Mike Gathergood (G4KFK) wrote:
Radium wrote:
Mike Gathergood (G4KFK) wrote:
Radium wrote:
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.

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.

What would these errors sound like?




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.

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?


Cheers
Mike

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.

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 he 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" wrote in message
oups.com...

Mike Gathergood (G4KFK) wrote:
Radium wrote:
Mike Gathergood (G4KFK) wrote:
Radium wrote:
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.

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.

What would these errors sound like?




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.

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.

"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

"Radium" wrote in message
oups.com...
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.

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

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?

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