On Jun 22, 9:13�am, Randy Yates <ya...@ieee.org> wrote:> On 06/22/2011 08:32 AM, Mark wrote: > > > > > > > I think we first have to define what we mean by "optimal" > > > Are we talking about an AWGN channel only? or are we talking about > > more interesting channels? > > > The old ham radio RTTY systems used 2 OOK receivers combined and they > > worked well under conditions like: > > > 1) selective fading where F1 or F2 might dissappear > > > 2) co-channel interference where there might be a strong CW tone > > between F1 and F2. (depending on the baud rate and F1 to F2 spacing, > > there can be a decent sized "gap" between F1 and F2. �Two narrow OOK > > receivers can ignore what is in the gap while an FM demod cannot.) > > > It is interesting for me to see this discussion here, I can remember > > vividly being a kid about 13 years old first exposed to RTTY systems > > and looking at the schematic and wondering what makes the "optimal" > > RTTY receiver... why they used 2 AM detecotrs instead of one singel FM > > detector... �I'm glad to see it was not a simple question and I look > > forward to reading what answer the group develops... > > Mark, > > Indeed, this is a derivative of the old RTTY systems. It is the > GMDSS/DSC system. It is good to know I'm on the right track - I > had no idea this had been done before! > -- > Randy Yates � � � � � � � � � � �% "Watching all the days go by... > Digital Signal Labs � � � � � � �% �Who are you and who am I?" > mailto://ya...@ieee.org � � � � �% 'Mission (A World Record)',http://www.digitalsignallabs.com% *A New World Record*, ELO- Hide quoted text - > > - Show quoted text -I think there are some ARRL publications... the other funny thing is I reacall thinking about RTTY,..... what a waste of a perfectly good voice channel, who would want to type a message when they could speak it... how wrong I was... now we have millions of kids typing messages :-) Mark
FSK Versus OOK Demodulation
Started by ●June 21, 2011
Reply by ●June 22, 20112011-06-22
Reply by ●June 22, 20112011-06-22
On Wed, 22 Jun 2011 09:13:28 -0400, Randy Yates <yates@ieee.org> wrote:>On 06/22/2011 08:32 AM, Mark wrote: >> I think we first have to define what we mean by "optimal" >> >> Are we talking about an AWGN channel only? or are we talking about >> more interesting channels? >> >> >> The old ham radio RTTY systems used 2 OOK receivers combined and they >> worked well under conditions like: >> >> 1) selective fading where F1 or F2 might dissappear >> >> 2) co-channel interference where there might be a strong CW tone >> between F1 and F2. (depending on the baud rate and F1 to F2 spacing, >> there can be a decent sized "gap" between F1 and F2. Two narrow OOK >> receivers can ignore what is in the gap while an FM demod cannot.)I'm guessing usually that CW tone will be the receiver's LO? ;) That is a neat way to pull in a signal in a receiver with poor LO isolation.>> It is interesting for me to see this discussion here, I can remember >> vividly being a kid about 13 years old first exposed to RTTY systems >> and looking at the schematic and wondering what makes the "optimal" >> RTTY receiver... why they used 2 AM detecotrs instead of one singel FM >> detector... I'm glad to see it was not a simple question and I look >> forward to reading what answer the group develops... > >Mark, > >Indeed, this is a derivative of the old RTTY systems. It is the >GMDSS/DSC system. It is good to know I'm on the right track - I >had no idea this had been done before!Everything has been done before! There is nothing new under the sun! ;) Sure seems like that most of the time. Eric Jacobsen http://www.ericjacobsen.org http://www.dsprelated.com/blogs-1//Eric_Jacobsen.php
Reply by ●June 22, 20112011-06-22
On Jun 22, 1:42�pm, eric.jacob...@ieee.org (Eric Jacobsen) wrote:> On Wed, 22 Jun 2011 09:13:28 -0400, Randy Yates <ya...@ieee.org> > wrote: > > > > > > >On 06/22/2011 08:32 AM, Mark wrote: > >> I think we first have to define what we mean by "optimal" > > >> Are we talking about an AWGN channel only? or are we talking about > >> more interesting channels? > > >> The old ham radio RTTY systems used 2 OOK receivers combined and they > >> worked well under conditions like: > > >> 1) selective fading where F1 or F2 might dissappear > > >> 2) co-channel interference where there might be a strong CW tone > >> between F1 and F2. (depending on the baud rate and F1 to F2 spacing, > >> there can be a decent sized "gap" between F1 and F2. �Two narrow OOK > >> receivers can ignore what is in the gap while an FM demod cannot.) > > I'm guessing usually that CW tone will be the receiver's LO? � ;) � > >no not the LO but just interference on the frequency.... ham channels are not assigned like commercial channels so it can be a bit if a free for all. Mark
Reply by ●June 22, 20112011-06-22
On Wed, 22 Jun 2011 12:05:27 -0700 (PDT), Mark <makolber@yahoo.com> wrote:>On Jun 22, 1:42=A0pm, eric.jacob...@ieee.org (Eric Jacobsen) wrote: >> On Wed, 22 Jun 2011 09:13:28 -0400, Randy Yates <ya...@ieee.org> >> wrote: >> >> >> >> >> >> >On 06/22/2011 08:32 AM, Mark wrote: >> >> I think we first have to define what we mean by "optimal" >> >> >> Are we talking about an AWGN channel only? or are we talking about >> >> more interesting channels? >> >> >> The old ham radio RTTY systems used 2 OOK receivers combined and they >> >> worked well under conditions like: >> >> >> 1) selective fading where F1 or F2 might dissappear >> >> >> 2) co-channel interference where there might be a strong CW tone >> >> between F1 and F2. (depending on the baud rate and F1 to F2 spacing, >> >> there can be a decent sized "gap" between F1 and F2. =A0Two narrow OOK >> >> receivers can ignore what is in the gap while an FM demod cannot.) >> >> I'm guessing usually that CW tone will be the receiver's LO? =A0 ;) =A0 >> >> >no not the LO but just interference on the frequency.... >ham channels are not assigned like commercial channels so it can be a >bit if a free for all. > >MarkFor a system that employs direct conversion to baseband any LO leakage will wind up at DC, or potentially right between the tones. That was what it made me think of, that's all. ;) Eric Jacobsen http://www.ericjacobsen.org http://www.dsprelated.com/blogs-1//Eric_Jacobsen.php
Reply by ●June 23, 20112011-06-23
steveu wrote:>>On 06/21/2011 02:13 PM, Vladimir Vassilevsky wrote: >>>>I suspect that any FSK you're going to run across today will be of >>continuous phase, but I'm not going to _trust_ that unless I see it in >>the specification! > > Many modern implementations of the PSTN FSK modems still do not ensure > continuous phase, so your lack of trust is fully justified.I have some difficulty believing in this, but hey, what is the problem. Make two independent PLLs, combine their outputs. Sync still operates jointly. Same applies to OP's selectively faded channel. The additional output from the PLLs would be the channel estimate. Vladimir Vassilevsky DSP and Mixed Signal Design Consultant http://www.abvolt.com
