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...
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