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

[Radium]

Hi:

Hypothetical situation: a PCM audio signal [24-bit and monoaural] is
transmitted through an analog 3 Mhz AM carrier, an AM receiver on the
other end [tuned to 3 Mhz] picks up the signal, and the reciever is
attached to a device that can recieve, process, and decode the PCM
audio back to analog and then send it to a loudspeaker. However -- in
this theoretical situation -- the environment is filled with EMI, RFI,
and heterodynes that affect all AM stations.

My question: Will the received PCM audio signal remain noticeably
"clean" to the listener or will he/she notice the EMI, RFI, and
heterodynes affecting the audio?

I ask because I think -- but definitely don't know -- that because the
received signal is digital, it is less likely that the EMI, RFI, and
heterodynes would cause noticeable auditory disruptions when compared
to analog. Do I guess correct?


Thanks,

Radium

 

[Meat Plow]

Hi:

Hypothetical situation: a PCM audio signal [24-bit and monoaural] is
transmitted through an analog 3 Mhz AM carrier, an AM receiver on the
other end [tuned to 3 Mhz] picks up the signal, and the reciever is
attached to a device that can recieve, process, and decode the PCM audio
back to analog and then send it to a loudspeaker. However -- in this
theoretical situation -- the environment is filled with EMI, RFI, and
heterodynes that affect all AM stations.

My question: Will the received PCM audio signal remain noticeably "clean"
to the listener or will he/she notice the EMI, RFI, and heterodynes
affecting the audio?

I ask because I think -- but definitely don't know -- that because the
received signal is digital, it is less likely that the EMI, RFI, and
heterodynes would cause noticeable auditory disruptions when compared to
analog. Do I guess correct?

I suppose that is possible especially if the reciever/decoder can buffer
the data.

 

[Mike Gathergood (G4KFK)]

My question: Will the received PCM audio signal remain noticeably
"clean" to the listener or will he/she notice the EMI, RFI, and
heterodynes affecting the audio?

I ask because I think -- but definitely don't know -- that because the
received signal is digital, it is less likely that the EMI, RFI, and
heterodynes would cause noticeable auditory disruptions when compared
to analog. Do I guess correct?

You guess correct, assuming that FEC is applied to the digital signal
before it is used to modulate the transmitter.

But you wouldn't do it that way anyway. Raw PCM is too bandwidth
inefficient. You'd use MPEG layer 2, or apt-X, or something like that
to reduce the bandwidth without noticeably degrading the audio quality.
You'd probably also multiplex several different channels (programmes)
together onto one RF carrier as well, to make better statistical use of
the RF bandwidth.

Cheers
Mike

 

[Radium]

You guess correct,
assuming that FEC is applied to the digital signal
before it is used to modulate the transmitter.

What is FEC?

But you wouldn't do it that way anyway. Raw PCM is too bandwidth
inefficient.

Isn't 3 Mhz enough to transmit a data rate of 1.06 mbps?

44,100 X 24 = 1,058,400

 

[Mike Gathergood (G4KFK)]

What is FEC?

Forward Error Correction. Google it

Isn't 3 Mhz enough to transmit a data rate of 1.06 mbps?

Yes - but the chipsets to compress the digitised audio are much cheaper
than the notional value of the bandwidth you would be trashing with
your 24bit PCM.

Cheers
Mike

 

[Radium]

You guess correct, assuming that FEC is applied to the digital signal
before it is used to modulate the transmitter.

What if FEC is not used?

 

[Brian Reay]

Forward Error Correction. Google it

Just to add to Mike's comment, FEC works by send the same message several
times- in simple terms, in the hope that one with get through correctly. It
is simple to implement, your recieving system just needs to be able to
identify a correct message and use it, not spot a bad message and initiate
either a request for resend or applly some sort of correction method
(assuming there is error correcting code in the message).

 

[[email protected]]

Just to add to Mike's comment, FEC works by send the same message several
times- in simple terms, in the hope that one with get through correctly. It
is simple to implement, your recieving system just needs to be able to
identify a correct message and use it, not spot a bad message and initiate
either a request for resend or applly some sort of correction method
(assuming there is error correcting code in the message).

These comms are in one direction, so you don't resend a packet as there
is no way to make such a request. In practice, the codes have both
error detection and correction capabilities, so to the degree the
coding allows, the signal can be corrected with the bits that were
received. Note nobody mentioned a modulation scheme for sending this
data.

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.]

 

[Mike Gathergood (G4KFK)]

What if FEC is not used?

With no FEC, your receiver would be more prone to those errors that you
were worried about in the first place.

Just to amplify on this, anything that you can do to reduce the
bandwidth of the digital signal before it's used to modulate the
transmitter will help. One very big advantage is that you can wind down
the bandwidth of the receiver, thus reducing the level of background
noise in the system.

Why did you choose 3MHz? What's the application?

