[Dspforum] demodulator performance/frame synchronization

[Ryan Merton]

Michael,

I did not use EDMA for this lab.  I thought about it, but as Joseph
said, the added hassle of dealing with baud boundaries across EDMA
frames would have been annoying.  As it is I have enough processing
time to run all of the code for each sample using polling (without any
optimization yet).

You just need to come up with a good way to make a decision on where a
latch/release occurred and where the baud boundary is and then make a
decision every 8 samples after that for however many bits there are.

Also, I found that the DSPLIB functions weren't very useful as the FFT
stuff requires at least 32 points and complex samples.  I just wrote
the literal implementation for the two bins that I cared about from
the description in 'help fft' in Matlab.  I'm certain it's painfully
slow to calculate even just those two with unoptimized C, but like I
said... it's fitting in the time constraints.

-Ryan

On Sun, Nov 16, 2008 at 4:46 PM, Joseph Haber <josephhaber at gmail.com> wrote:
> Mike,
>
> Don't make any presumptions that a start falls on an "8" boundry -- in fact,
> that concept doesn't exist until a start notification is found.
>
> My first instinct is that EDMA makes this more complicated than you want --
> you would have to be careful about processing across two sequential
> "blocks", which seems like a pain to me.
>
> Joseph
>
> On Sun, Nov 16, 2008 at 4:39 PM, michael norris <mnorris at gmail.com> wrote:
>>
>> Instructors/students,
>>
>> Can we assume that a frame begins on an 8 (f_sample/f_symbol) sample
>> boundary? In the general case, I don't think we can. If we can't, then it
>> seems we need to compute an 8-point FFT per sample in order to synch the
>> frame.
>>
>> Once we've synched on the frame, we could then compute the FFT once per 8
>> samples. Then we would benefit from interleaving the sampling and
>> computation with EDMA. But if our computation time must already be under one
>> sampling time, then what benefit is there to implementing EDMA with this
>> lab?
>>
>> (Maybe it prevent us from missing data during the printf() call. But do we
>> care about this?)
>>
>> Could someone add a little more about detecting the frame boundary?
>> Assuming we are sampling once-at-a-time to do the synchronization, where in
>> the start bit's 8 samples do we fall? Are we one past the half-way sample,
>> i.e. sample 5/8?
>>
>> We should have plenty of time to do our 8-point FFT with this board. I'm
>> just curious where EDMA could benefit us here.
>>
>> Thanks,
>> -Michael
>>
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