Mohinder,
Do not quickly jump to conclusions. What I attempted to do was to explain in simple layman's terms what the GRTS FM station is supposed to be. As Beran has rightly pointed out, simul-cast systems have the capability to simultaneously broadcast both analogue and digital information through the same transmission medium. It is also apparent that digital systems also require less power and travel shorter distance than analogue signals of the same power. Because of this limtation, regenerators are used at regular intervals to recreate the original digital signals.
I assume you know that for decades the US military have been using spread spectrum technology to send digital information. A spread spectrum system is one in which the transmitted signal is spread over a wide frequency band, much wider, in fact, than the minimum bandwidth required to transmit the information being sent. This technology is used in cordless phones as well as to send control signals to probes and satelites.
Dampha, any engineer worth his salt should be able to differentiate between digital and analogue signals and systems. Do you remember how I have described some of you as behaving as if the world is digital to the extend that everything is an all or nothing issue to you?
Yus, your opinion of me does not matter to at all!
Have a good day, Gassa.
Please enjoy this article on spread spectrum technology Curtesy of Digital communications by Harold E. Price.
Spread spectrum communications cannot be said to be an efficient means of utilizing bandwidth. However, it does come into its own when combined with existing systems occupying the frequency. The spread spectrum signal being "spread" over a large bandwidth can coexist with narrowband signals only adding a slight increase in the noise floor that the narrowband receivers see. As for the spread spectrum receiver, it does not see the narrowband signals since it is listening to a much wider bandwidth at a prescribed code sequence.
Here are five types of spread spectrum techniques:
Direct Sequence Systems - Direct sequence is perhaps one of the most widely known and utilized spread spectrum systems and it is relatively simple to implement. A narrow band carrier is modulated by a code sequence. The carrier phase of the transmitted signal is abruptly changed in accordance with this code sequence. The code sequence is generated by a pseudorandom generator that has a fixed length. After a given number of bits the code repeats itself exactly. The speed of the code sequence is called the chipping rate, measured in chips per second (cps). For direct sequence, the amount of spreading is dependent upon the ratio of chips per bit of information. At the receiver, the information is recovered by multiplying the signal with a locally generated replica of the code sequence.
Frequency Hopping Systems - In frequency hopping systems, the carrier frequency of the transmitter abruptly changes (or hops) in accordance with a pseudo random code sequence. The order of frequencies selected by the transmitter is dictated by the code sequence. The receiver tracks these changes and produces a constant IF signal.
Time Hopping Systems - A time hopping system is a spread spectrum system in which the period and duty cycle of a pulsed RF carrier are varied in a pseudorandom manner under the control of a coded sequence. Time hopping is often used effectively with frequency hopping to form a hybrid time-division, multiple-access (TDMA) spread spectrum system. Each burst consists of k bits of data and the exact time each burst is transmitted is determined by a PN sequence.
Pulsed FM (Chirp) Systems - A pulsed FM system is a spread spectrum system in which a RF carrier is modulated with a fixed period and fixed duty cycle sequence. At the beginning of each transmitted pulse, the carrier frequency is frequency modulated causing an additional spreading of the carrier. The pattern of the frequency modulation will depend upon the spreading function which is chosen. In some systems the spreading function is a linear FM chirp sweep, sweeping either up or down in frequency.
Hybrid Systems - Hybrid systems use a combination of spread spectrum methods in order to use the beneficial properties of the systems utilized. Two common combinations are direct sequence and frequency hopping. The advantage of combining the two methods is to capitalize on characteristics that are not available from a single method.
There is a time in the life of every problem when it is big enough to see, yet small enough to solve. -Mike- Levitt-
>From: Mohinder Bains <
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>Reply-To: The Gambia and related-issues mailing list <
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>To:
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>Subject: Re: Daft or Inept
>Date: Thu, 28 Feb 2002 12:47:24 +0000
>
>Jungle Sunrise wrote
>"
>What I meant to write was that all FM transmissions are NOT digital.
>I
>could not send a rejoinder to rectify what I wrote because I had
>used up all
>my allowable postings.
>
>"
>
>The contents of your posting conveys otherwise which leads me to
>suspect you
>have a misconception of what digital Radio is. Being one of the
>best ( or
>rather the best) Engineer in the Gambia makes one wonder what your
>credentials are if you cannot even answer a simple question within
>your
>career domain. Woe betide the gambian if all her Engineers are your
>like.
>
>
>
>Mr Njie,
>
>Yus was right when he said
>
>"
>>one has to have a digital radio to receive >transmissions
>>from a digital radio station. "
>
>I think your impression of digital transmission is the opposite to
>what it
>is. It is more expensive and limited in distance ( but this can be
>overcome
>by strategically placed transmitters or by satellite. Digital
>signals are
>also susceptible to distortion and noise
>which, is usually minimized by building into receivers a
>centre-sampler
>circuitry for determining the threshold value for received signals (
>i.e
>anything above the threshold value is decoded as 1 and otherwise 0)
>since
>noise may degrade the signal to a great extend.
>
>Digital broadcasting requires the use of a large amount of bandwidth
>due to
>the encoding procedure involved ( i.e the modulation method,
>quantizing and
>coding of the signal etc). It would require a long posting to delve
>into
>the modalities of how this is achieved, which is best left for
>later.
>
>To obtain clarity of sound with digital systems, distance is a
>limiting
>factor and more complex circuitry is needed at the receiver to
>decode the
>signals. The current radio sets do not have the required circuitry
>for
>suitable reception.
>
>In Europe, digital radio receivers are selling at around £200 - £300
>per unit, which in my opinion would dent the pocket of any ordinary
>Gambian
>whether abroad or in the Gambia.
>
>May be in time, the cost of these units may be affordable to the
>masses and
>only then would the justification for setting up such stations be
>realised.
>Don't misunderstanding me on this point. What I am trying to say is
>that,
>if this stations are to be commercially viable, they need to reach a
>wide
>audience, and the only way to achieve this is if these audience can
>tune
>in.
>
>I think Yus would enlightened further on your questions. However,
>the
>basics of AM and FM (for Analog transmission) follows:
>
>AM modulation produces two sidebands (Upper & Lower) after
>modulation and
>both sidebands carry the same information. sometimes one of the
>sideband is
>suppressed to save on transmitting bandwith. This technique is
>known as
>Suppressed-Single-Sideband (SSB).
>
>
>FM Modulation on the other hand produces a series of sidebands on
>both sides
>of the carrier( the frequencies of these sidebands are scaled
>according to
>their distance from the carrier ), and since all these sidebands
>carry the
>same information, one tends to tune get a clear reception.
>
>I think this enlightened you on these aspect.
>
>Thanks
>Mohin
>
>
>
>
>
>
>
>
>
>
>
>
>
>
>
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