|
|
Posted by Green Xenon [Radium] on 09/25/07 04:28
On Sep 23, 2:36 pm, "Brian Gregory [UK]" <n...@bgdsv.co.uk> wrote in
http://groups.google.com/group/uk.tech.digital-tv/msg/6a386fec832f1ef5 :
> "Green Xenon [Radium]" <gluceg...@excite.com> wrote in
messagenews:46f6d850$0$11068$4c368faf@roadrunner.com...
>
> > Hi:
> > One major reason the luminance signals of
> > television are broadcasted on an
> > AM-carrier instead of FM is
> > because FM requires large amounts of
> > bandwidth. Is there a way to use FM video
> > without hogging so much
> > bandwidth?
> > Quotes from
> >
http://groups.google.com/group/sci.electronics.basics/msg/0c013cf5371da8dc?hl=en&
:
> >
> >
> > >Multiple-level quadrature modulation,
> > >"constellation modulation",
> > >is most common for packing
> > >lots of bits per Hz of bandwidth.
> > >The more you pack,
> > >the better the s/n ratio has to be.
> > >http://en.wikipedia.org/wiki/Quadrature_amplitude_modulation
> > >http://en.wikipedia.org/wiki/Constellation_diagram
> > Does this mean that Quadrature Modulation
> > and Constellation Modulation
> > can -- at least in theory -- be applied
> > to FM video so that excessive
> > bandwidth is not needed? If so, then
> > what would be the minimum
> > radio-frequency required to transmit the video signal?
> No.
>
> These are like special modified forms of AM.
>
> You can only use these to reduce
> bandwidth of an FM signal in the same way
> that you can use AM to reduce
> the bandwidth of an FM signal.
> It just won't
> be FM any more.
Couldn’t the video signal first be converted to QAM, multiple-level
quadrature modulation, and/or constellation modulation and then
transmitted on a FM radio carrier? QAM/QM/CM on FM?
AFAIK, the QAM/multiple-level-QM/ constellation-modulation version of
the video signal won’t require as much bandwidth as the original video
signal. The original video signal is first converted to bits and symbols
and then transmitted on an FM radio carrier. To make the best use of
bandwidth, limit the baud to only 1-symbol-per-second but pack as many
bits-per-symbol as feasible.
Quotes from http://en.wikipedia.org/wiki/Baud :
“Conveying more than one bit per symbol has advantages. It reduces the
time required to send a given quantity of data, and allows modern
modems, FDDI and 100/1000 Mbit/s Ethernet LANs, and others, to achieve
high data rates. An optimal symbol set design takes into account channel
bandwidth, desired information rate, noise characteristics of the
channel and the receiver, and receiver and decoder complexity. A typical
2400-bit/s modem transmits at 600 baud (600 symbol/s), where each
quadrature amplitude modulation symbol carries four bits of information.
1000 Mbit/s Ethernet LAN cables use many wire pairs and many bits per
symbol to encode their data payloads. 1000BASE-T uses four wire pairs
and two data bits per symbol to get a symbol rate of 125MBd.”
Quotes from
http://www.physics.udel.edu/wwwusers/watson/student_projects/scen167/thosguys/baud.html
:
“So the technique is to try and "pack" as many bits as you can into 1 baud.”
So it’s always best to use the most amounts of bits-per-baud but no more
than 1 baud
bps = baud * number of bits per baud.
Remember, keep the baud no more than one-symbol-per-second but at the
same time, convey as many bits-per-symbol as mathematically and
physically possible.
Then, hopefully, it will be possible to transmit/receive video signals
on FM carriers with radio-frequencies below the medium-wave band --
without stealing bandwidth from existing stations using those
frequencies -- or frequencies near those stations.
http://en.wikipedia.org/wiki/Medium-wave
MW = from 530 kHz up to 1610 kHz
Navigation:
[Reply to this message]
|