Decoding Pulse Telephone

[richard]

most dials have a 66/33 mark/space ratio and fixed impulse rate of 10 impulses / second , there is also a fixed minimum interdigital pause between digits built into the dial mechanism . a pause in dialing longer than the 66/33 ratio will always indicate a new digit or the end of dialing

[Ioannis]

Thanks for the info. Did not knew there is a 66/33 ratio. Soon the scope will reveal the case.

Ioannis

[keithdoxey]

Its a long time since I did my BT apprenticeship (40 years!) and I no longer have any of my notes but this thread triggered something in my brain about the minimum Inter Digit Pause being 450mS as that is the amout of time that was needed for a strowger switch to detect the end of the dialled digit and sweep the bank looking for a free outlet. If its a rotary dial tone you wont be able to get the minimum below about 1 second as thats how long it takes to turn the dial for the next digit but if its Pulse Dial push button phone then the minimum Inter Digit Pause will be determined by the dialling chip in the phone.

[Ioannis]

OK, Thanks Keith.

The first pulse does not seem to be different than the rest.

The pause between digits is between 500-750ms according to the phone.

Each pulse is 1Hz or 600/400msec ratio.

Ioannis

[Art]

If I recall correctly there’s a switch to tell the dial is in the home position to tell it was the end of a set of pulses,
or that a set of pulses is about to arrive if you want to look at it that way.

You can avoid any debounce hardware. All you have to do is pick whether you’re going to count rising or falling edges,
and look for a decent consecutive samples of the switch in one position, and then the same number of samples indicating
the switch is in the opposite position to qualify a rising or falling edge of the dial switch.

[Ioannis]

Thats interesting. I see two pair of wires coming out of the dial. So one pair is sure the digit pulser, the other might be exactly that, the home position. Maybe they used that to mute the earpiece?

Sure I can make good use to it.

Ioannis

[Art]

The other wire might be for the rotary switch normally open and closed positions.
I found code I did if it helps. This was to time dialling zero (ten) to calibrate a dial.
The LED should land int he middle, but most dials are a little off.
It happens to know the number dialled too, though the hardware wasn’t for displaying it.

'
'
'***********************************************************************************
'*                                                                                 *
'*                          [email protected]                               *
'*                             Brek Martin 2013-14                                 *
'***********************************************************************************
'
'
DEFINE OSC 20								' but we are really using 8 MHz
DEFINE NO_CLRWDT							' watchdog is cleared manually
LCD_DATAUS CON 50							' LCD timing
LCD_COMMANDUS CON 2000						'
'
'INTCON.4 = 1									'enable portb.0 interrupt
'OPTION_REG.6 = 0								'interrupt on falling edge of portb.0
'
'
INCLUDE "Elapsed.bas"							'
'
'
'
' execution time!
'
CMCON = 7	' set portb to digital
trisb.7 = 0	' set LED outputs
trisb.6 = 0	'
trisb.5 = 0	'
trisb.4 = 0	'
trisb.3 = 0	'
trisb.2 = 0	'
trisb.1 = 0	'
trisb.0 = 1 ' set rotary dial switch input (pulse)
trisa.1 = 1 ' set rotary dial switch input (home)
'
@ clrwdt			; clear watchdog timer manually
'
flashrate var byte
timea var byte
index var byte	' declare values for LED test startup
indexb var byte
bpattern var byte
dialcount var byte
'
oldlast var byte
'
errstart:
'
@ clrwdt			; clear watchdog
bpattern = %00000001
dialcount = 0
flashrate = 8
timea = 64
'
FOR indexb = 0 TO 6  ' fancy led test startup
FOR index = 0 TO 6
@ rlf _bpattern
portb = bpattern
pause flashrate
NEXT index
FOR index = 0 TO 6
@ rrf _bpattern
portb = bpattern
pause flashrate
NEXT index
flashrate = flashrate - 1
NEXT indexb
portb = 0
'
@ clrwdt			; clear watchdog timer manually
'
extint = 0			'reset external variables
freq = 0
ticcnt = 0
'
ready var bit		'declare internal variables
ready = 0
counting var bit
counting = 0
thetime var byte
thetime = 0
'
lasta var byte'		'bit to check for last rising edge
lasta = 0
'
'
Gosub ResetTime		'Reset Time 00:00:00
'
'
'
cycle:				' main routine - 3D Cube user program
@ clrwdt			; clear watchdog timer manually
'
'
IF porta.1 = 1 && counting = 0 THEN
portb = %11111110 ' dial not home indicator
ready = 1 ' set ready to count time status
thetime = 0
bpattern = 0
'
ELSE
'
IF counting = 1 THEN
'
IF portb.0 = 0 THEN
portb = %11111110 ' flash LEDs on for break
pauseus 50
portb = 0
ELSE
portb = 0		' flash LEDs off for make
ENDIF
'
ELSE
portb = 0
ENDIF
ENDIF
'
'
IF ready = 1 THEN ' check ready to count status
IF portb.0 = 0 THEN ' look for first falling edge
gosub StartTimer	'start clock timer
thetime = 0
dialcount = 0
timea = 64
counting = 1
ready = 0  ' lock routine
ENDIF
ENDIF
'
'
IF counting = 1 THEN ' check counting and dial at home position
'
IF portb.0 = 1 && lasta > 3 THEN ' look for last rising edge otherwise is overwritten
thetime = Seconds*100
thetime = thetime + Ticks
ENDIF
'
oldlast = lasta
'
IF portb.0 = 0 THEN		' dial switch debounce in software
lasta = lasta + 1
ELSE
lasta = 0
ENDIF
'


