In AM, the carrier wave level varies with the modulating signal.
In FM, the carrier wave frequency varies with the modulating signal.

In PCM, does the carrier wave switch on/off (pulse)
I struggling to visualise it.

Thanks.

 

Do these links help? - http://en.wikipedia.org/wiki/Pulse-code_modulation

http://telemetry-products.com/sites/default/files/PCM_Tutorial.pdf

http://www.tech-faq.com/pulse-code-modulation.html

 

PCM may simply generate a bit-stream without necessarily using any carrier wave, but the bit-stream can be used to modulate a carrier for various transmission or recording purposes.

Simple on/off modulation or Amplitude Shift Keying of a carrier is one possibility, and this was used fairly extensively in earlier generations of fibre-optic telecommunications systems (here the carrier would be obtained from an IR LED or laser). Some later systems use more advanced modulation.

A number of other amplitude, frequency or phase modulation schemes can be used to carry PCM signals. In order to obtain the best possible use of limited available bandwidth, complex modulation systems such as QAM are sometimes used.
http://en.wikipedia.org/wiki/QAM

Finally, in cases where we want to try to send relatively broadband digital signals over old-fashioned copper wire pairs, some really elaborate schemes using multiple carrier frequencies may be used. Each carrier handles only part of the data, and the data has to be reassembled on reception.

 

Thanks guys.


Apologies for the total digression...
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Im not a scholar of American history, but I admire LBJ - have many Presidents done more for the poor, elderly and discriminated.
His critics use Vietnam as an excuse not a reason.