Reply by Vladimir Vassilevsky●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
Reply by Eric Jacobsen●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.
>
>Mark
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 Eric Jacobsen●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!
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
Reply by Randy Yates●June 22, 20112011-06-22
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://yates@ieee.org % 'Mission (A World Record)',
http://www.digitalsignallabs.com % *A New World Record*, ELO
Reply by Mark●June 22, 20112011-06-22
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
Reply by steveu●June 22, 20112011-06-22
>On 06/21/2011 02:13 PM, Vladimir Vassilevsky wrote:
>>
>>
>> Randy Yates wrote:
>>
>>> With the right filtering, an FSK signal can be viewed as two
>>> complementary OOK (on-off keyed) signals.
>>
>> There is more than that. The phase is continuous.
>
>Not necessarily, unless phase continuity is specified (and the
>specification complied to). Old teletype FSK was sometimes accomplished
>with a pair of audio-frequency oscillators, their outputs switched, and
>fed to a SSB transmitter.
>
>>> Is the optimal FSK
>>> demodulator more optimal, less optimal, or equivalent to two optimal
>>> OOK demodulators with their outputs combined?
>>
>> Optimal FSK demodulator is more optimal then two OOK demodulators as it
>> makes use of the continuity of the phase.
>
>Even without phase continuity it's more optimal in that it doesn't make
>the receiver guess at a zero level -- for the case that I remember, a
>sufficient statistic is the difference in signal strengths out of a pair
>of matched filters. In as much as the two signals are nominally of the
>same amplitude, the sufficient statistic was a signed number with the
>decision point at 0.
>
>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.
Steve
Reply by Vladimir Vassilevsky●June 22, 20112011-06-22
Randy Yates wrote:
> On 06/21/2011 09:20 PM, Randy Yates wrote:
>
>> [...]
>> 2) decode both as OOK and somehow "optimally" combine them.
>
>
> PS: Assume you have a weighting vector w, -1 <= w <= +1, that
> indicates the relative "goodness" of each OOK, 0 being equally
> good.
Then the constellation points are (w+1,0) and (0, w-1), on assumption
that the FSK is orthogonal. Now connect the points with a piece of
straight line and build a middle perpendicular to that piece. This
perpendicular is the decision border between "1" and "0".
Vladimir Vassilevsky
DSP and Mixed Signal Design Consultant
http://www.abvolt.com
Reply by Tim Wescott●June 21, 20112011-06-21
On 06/21/2011 07:27 PM, Randy Yates wrote:
> On 06/21/2011 06:41 PM, Tim Wescott wrote:
>> [...]
>> Even without phase continuity it's more optimal in that it doesn't
>> make the receiver guess at a zero level
>
> Tim, what "zero level" are you referring to? Or did you mean "guess at a
> threshold?"
Guess at a threshold -- actually, for decent OOK reception, the receiver
needs to make a guess at the pulse amplitude, so it can slice it
effectively.
>> 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!
>
> Yes, I neglected to specify that we should consider this
> discontinuous-phase
> FSK.