PAM in digital = AM in analog PFM in digital = FM in analog PCM in digital = x in analog What is "x"? Pulses are digital. Codes are also digital.
Modulation Schemes
Started by ●March 30, 2004
Reply by ●March 30, 20042004-03-30
I think pulse code modulation means varying phase to indicate a change in frequency. This is interesting because the phase can be hard to detect. I suppose the signal has constant amplitude and constant frequency. Then the time of the signal can be adjusted with a reactive component. The key is detecting what the phase is and what it used to be. I supose you could determine the voltage at a certain time. PCM might be useful, but I think it is very similar to PAM and PWM.
Reply by ●March 30, 20042004-03-30
Curious wrote:> PAM in digital = AM in analog > > PFM in digital = FM in analogAs far as I know, that's PPM (pulse position modulation).> PCM in digital = x in analog > > What is "x"?CW Morse code and maybe RTTY.> Pulses are digital. Codes are also digital.So is my calculator and my alarm clock. Jerry -- Engineering is the art of making what you want from things you can get. �����������������������������������������������������������������������
Reply by ●March 30, 20042004-03-30
Curious wrote:> PAM in digital = AM in analog > > PFM in digital = FM in analog > > PCM in digital = x in analog > > What is "x"? > > Pulses are digital. Codes are also digital.This reminds me of discussions I have been in related to MODEM's, and which systems modulate the signal, as opposed to a "pure digital" system. The last one I remember was describing ethernet, claiming that it was not modulating when put on the wire, but uses Manchester coding instead. My answer was that it doesn't use Manchester coding but synchronous phase modulation with only 0 and 180 degree phases allowed. You can't put bits on a wire without modulating something. The result from the PCM coding is likely used to AM, FM, or PM modulate something. For a CD, after going through a variety of complicated coding systems the result is amplitude modulating of a LASER beam, with only two amplitudes allowed. -- glen
Reply by ●March 31, 20042004-03-31
Kevin Weddle wrote:> I think pulse code modulation means varying phase to indicate a > change in frequency. This is interesting because the phase can be > hard to detect. I suppose the signal has constant amplitude and > constant frequency. Then the time of the signal can be adjusted > with a reactive component. The key is detecting what the phase is > and what it used to be. I supose you could determine the voltage > at a certain time. PCM might be useful, but I think it is very > similar to PAM and PWM.IMHO, I'd describe PCM as "mapping a code to another". By PCM we can create NRZ or RZ or differential Manchester code (Google will explain) ... of the pulse train. If we want to transport it through analog media, we'll feed this code stream into an ASK, FSK, PSK modulator... So we have either PAM = pulse-train->ASK->Modulator->analog signal, or we could have PCM/ASK = pulse-train->PCM->ASK->Modulator->analog signal. The same works with PFM or PPM. Now, if I'd have to find an analog counterpart to PCM, I'd propose a frequency shift on an AM signal as one example. Bernhard
Reply by ●March 31, 20042004-03-31
On Tue, 30 Mar 2004 17:05:20 GMT, glen herrmannsfeldt <gah@ugcs.caltech.edu> wrote:>Curious wrote: > >> PAM in digital = AM in analog >> >> PFM in digital = FM in analog >> >> PCM in digital = x in analog >> >> What is "x"? >> >> Pulses are digital. Codes are also digital. > > >This reminds me of discussions I have been in related >to MODEM's, and which systems modulate the signal, >as opposed to a "pure digital" system. > >The last one I remember was describing ethernet, claiming >that it was not modulating when put on the wire, but uses >Manchester coding instead. My answer was that it doesn't >use Manchester coding but synchronous phase modulation with >only 0 and 180 degree phases allowed.Manchester coding, as used in Ethernet, uses five symbols: CD0, CD1, CVL, CVH, and IDL. It's a bit of a stretch to describe the entire set as phase modulation of a carrier (particularly IDL, which has a constant level), but I guess you could if you wanted to. BTW: only low speed (<= 10Mb/s) Ethernet uses Manchester coding. More recent Ethernet rates use 4B/5B, 8B/10B or 64B/66B coding. In all cases, the coding provides DC balance and a minimum transition density (for reliable clock recovery). Regards, Allan.
