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[aprssig] an off-topic query about statistical analysis in digital modes

Scott Miller scott at opentrac.org
Fri Sep 21 13:52:28 UTC 2007


Yeah, Varicode is just a simple static Huffman code for efficiency, not 
reliability.  I've actually written PSK31 code for the OpenTracker, but 
I'm not sure if anyone actually uses it.  I haven't been able to 
generate the proper checksums for PropNet^31 operation.

Scott
N1VG

Steve Noskowicz wrote:
> 
> Perhaps you know that PSK31 has variable length
> symbols with more common ones (letters e, t, a, o, i,
> n, s))  having shorter codes.
> 
> 73, Steve, K9DCI
> 
> 
> --- Scott Miller <scott at opentrac.org> wrote:
> 
>> http://www.tapr.org/pub_wdcdat.html
>>
>> Wireless Digital Communications: Design and Theory,
>> by Tom McDermott, N5EG.
>>
>> I'm sure there are plenty of other good texts out
>> there, but this one 
>> covers a lot of ground that's of particular interest
>> to hams.
>>
>> Also see the Wikipedia articles for convolutional
>> code, viterbi decoder, 
>>   and Hamming distance.  They have some good
>> information and plenty of 
>> references.
>>
>> Scott
>> N1VG
>>
>> w0ep at frii.com wrote:
>>> Is there a good book to learn about these things?
>>>
>>>
>>> On Thu, 20 Sep 2007, Scott Miller wrote:
>>>
>>>> I think what you're describing is basically a
>> convolutional code.
>>>> Such things work better when you've got
>> side-channel information to work
>>>> from - like if you know one particular bit was
>> closer to the decision
>>>> threshold than the others, you know that one's
>> the most likely to be wrong.
>>>> In APRS you might be able to recover more data by
>> knowing that certain
>>>> fields are always digits, or always alphanumeric,
>> and using that
>>>> knowledge to try flipping specific bits to see if
>> the new message
>>>> matches the checksum.
>>>>
>>>> Of course, you're still better off encoding it
>> with a proper
>>>> convolutional code in the first place...
>>>>
>>>> Scott
>>>> N1VG
>>>>
>>>> w0ep at frii.com wrote:
>>>>> We've kind of been on an off-topic swing and
>>>>> I know this is a good place to find people that
>> know
>>>>> about digital data transmission so:
>>>>>
>>>>> Are there any digital data transmission systems
>> that
>>>>> use a dictionary of expected terms and
>> statistical
>>>>> analysis that assigns probability to incoming
>>>>> raw data to match against the dictionary?
>>>>>
>>>>> Sort of like what my BlackBerry does with
>> SureType mode?
>>>>>
>>>>>
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>>>>>
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>>>>>
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> 
> 
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