[aprssig] "Best" packet decoder solution

Phil N6TCT phil_aprssig at lapsley.org
Mon Dec 27 19:02:33 CST 2010

That's awesome, Scott!

The two things I've wanted to see in an APRS decoder for a while have
been adaptive equalization and soft decoding.

Adaptive equalization would take care of "twist" and the
pre-emphasis/de-emphasis problem: as the receiver is syncing up it
figures out what kind of spectral equalization it needs to apply to
address this particular transmitter.

For soft decoding, you don't make "hard" (1/0) decisions for each
received bit.  Rahter, you make a soft 1/0 guess for each bit and keep
track of the confidence values of each.  Then when it's time to do a CRC
check, if the packet passes, great.  If it fails, you find the bit
you're least confident of, flip it, and run the CRC check again.  Repeat
this process until you run out of time and/or bits.  :-)

Of course, both of these approaches demand lots of MIPS and aren't
trivial to implement, but they will increase successful decode rate and
make things more robust.


Scott Miller wrote:
> The OTUSB does use DSP for demodulation... so does the OT1+, but in a
> more simplistic way.  The algorithm is stripped down a bit to use the
> HCS08's 8x8 unsigned multiply, but it does pretty well against the
> test track - I got over 930 decoded.
> The 32-bit system will have better dynamic range, and will have enough
> horsepower that I could have it run multiple algorithms in parallel in
> case one's better in certain circumstances.
> Scott
> N1VG
> On 12/27/2010 3:39 PM, Keith VE7GDH wrote:
>> Andrew VK4TEC wrote...
>>> DSP ?
>> That stands for Digital Signal Processing. The TT4 uses DSP.
>> I understand that Scott N1VG has an evaluation board with a 32-bit MCU
>> capable of 76 MIPS that has some DSP instructions,
>> but mentioned that it will take some months to finish developing
>> the code. Someone said to Scott "now that you have gone all DSP
>> on us..." I'm not sure if that meant that the OTUSB already uses
>> DSP. I haven't seen any direct reference indicating that it did.
>> http://en.wikipedia.org/wiki/Digital_signal_processing
>> http://en.wikipedia.org/wiki/Digital_signal_processor
>> Incidentally, Scott sent me an OTUSB a few days ago. I've played
>> with a bit, but will have time to give it more of a workout in a few
>> days. It was a pleasant surprise to find it in the mailbox without
>> having actually ordered one!
>> 73 es cul - Keith VE7GDH
>> -- 
>> "I may be lost, but I know exactly where I am!"
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