[GreenKeys] ITTY/RTTY Sampling tests

[jim at k6ccc.org]

I have dealt with digital audio sampling for decades.  I have taught T1 theory (which includes the analog to digital conversion and back).
The standard statement is that in order to properly convert A to D, the sampling rate needs to be at least twice the audio frequency.  So an 8KHz sampling rate should work just fine.  That is what almost ALL telephone transmission uses (and has for decades).  Now, there can be some interesting gotchas at very specific frequencies where the sampling frequency is an EXACT multiple of the audio frequency.
I don't understand what was going on that resulted in a frequency shift of 580 hertz.  That does not make much sense.  It almost sounds like one end was trying to sample at one rate and the other end was trying to sample at another.  That won't work.


73
Jim Walls
K6CCC


-----Original Message-----
From: "Paul Heller" <paul0926 at comcast.net>
Sent: Thursday, September 23, 2021 15:24
To: "Harold Hallikainen" <harold at w6iwi.org>
Cc: "Greenkeys" <greenkeys at mailman.qth.net>
Subject: [GreenKeys] ITTY/RTTY Sampling tests

A report. Bill, Nick and I did some testing today. 8K did not work. None of us could receive it, but eventually Nick got close. Nick put forth a very interesting theory about what was happening.

Nick measured: 
	57 baud, 1545/1675 tones if sampling at 8kHz
	75 baud, 2125/2295 tones if sampling at 11kHz or 22kHz

All tests were 8 bit.

Conclusion: 11kHz is the minimum sampling rate. 8bit is ok, do not need 16bit  (so that saves bandwidth!)

Here is the theory about what might be going on. Per Nick:

	I am thinking maybe you are sending things at 8/11 speed. 
	New 8kHz sampling, Old 11kHz sampling = approx 0.73
	0,73 x 75 baud = approx 50 baud
	0.73 x 2125 mark = 1545 mark which is about what my Dovetron is showing….
	And I think 1675 space which is 0.73 x 2295

	Not the intended result but sorta cool........


So the slower sampling dropped both the frequency and the baud rate. Well how about that folks?

Again, I’m using darkice for the audio sampling. Your mileage may vary.

ITTY 100 is back up and running as it was before (except the for the drop from 16 to 8 bit). I will probably drop ITTY and AUTOSTART down to 8bit as well. Should reduce everyone’s internet usage.

Paul



> On Sep 23, 2021, at 9:58 AM, Harold Hallikainen <harold at w6iwi.org> wrote:
> 
> Methods of doing AFSK Internet broadcasts like ITTY are interesting. I
> thought the idea of splicing together mark and space tone files was very
> clever.
> 
> I wonder about the need for audio compression. I did a microcontroller
> project that streamed live audio using ADPCM and PCM. The PCM used 16 bit
> samples at 44.41 kHz. But, I think for RTTY, 8 bit samples at 8 kHz should
> be fine (since that's what circuit switched telephone uses). In my
> application, on receiving the HTTP request, I sent the WAV header with a
> very large file size, then just started sending the samples.
> 
> As mentioned, I do think the splicing of tone files is very clever!
> Another method that would be interesting is direct digital synthesis.
> Depending on the desired frequency, each successive sample would be some
> number of steps through a sine table from the previous sample.
> 
> Anyway, interesting stuff!
> 
> Harold
> 
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