[GreenKeys] FSK Audio files for Test and Evaluation

Fri May 2 00:52:43 EDT 2025

Great! I've never done anything in Python. My NYT stuff is all in PHP. I'm
using a PHP direct digital synthesis to generate the tones. I use the same
code in C in the DSP TU ( https://w6iwi.org/rtty/DspTU2/ ). I've always
been impressed with DDS. I remember seeing an article about it in the
early 1970s or so. It was constructed with a full adder, a latch, and a
sine lookup ROM driving a DAC. REALLY CLEVER! And now, in software, it's
just a few lines of code ( see
https://github.com/HaroldHallikainen/DSP_TU2/blob/master/src/dds.c ).

I'm not doing compression on my NYT feed so I don't have to decompress it
in the DSP TU. I might eventually add some sort of compression. The DSP TU
is running on a PIC32MZ microcontroller. I have seen that MP3 decoding is
possible on these chips, though I have never tried it. I DID to some ADPCM
encoding years ago on a PIC32MX. That gives about 4 to one compression,
which would bring my 64 kbps down to 16 kbps.

It would be interesting to do one bit per sample, bringing us down to 8
ksps. But, looking at
https://homspace.nl/samplerbox/WAVE%20File%20Format.htm , it looks like
the minimum would be 8 bits per sample to get the block align to work:

The wBlockAlign field should be equal to the following formula, rounded to
the next whole number:

wChannels * (wBitsPerSample / 8)

Essentially, wBlockAlign is the size of a sample frame, in terms of bytes.
(eg, A sample frame for a 16-bit mono wave is 2 bytes. A sample frame for
a 16-bit stereo wave is 4 bytes. Etc).

Anyway, great work! We are getting a lot of audio feeds.

Harold
https://w6iwi.org

On Thu, May 1, 2025 9:07 pm, John Spigel wrote:
> Thanks Harold. Much of this codeing is new to me and I only got into the
> workings of audio compression in the last day or so. I'm using ffmpeg. My
> code can batch process so will run quickly through multiple files I
> process first through my text editor.
>
> The positive responses posted here on greenkeys encouraged me to add 50
> Baud EU FSK audio to the 5bits.net site which may be useful. (And it was
> easy)
>
> 73, John W1AN
>
>
> On 01-May-25 23:39, Harold Hallikainen via GreenKeys wrote:
>
>> They work great! I tested the 45 baud 170 Hz and 850 Hz shift. Both
>> printed perfectly.
>>
>> I also ran one of the files through
>> https://www.maztr.com/audiofileanalyzer and find that you are doing 8
>> ksps, 16 bits per sample, and through MP3 compression, sending at 20.2
>> kbps.
>>
>> Very nice! My New York Time feed ( https://w6iwi.org/rtty/audio/nyt/ )
>> also runs at 8 ksps but with 8 bit samples and no compression for 64
>> kbps.
>>
>> Harold
>>
>>
>>
>> On Thu, May 1, 2025 11:43 am, John Spigel wrote:
>>
>>> From software I'm creating I've put together a couple of FSK audio
>>> files for outside evaluation of audio and decode quality. The tones
>>> themselves are generated in python. These are large 60M mp3 files. If
>>>  able, please open, test and let me know if they are good to go or
>>> need some further filtering. I also may be able to compact further.
>>> Audacity
>>> says the noise is about 60db down, but that's not what the real world
>>> may see. There is both 170 and 850 cps shift. Thanks! And I'll have
>>> more of these soon.
>>>
>>> https://5bits.net
>>>
>>>
>>>
>>> 73, John W1AN
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>
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>>>> Jordan Spencer Cunningham's GreenKeys Search Tool:
>>>> https://teletype.net/gksearch
>>>> 2002-to-present greenkeys archive:
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>>>> 1998-to-2001 greenkeys archive:
>>>> http://mailman.qth.net/archive/greenkeys/greenkeys.html
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>>>>
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