[GreenKeys] [External] HF radio relating to DSP TU Update

[Harold Hallikainen]

Thanks! I've heard of go, but this is the first code I've actually seen. I
know code is supposed to be "self documenting," but I use a lot more
comments. Looking at this, it appears that the calculations are all
integer while I used doubles. This results in stuff like this:

// the bandpass filter is picky about the sample rate, and the output is
perfectly
	// balanced when the sample rate is 3x the center frequency, so lets
divide the sample
	// rate down to as close to that as we can get.


I am able to just pass the double audio samples into filters.

I also see in the modulator that when it starts up, a sine table is
created as an array of 16384 16 bit integers. I was considering making a
sine table, but I read somewhere that it's probably just as fast to
calculate the sine as to look it up. So, I calculate the sine during each
DDS sample. As it is, with the demodulator, automatic threshold control,
AGC, and the DDS modulator, I'm using about 5% of the CPU cycles.

Anyway, it IS interesting to read other peoples code (and schematics).
THANKS for sending it. I will add it to my RTTY references.

Harold
https://w6iwi.org/rtty


On Thu, July 7, 2022 6:59 pm, Jeffrey Golas wrote:
> Harold, came across this today, not sure if it helps.
>
> http://code.beefchicken.com/keelan/gortty
>
> Jeff KC3GJX
>
>> On Jul 6, 2022, at 11:33 PM, Harold Hallikainen via GreenKeys
>> <greenkeys at mailman.qth.net> wrote:
>>
>> 
>>
>>> On Wed, July 6, 2022 8:03 pm, Jones, Douglas W wrote:
>>> From: Jim Haynes [jhhaynes at earthlink.net] -- Wednesday, July 6, 2022
>>> 8:46
>>> PM
>>>
>>>> I think the most successful attack on the multipath problem was a
>>>> system
>>>> developed at MIT called RAKE, which put the signal through a delay
>>>> line
>>>> and then tried to take signals from several taps on the line and
>>>> combine
>>>> them constructively.
>>>
>>> See:  Clarence W. Hansell, "Method and Means for Reducing Multiple
>>> Signals," U.S. Patent 2,310,692, granted Feb. 9, 1943, applied for in
>>> 1939.  Yep, patent does exactly what you describe, using short hunks of
>>> coax as delay lines.  The problem they were working on at RCA at the
>>> time
>>> was multipath interference in TV signals, a common source of ghost
>>> images.
>>>
>>> I found that patent when I was working on the analog delay line
>>> Wikipedia
>>> page:
>>> -- https://en.wikipedia.org/wiki/Analog_delay_line
>>>
>>> That page is still over 50% my work, despite the passage of time.
>>>
>>>         Doug Jones
>>>         jones at cs.uiowa.edu
>>
>> That's great! The combining signals from several taps on a line reminds
>> me
>> of a DSP Finite Impulse Response filter which, has memory for a series
>> of
>> samples, starting from the latest, then one earlier, then one earlier,
>> etc., then multiplying the contents of each of those samples by a
>> coefficient and adding the results. So, if you know the transfer
>> function
>> of the path (with reflections), you can do the inverse of it in the FIR
>> filter. With HF, it seems tricky since the path is always changing. But
>> I
>> guess now and then you could transmit a known "sync" signal to determine
>> the transfer function of the path at that time and adjust the filter as
>> appropriate.
>>
>> More fun stuff I don't remember from school!
>>
>> Harold
>> https://w6iwi.org
>>
>>
>>
>>
>> --
>> https://w6iwi.org
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