Cheers
Mike

 

[Mike Gathergood (G4KFK)]

Just to add to Mike's comment, FEC works by send the same message several
times- in simple terms, in the hope that one with get through correctly. It
is simple to implement, your recieving system just needs to be able to
identify a correct message and use it, not spot a bad message and initiate
either a request for resend or applly some sort of correction method
(assuming there is error correcting code in the message).

FEC is generally used in applications where you don't have time to
request a retransmission of an errored packet, and/or where the
transmission path is simplex, and/or where there are multiplex
receivers for a single transmitter.

If you have the luxuries of time and a full-duplex point-to-point
environment, ARQ is better.

Cheers
Mike

 

[Radium]

With no FEC, your receiver would be more prone to those errors that you
were worried about in the first place.

What would these errors sound like?

Just to amplify on this, anything that you can do to reduce the
bandwidth of the digital signal before it's used to modulate the
transmitter will help. One very big advantage is that you can wind down
the bandwidth of the receiver, thus reducing the level of background
noise in the system.

Why did you choose 3MHz?

44,100 X 24 = 1,058,400

1,058,400 bps requires that the frequency of the carrier be at least
2,646,000 Hz. To make it safe, use 3 MHz.

What's the application?

Well, my application was more to do with reception than transmission.

I'd like to know what I would hear on a 3MHz AM carrier whose receiver
[both the AM and the linear PCM part] is at its maximum bandwidth. The
3 Mhz AM receiver is attached to a linear-PCM receiver [once again,
both receivers have the maximum bandwidth possible for them]. The
linear-PCM receiver is attached to a DAC which converts the linear-PCM
signal to analog. 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?

 

[Geoffrey S. Mendelson]

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.

 

[Mike Gathergood (G4KFK)]

What would these errors sound like?

An error in a PCM system would manifest itself as a difference between
what you put in at the analogue input to the transmitter, and what you
got out of the analogue output of the receiver.

The magnitude and polarity of the difference would depend entirely on
whether the bit error was the MSB (polarity would be wrong), or one of
the LSBs (the amplitude would be wrong). It wouldn't "sound" like
anything in particular.

Well, my application was more to do with reception than transmission.

I'd like to know what I would hear on a 3MHz AM carrier whose receiver
[both the AM and the linear PCM part] is at its maximum bandwidth. The
3 Mhz AM receiver is attached to a linear-PCM receiver [once again,
both receivers have the maximum bandwidth possible for them]. The
linear-PCM receiver is attached to a DAC which converts the linear-PCM
signal to analog. 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?

Have a look here: http://www.imdb.com/title/tt0375210/

Seriously though, I have no idea. Why don't you try it and post the
results here?

Cheers
Mike

 

[Radium]

An error in a PCM system would manifest itself as a difference between
what you put in at the analogue input to the transmitter, and what you
got out of the analogue output of the receiver.

The magnitude and polarity of the difference would depend entirely on
whether the bit error was the MSB (polarity would be wrong), or one of
the LSBs (the amplitude would be wrong). It wouldn't "sound" like
anything in particular.

What about the heterodyne tones present on analog AM radio? Would they
be audible on a linear-PCM receiver that receives PCM signals on an AM
station?

Well, my application was more to do with reception than transmission.

I'd like to know what I would hear on a 3MHz AM carrier whose receiver
[both the AM and the linear PCM part] is at its maximum bandwidth. The
3 Mhz AM receiver is attached to a linear-PCM receiver [once again,
both receivers have the maximum bandwidth possible for them]. The
linear-PCM receiver is attached to a DAC which converts the linear-PCM
signal to analog. 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?

Have a look here: http://www.imdb.com/title/tt0375210/

Seriously though, I have no idea. Why don't you try it and post the
results here?

Easy for you to ask. I doubt any store has the device. And the
equipment required to amplify .00000000001 dB to an audible level would
take up the entire room.

So the best I could do -- at least for the moment -- is guess.

I am aware though that just because the PCM-receiver is digital does
not mean its completely immune to heterodynes, EMI, or RFI. If the
heterodyne, EMI or RFI has a waveform that sufficiently resembles a PCM
signal, it may very well be picked up by the PCM-receiver that is
connected to the AM receiver.

Physically, the digital reciever is still an electronic device and
hence it has some reception of EMI, RFI, and heterodynes. Its just not
affected as much as an analog receiver would be.

Cheers
Mike

I've seen that "white noise" movie. But thats more like Sci-Fi. Yet it
is one thing that gave me the interest to hear whatever is buried DEEP
in background noise.

 

[Don Bowey]

An error in a PCM system would manifest itself as a difference between
what you put in at the analogue input to the transmitter, and what you
got out of the analogue output of the receiver.

The magnitude and polarity of the difference would depend entirely on
whether the bit error was the MSB (polarity would be wrong), or one of
the LSBs (the amplitude would be wrong). It wouldn't "sound" like
anything in particular.