Reply by ●March 31, 20042004-03-31
Allan Herriman wrote: (snip) (I wrote)>>The last one I remember was describing ethernet, claiming >>that it was not modulating when put on the wire, but uses >>Manchester coding instead. My answer was that it doesn't >>use Manchester coding but synchronous phase modulation with >>only 0 and 180 degree phases allowed.> Manchester coding, as used in Ethernet, uses five symbols: CD0, CD1, > CVL, CVH, and IDL. It's a bit of a stretch to describe the entire set > as phase modulation of a carrier (particularly IDL, which has a > constant level), but I guess you could if you wanted to.Well, while it is transmitting, anyway.> BTW: only low speed (<= 10Mb/s) Ethernet uses Manchester coding. > More recent Ethernet rates use 4B/5B, 8B/10B or 64B/66B coding. > In all cases, the coding provides DC balance and a minimum transition > density (for reliable clock recovery).Yes, 10Mb/s is what I was discussing. There seem to be some people that believe that digital is good, analog is bad, and that converting digital signals to analog (through modulation) is also bad. -- glen
Reply by ●March 31, 20042004-03-31
On Wed, 31 Mar 2004 09:01:30 GMT, glen herrmannsfeldt <gah@ugcs.caltech.edu> wrote:>Allan Herriman wrote: > >(snip) >(I wrote) >>>The last one I remember was describing ethernet, claiming >>>that it was not modulating when put on the wire, but uses >>>Manchester coding instead. My answer was that it doesn't >>>use Manchester coding but synchronous phase modulation with >>>only 0 and 180 degree phases allowed. > >> Manchester coding, as used in Ethernet, uses five symbols: CD0, CD1, >> CVL, CVH, and IDL. It's a bit of a stretch to describe the entire set >> as phase modulation of a carrier (particularly IDL, which has a >> constant level), but I guess you could if you wanted to. > >Well, while it is transmitting, anyway. > >> BTW: only low speed (<= 10Mb/s) Ethernet uses Manchester coding. >> More recent Ethernet rates use 4B/5B, 8B/10B or 64B/66B coding. >> In all cases, the coding provides DC balance and a minimum transition >> density (for reliable clock recovery). > >Yes, 10Mb/s is what I was discussing. > >There seem to be some people that believe that digital is good, >analog is bad, and that converting digital signals to analog >(through modulation) is also bad.Even those xB/yB codes, which at first appear very digital in nature, have restrictions on the output spectrum on the line, which is typically achieved with FIR filters and some sort of DAC inside an ASIC. Even something obviously digital like NRZ can be regarded as an analog signal. Here's an NRZ eye diagram I measured earlier this week. http://fractional-divider.tripod.com/eye.pdf (Notice the timebase: 16.8 ps/division) Regards, Allan.
Reply by ●March 31, 20042004-03-31
"Curious" <curious11112001@yahoo.com> wrote in message news:34a4f456.0403292145.1bd4f7f1@posting.google.com...> PAM in digital = AM in analogNot really... PAM is an analog signal modulating the analogue amplitude of a train of pulses.. so it isn't purely digital. PAM in the pure-digital domain would actually be PCM (see below) Whereas AM is one signal modulating the amplitude of another carrier. AM can happen in the analogue domain (in discrete circuitry) or in the digital domain (inside a DSP using sampled signals).> PFM in digital = FM in analog > > PCM in digital = x in analog > > What is "x"?Forgive me if I'm wrong, but I think the most obvious answer is... PCM = Pulse Code Modulation = standard, run of the mill, digitally sampled analog waveform (not modulated). Pulses of codes = a pulse of a certain "code value" at sampling intervals... Therefore x = . and PCM in digital = analog in analog. :) Or, am I just stating the obvious... Was "Curious" asking something more complicated? Cheers, Chris "Curious" <curious11112001@yahoo.com> wrote in message news:34a4f456.0403292145.1bd4f7f1@posting.google.com...> PAM in digital = AM in analog > > PFM in digital = FM in analog > > PCM in digital = x in analog > > What is "x"? > > Pulses are digital. Codes are also digital.
Reply by ●March 31, 20042004-03-31
glen herrmannsfeldt wrote: ...> There seem to be some people that believe that digital is good, > analog is bad, and that converting digital signals to analog > (through modulation) is also bad.I wonder if those people wish for digital ears. Hey: digital fingers! Jerry -- Engineering is the art of making what you want from things you can get. �����������������������������������������������������������������������
