Pulse Modulation Transmitter and Receiver Questions

[reyge]

In the modulator side (whether PPM, PWM, PCM, or Delta Modulation) , a specific timeslot is alloted for one discrete level right? So, lets say i have 8 discrete levels and i have to represent them in bits ( so i'll need 3 bits). If i am going to transmit one discrete level, i have to send the 3 bits serially ( MSB or LSB first? ).

Now, in the demodulator / decoder side, how does one distinguish this group of 3 bits from the other 3 bits that represent another discrete level? Is there some kind of a synchronization like a clock that is transmitted together with the pulse modulated signal?

thanks. i could really use some help

 

[A.Anand Srinivasan]

generally a preamble signal is sent at the beginning of transmission for synchronisation... this preamble signals is a predecided signal and is known by the demodulator too... so after the preamble ends the demodulator knows it is the starting of a discrete level and then it can find each discrete level by itself since each is 3 bit long....

 

[reyge]

woow thanks! Can you give me an example of a preamble signal? is there a general signal for that?

 

[A.Anand Srinivasan]

in certain systems there would be a particular input that would never occur and that discrete level would be used as preamble....

consider 3 bit system whichs all 0 discrete level is not used for the sake of avoiding dc wander... now a stream of continuous 0's ending with a 1 can be used as a preamble.... a 1 after a stream of continuous 0's would mean that the preamble has ended for that particular system....

 

[reyge]

oh.. so in such a system, the 3-bit would only accomodate 7 discrete levels? can you explain dc wander?

thanks.

 

[A.Anand Srinivasan]

ya those systems would often try to avoid the 111 discrete level also...
the all 0 and all 1 values are avoided due to dc wander...

when a continous stream of 1 or 0 occurs there is a possibility that the output can droop in value and occasionally switch to the opposite value... this drooping is mainly due to the reactive components in the circuit.... this is called dc wander....