[GreenKeys] ITTY comments and demodulator

Sat Sep 26 00:04:30 EDT 2026

Thanks for the response! Many UARTS do multiple samples per bit and use
the state of the majority as the state of the bit. I considered doing this
in the software uart in the DSP TU, but decided to just sample at the
center of the bit time since the demodulated data is going through a low
pass filter at 1.7 times the baud rate. There may be some inadvertant
positive feedback around the comparator, and, if you are not seeing noise
on the transitions, all is well!

On the NE555 as an astable multivibrator, I just tied pins 2 and 6
together for the input and took the output on pin 4 (going from memory
here!) to form an inverting schmitt trigger with input thresholds at 1/3
and 2/3 Vcc. A resistor around it with a capacitor to ground on the input
makes an astable multivibrator with fewer parts. One of my design
philosophies is that the ideal design has zero parts, so I really try to
reduce parts count.

By the way, some audio to run through the demodulator is available at
https://w6iwi.org/rtty/audio/ .

Harold

On Fri, September 25, 2026 8:36 pm, Dale Hughes wrote:
> Hi Harold,
>
>
> Many thanks for your comments and interesting information. Yes, the
> design is old fashioned (like it's creator hi hi). The reason I did it
> that way was because I had all of the chips on hand and I could build it
> over a weekend. The XR2211 wasn't available locally, so it was good to
> finally use the chips I had carefully stored away years ago...
>
> I did think of using an Arduino or something similar, but I also liked
> the idea of having some hardware that is (sort of) period appropriate.
>
> I did check the transitions on the comparator output and there is no
> evidence of any oscillation during the signal element transitions, though
> adding some positive feedback would be beneficial and I may do that if
> any problems do arise. I guess the UART will also filter out any spurious
> signals as well.
>
> One final thing, in the UART circuit you sent the receive clock is
> generated using an interesting configuration for the 555 timer. I've not
> seen that arrangement before so I will try it out.
>
> Regards
>
>
> Dale VK1DSH
>
>
>
> On Fri, 25 Sept 2026, 14:37 Harold Hallikainen, <harold at w6iwi.org> wrote:
>
>
>> Thanks for posting the schematic! Lots of hardware!
>>
>>
>> The UART section remnds me of a design I did back in 1980 (
>>
>>
>> https://bh.hallikainen.org/uploads/HaroldHallikainen/20130416125339_Par
>> t1.pdf ).
>> As noted on that schematic, the design originally used the General
>> Instrument AY5-1013 UART which required +5V and -12V, so there is a
>> charge pump to generate the -12V using an NE555. Later production used
>> the Intersil  6402 UART which only required +5V. Note that U5 generates
>> the Reset Data Available pulse based on the incoming Data Available
>> signal. This also provides a strobe to latches that hold the data out of
>> the UART on the display board (
>> https://bh.hallikainen.org/uploads/HaroldHallikainen/Harold_009.pdf ).
>>
>>
>> Note that I used the XR2211 as the PLL FSK demodulator, while your
>> circuit uses the NE565. The XR2211 has the data comparator within it and
>> also has a quadrature detector that is used for carrier sense for mark
>> hold. The data comparator typically has some positive feedback (making
>> it a Shcmitt trigger). I did make a demodulator using the NE565 that
>> drove a Teletype model 15. I had no hysteresis on the data comparator
>> and could hear the selector magnets sing as tone leaded through (actully
>> two times tone out of the phase detector). On the transmit side, the
>> XR2206 was a great chip.
>> It could generate a square wave, triangle wave, or sine wave. It
>> directly supported FSK. It also supported linear FM and double sideband
>> suppressed carrier AM. The DSBSC mixer (analog multiplier) was used in a
>> subcarrier demodulator (subcarriers between 26 kHz and 110 kHz) at
>> https://bh.hallikainen.org/uploads/HfDrc190.pdf#page=298 . The XR2206
>> oscillator acted as a local oscillator that was sent to the analog
>> multiplier along with the incoming subcarrier. The incoming subcarrier
>> was upconverted to 455 khz where it went through an IF amplifier with a
>> ceramic filter, then a ceramic discriminator to demodulate the audio.
>>
>> By the way, those schematics (which are pretty hard to read) were
>> generated by the Dasoft CAD software that ran under CP/M. It did
>> schematic capture and PCB layout.
>>
>> The NE565, XR2206, and XR2211 were all amazing chips, but are now no
>> longer manufactured.
>>
>> Today, I'd be tempted to do this all in one chip (a microcontroller)
>> like in the https://w6iwi.org/rtty/DspTU3/ , but that may be cheating.
>>
>> Harold
>> https://w6iwi.org
>>
>>
>>
>>
>> On Thu, September 24, 2026 1:19 am, Dale Hughes via GreenKeys wrote:
>>
>>> Hello folk,
>>>
>>>
>>>
>>> Greetings from VK1 (Canberra, Australia). I've been printing some of
>>> the ITTY streaming news which has been fun and interesting. Many
>>> thanks to those people who provide the excellent and useful service.
>>>
>>> Previously I printed the 45 baud version of the news using my old
>>> RTTY
>>> modem with a Kleinschmidt TT4C machine, however I have now updated the
>>>  system so I can use my Teletype Model 28RO which otherwise wasn't
>>> getting a lot of use.
>>>
>>> The new demodulator uses a PLL device and includes a UART to
>>> regenerate and upconvert the baud rate of the demodulated characters,
>>> so I can now easily print the 45, 50 and 75 baud services to a single
>>> machine running at 75 baud. The PLL can be easily tuned to the various
>>> tones that are
>> used
>>> using a front panel control. Attached is the schematic of the new
>>> unit,
>> it
>>> is easy to build and it works very well. I have written up the design
>>>  which includes the various source documents like application notes
>>> and datasheets and I am happy to send the file to anyone who is
>>> interested. It's about 12
>>> Megabytes so I haven't attached it to this post.
>>>
>>>
>>>
>>> Regards
>>> Dale VK1DSH
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>>
>>
>> --
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>>
>>
>

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