[aprssig] "Best" packet decoder solution
scott at opentrac.org
Tue Dec 28 13:30:18 CST 2010
I sent you some details on the algorithm off-list, Phil. It's pretty
simple. It could do soft decision, though. I've thought about that in
the past, but didn't have confidence values to work with and now that I
do, I probably don't have the clock cycles to spare for it. Not to
mention the RAM. I'm going to have to wait for the 32-bit platform for
Adaptive equalization would be great. Afraid I don't know how to go
about doing that in a resource-constrained system, though.
On 12/27/2010 5:02 PM, Phil N6TCT wrote:
> 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.
>> 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.
>>> 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!"
>>> aprssig mailing list
>>> aprssig at tapr.org
>> aprssig mailing list
>> aprssig at tapr.org
> aprssig mailing list
> aprssig at tapr.org
More information about the aprssig