowey]

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

 

[Michael A. Terrell]

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

 

[Radium]

The answer is this:

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

Including heterodynes?

 

[Radium]

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.

 

[Samuel Hunt]

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

 

[Tim Williams]

You need to do a lot of studying on how to use the dB.

I know full well what a logarithm is; don't patronize me.

My point was that some otherwise ambiguous dB scales (at least one) have a
defined absolute basis.

Tim

 

[Telamon]

I know full well what a logarithm is; don't patronize me.

My point was that some otherwise ambiguous dB scales (at least one) have a
defined absolute basis.

Like Michael stated dB is a logarithmic reference-less ratio value.

You can use dB for things like amplifiers that have a gain or
attenuators that have a loss for example. The gain of an amplifier can
be expressed in dB because the reference value is the input value of
the amplifier, which will allow you to calculate the output value but
if you are speaking of a value of power or voltage by itself then you
need an absolute scale with a reference quantity.

Absolute scales would be dBV, dBuV, and dBm. In those terms the
reference is 1 volt, 1 micro-volt (0.000001 volt), and 1 milliwatt
(.001 watt).

The reason you need a reference value is noise prevents you from
measuring 0 Volts and 0 watts accurately so you need to use a small
reference value in its place and so everyone agreed to use these
values.

Definitions are:
dBV = 20 * log (volts / 1 )
dBuV = 20 * log ( volts / 0.000001 )
dBm = 10 * log ( power / .001 )

So for example:
30 dBm = 1.0 watt
0 dBm = 0.001 watt (the reference value)
-30 dBm = 0.000001 watt

Most of the time in radio dBm, dBuV, and watts are used.

 

[Michael A. Terrell]

I know full well what a logarithm is; don't patronize me.

My point was that some otherwise ambiguous dB scales (at least one) have a
defined absolute basis.

I'm not "Patronizing" you. You were using it in the wrong context.


--
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

 

[Brian Reay]

Absolute scales would be dBV, dBuV, and dBm. In those terms the
reference is 1 volt, 1 micro-volt (0.000001 volt), and 1 milliwatt
(.001 watt).

The reason you need a reference value is noise prevents you from
measuring 0 Volts and 0 watts accurately so you need to use a small
reference value in its place and so everyone agreed to use these
values.

Given you are discussing use of the dB, I think the above leaves a bit out.
You can't use 0W as your reference because, given the definition of the dB,
you'd need to divide by 0 which, as I'm sure you know, isn't acceptable.

 

[jasen]

On 9/12/06 3:57 PM, in article
[email protected], "Radium"
<[email protected]> wrote:

In fringe areas, analog cell phones could be well understood despite the
poor signal-to-noise ratio. Digital cell phones in a fringe area just quit
working, or lose sync and you hear bits of other conversations.

with GSM phones I got a "guitar sounding" effect in marginal areas

Bye.
Jasen

 

[jasen]

Whats stops a .00000000001 dB signal from existing?

Nothing, it's just indistinguishable from a 0db signal.

In other words it's about half the amplitide of a 3db signal...

It's a logarythmic scale.

Bye.
Jasen

 

[Michael A. Terrell]

Nothing, it's just indistinguishable from a 0db signal.

In other words it's about half the amplitide of a 3db signal...

It's a logarythmic scale.

Bye.
Jasen

No, without a reference, its use is meaningless. He was trying to
use dB for an absolute signal level, not a ratio.


--
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

 

[Tim Williams]

Like Michael stated dB is a logarithmic reference-less ratio value.

So what is acoustic dB?
http://home.new.rr.com/trumpetb/audio/dBexamp.html
SPL, I should say.

There it lies, x dB on the left scale.

Tim

 

[Michael A. Terrell]

So what is acoustic dB?
http://home.new.rr.com/trumpetb/audio/dBexamp.html
SPL, I should say.

There it lies, x dB on the left scale.

That is a SPL chart with the dB levels referenced to 0 dB SPL =
0.0002 µbar.


--
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

 

[Michael A. Terrell]

So what is acoustic dB?
http://home.new.rr.com/trumpetb/audio/dBexamp.html
SPL, I should say.

There it lies, x dB on the left scale.

Read this Sound Level Meter manual if you want to understand that
chart.

<http://rsk.imageg.net/graphics/uc/rsk/Support/ProductManuals/3304050_PM_EN.pdf>


--
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