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

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

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

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

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

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

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

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

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

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

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

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

"Michael A. Terrell" wrote in message
...
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

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

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

Hook up your computer with SETI.You might hear ET out there.Seriously.
cuhulin

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

In article ,
"Tim Williams" wrote:

"Michael A. Terrell" wrote in message
...
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.

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 a
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.

--
Telamon
Ventura, California

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

Tim Williams wrote:

"Michael A. Terrell" wrote in message
...
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.

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

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