Month: December 2013

Posted on

The Baudot code

http://www.dataip.co.uk/Reference/BaudotTable.php

Binary
Decimal
Hex
Octal
Letter
U.S.
Figures
CCITT No.2
Figures
00000
0
0
0
N/A
N/A
N/A
00001
1
1
1
E
3
3
00010
2
2
2
LF
LF
LF
00011
3
3
3
A
–
–
00100
4
4
4
Space
Space
Space
00101
5
5
5
S
BELL
‘
00110
6
6
6
I
8
8
00111
7
7
7
U
7
7
01000
8
8
10
CR
CR
CR
01001
9
9
11
D
$
WRU
01010
10
A
12
R
4
4
01011
11
B
13
J
‘
Bell
01100
12
C
14
N
,
,
01101
13
D
15
F
!
!
01110
14
E
16
C
:
:
01111
15
F
17
K
(
(
10000
16
10
20
T
5
5
10001
17
11
21
Z
“
+
10010
18
12
22
L
)
)
10011
19
13
23
W
2
2
10100
20
14
24
H
#
£
10101
21
15
25
Y
6
6
10110
22
16
26
P
0
0
10111
23
17
27
Q
1
1
11000
24
18
30
O
9
9
11001
25
19
31
B
?
?
11010
26
1A
32
G
&
&
11011
27
1B
33
Figures Shift
Figures Shift
Figures Shift
11100
28
1C
34
M
.
.
11101
29
1D
35
X
/
/
11110
30
1E
36
V
;
=
11111
31
1F
37
Letters Shift
Letters Shift
Letters Shift

 

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Notes on underwater sound

Not a good idea to connect a piezo directly to the output of an iPod. Apparently the impedance is too low and it fried the audio circuit and battery. I should have just dropped it (the iPod) in the water instead of the piezo.

I was able to get excellent underwater sound using  a piezo for a mic – it was a little over an inch diameter, broken out of the radio shack black plastic case – with some hot glue to cover the wire connections – probably not necessary.

See this article about working with Radio Shack piezo transducers

http://www.edrums.info/radio_shack_piezo.htm

The speaker was a Dayton Audio Weatherproof Extreme Exciter – from Amazon – http://amzn.com/B0031K2XBA

And I used a small power amp to feed it. http://amzn.com/B0049P6OT

Note: the exciter also sounds great when its duct taped to a cymbal.

 

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RTTY encoding and decoding in Max

notes

Today I was able to get an AFSK (audio frequency shift keying) system running in Max – sort of – It encodes text into ASCII bits and decodes the signal back into text – with a clock set at around 30ms (32 bits/second) – but there is no clock synchronization yet. Or stop bits, etc., The patch just uses the transmitter clock to sync the receiver (cheating)

Listen to an example of the word ‘hello’ at 32 bits/sec

local file is in max teaching examples/rtty-sim5.maxpat

Next step will be to get receiver sync happening – then make it conform to RTTY standard – probably a few days effort for this, but at least this is a proof of concept.

The synchronization may need to happen at the sample level (gen~) because it requires finding the beginning and end of bits – in order to set the clock pulses accurately.

 

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Nonoo radio DSP tutorials (RTTY)

By Norbert Varga

http://dp.nonoo.hu/projects/ham-dsp-tutorial/

From the IIR RTTY tutorial, a Paper on decoding RTTY in Linux:

http://www.ele.uva.es/~jesus/rtty/rtty-2.0/doc/rtty.pdf

Other resources on RTTY / FSK decoding

  1. RTTY decoder application and paper describing the method.
  2. RTTY diddles, about the protocol
  3. DPLL theory
  4. Bit banging, simple bit synchronization
  5. UART character recovery
  6. FSK signals and demodulation
  7. FSK demodulation theory
  8. Receiver sync theory
Getting started on RTTY

http://www.aa5au.com/gettingstarted/rtty_diddles_technical.htm