What about the heterodyne tones present on analog AM radio? Would they
be audible on a linear-PCM receiver that receives PCM signals on an AM
station?

Well, my application was more to do with reception than transmission.

I'd like to know what I would hear on a 3MHz AM carrier whose receiver
[both the AM and the linear PCM part] is at its maximum bandwidth. The
3 Mhz AM receiver is attached to a linear-PCM receiver [once again,
both receivers have the maximum bandwidth possible for them]. The
linear-PCM receiver is attached to a DAC which converts the linear-PCM
signal to analog. 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?

Have a look here: http://www.imdb.com/title/tt0375210/

Seriously though, I have no idea. Why don't you try it and post the
results here?

Easy for you to ask. I doubt any store has the device. And the
equipment required to amplify .00000000001 dB to an audible level would
take up the entire room.

There is no such thing as a .00000000001 dB signal

So the best I could do -- at least for the moment -- is guess.

I am aware though that just because the PCM-receiver is digital does
not mean its completely immune to heterodynes, EMI, or RFI. If the
heterodyne, EMI or RFI has a waveform that sufficiently resembles a PCM
signal, it may very well be picked up by the PCM-receiver that is
connected to the AM receiver.

Physically, the digital reciever is still an electronic device and
hence it has some reception of EMI, RFI, and heterodynes. Its just not
affected as much as an analog receiver would be.

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.

 

[Radium]

Radium wrote:
What would these errors sound like?

An error in a PCM system would manifest itself as a difference between
what you put in at the analogue input to the transmitter, and what you
got out of the analogue output of the receiver.

The magnitude and polarity of the difference would depend entirely on
whether the bit error was the MSB (polarity would be wrong), or one of
the LSBs (the amplitude would be wrong). It wouldn't "sound" like
anything in particular.

What about the heterodyne tones present on analog AM radio? Would they
be audible on a linear-PCM receiver that receives PCM signals on an AM
station?

Well, my application was more to do with reception than transmission.

I'd like to know what I would hear on a 3MHz AM carrier whose receiver
[both the AM and the linear PCM part] is at its maximum bandwidth. The
3 Mhz AM receiver is attached to a linear-PCM receiver [once again,
both receivers have the maximum bandwidth possible for them]. The
linear-PCM receiver is attached to a DAC which converts the linear-PCM
signal to analog. 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?

Have a look here: http://www.imdb.com/title/tt0375210/

Seriously though, I have no idea. Why don't you try it and post the
results here?

Easy for you to ask. I doubt any store has the device. And the
equipment required to amplify .00000000001 dB to an audible level would
take up the entire room.

There is no such thing as a .00000000001 dB signal

Whats stops a .00000000001 dB signal from existing?

 

[Tim Williams]

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

 

[Ron Baker, Pluralitas!]

Hi:

Hypothetical situation: a PCM audio signal [24-bit and monoaural] is
transmitted through an analog 3 Mhz AM carrier, an AM receiver on the
other end [tuned to 3 Mhz] picks up the signal, and the reciever is
attached to a device that can recieve, process, and decode the PCM
audio back to analog and then send it to a loudspeaker. However -- in
this theoretical situation -- the environment is filled with EMI, RFI,
and heterodynes that affect all AM stations.

My question: Will the received PCM audio signal remain noticeably
"clean" to the listener or will he/she notice the EMI, RFI, and
heterodynes affecting the audio?

Depends.
What is the transmitter power? What is the separation
between the transmitter and receiver?

I ask because I think -- but definitely don't know -- that because the
received signal is digital, it is less likely that the EMI, RFI, and
heterodynes would cause noticeable auditory disruptions when compared
to analog. Do I guess correct?

If there is a decent SNR you are correct.
If the SNR falls below a certain value then
the reverse is true.

 

[Michael A. Terrell]

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

dB without a reference is meaningless. How can you have a ratio
without a reference?


--
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

 

[Ron Baker, Pluralitas!]

With no FEC, your receiver would be more prone to those errors that you
were worried about in the first place.

What would these errors sound like?

Just to amplify on this, anything that you can do to reduce the
bandwidth of the digital signal before it's used to modulate the
transmitter will help. One very big advantage is that you can wind down
the bandwidth of the receiver, thus reducing the level of background
noise in the system.

Why did you choose 3MHz?

44,100 X 24 = 1,058,400

1,058,400 bps requires that the frequency of the carrier be at least
2,646,000 Hz. To make it safe, use 3 MHz.

What's the application?

Well, my application was more to do with reception than transmission.

I'd like to know what I would hear on a 3MHz AM carrier whose receiver
[both the AM and the linear PCM part] is at its maximum bandwidth. The
3 Mhz AM receiver is attached to a linear-PCM receiver [once again,
both receivers have the maximum bandwidth possible for them]. The
linear-PCM receiver is attached to a DAC which converts the linear-PCM
signal to analog. 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.

dB is a ratio, not a power.