IF lasta < oldlast THEN ' increment dial pulse counter
dialcount = dialcount + 1
ENDIF



'
IF porta.1 = 0 THEN
gosub StopTimer
counting = 0
'
timea = 10
'
IF dialcount < 10 THEN	' check that zero was dialed
goto errstart			' or error condition
ENDIF
'
Gosub ResetTime			'Reset Time 00:00:00
ENDIF
ENDIF
'
'
'
'
IF thetime > timea && thetime < 75 THEN	' LED time display
bpattern = %00000010 ' dial too fast
ENDIF
IF thetime > 74 && thetime < 85 THEN
bpattern = %00000100
ENDIF
IF thetime > 84 && thetime < 95 THEN
bpattern = %00001000
ENDIF
IF thetime > 94 && thetime < 105 THEN
bpattern = %00010000 ' ideal time
ENDIF
IF thetime > 104 && thetime < 115 THEN
bpattern = %00100000
ENDIF
IF thetime > 114 && thetime < 125 THEN
bpattern = %01000000
ENDIF
IF thetime > 124 THEN
bpattern = %10000000 ' dial too slow
ENDIF
'
portb = bpattern
pauseus 500
portb = 0
'
'
goto cycle		' end main routine - do the next cycle
'
'

[AvionicsMaster1]

Though I doubt it, European phones may be different than in the USA but here in the USA only two wires, a pair, are needed for a phone to work.

The other pair at one time was used for enhancements and power. Those old phones often had several incandescent light bulbs that wouldn't allow the main line to function because the current draw was excessive. Typically nowadays those wires are used for a second line inside the house.

I think the home position you articulate is really time. If the handset is on the phone long enough the circuit resets and waits for the pulses or the tones. When I was younger you were cool if you didn't use the dial to call someone. You had to be talented enough to bang on the phone cradle the correct number of times to dial a phone. Ah the good old days.

I'm pretty sure all the pulses are the same time on/pause ratio. You just need to count the number of pulses and not wait too long to dial.

[keithdoxey]

Thats interesting. I see two pair of wires coming out of the dial. So one pair is sure the digit pulser, the other might be exactly that, the home position. Maybe they used that to mute the earpiece?

Sure I can make good use to it.

Ioannis

The dials in British phones had 5 contacts. 2 of them were the pulsing contacts which were normally closed and pulsed open for the dial pulses. The other three contacts were normally open and are referred to as the "Dial Off Normal" contacts. They placed a short across the earpiece to prevent acoustic shock and also a short across the carbon granule microphone to prevent the back EMF surges from welding the granules together.

http://www.britishtelephones.com/howdial.htm

Keith