Pulse Dialing Detection
=======================

**Pulse dialing** was the standard method of digit signaling in rotary telephone systems from the early 1900s through the 1980s. Each digit was transmitted as a series of timed **loop break pulses**, created by rapidly toggling the telephone's internal switch hook contacts as the dial rotated back to rest.

Overview
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Each digit generates a specific number of pulses:

- Digit “1” = 1 pulse
- Digit “2” = 2 pulses
- ...
- Digit “0” = 10 pulses

Each pulse consists of:

- **Break** (on-hook): ~60–70 ms (open loop)
- **Make** (off-hook): ~40 ms (closed loop)
- A ~700–800 ms pause after all pulses marks the end of the digit

.. figure:: /_images/dial_5_pulse_digits.png
   :alt: Dial pulse pattern for digit 5 with timing annotations
   :width: 600px
   :align: center

   **Pulse Dialing – Digit 5:** Five loop break pulses, each ~60 ms open and ~40 ms closed, followed by a post-digit pause.

.. note::
   **Why does "0" send 10 pulses?**  
   This legacy behavior comes from early switchgear that had no concept of “digit zero.” The switch simply counted pulses, so “0” was encoded as the maximum—**10 pulses**—and positioned last on the rotary dial.

Historical Evolution
--------------------

- **1900s–1940s:** Step-by-step switches and rotary selectors mechanically counted loop breaks.
- **1950s–1970s:** Electromechanical relays and early transistor circuits refined accuracy and reduced noise.
- **1980s–2000s:** Modems and fax machines began using microcontrollers to decode rotary pulses.
- **Present:** Embedded systems and GPIO edge detection provide precise, software-based pulse decoding.

Anti-Tinkle Suppression
-----------------------

One challenge with pulse dialing was the **"tinkle"** sound — faint bell rings caused by pulse transitions during dialing. This occurred when the ringer briefly saw inductive spikes from loop current changes.

To suppress tinkle:

- CO equipment muted the ringer path during dialing  
- Some phones included **anti-tinkle circuits** using rectifiers or spark gaps  
- Advanced PBX gear actively blocked ringers during loop break sequences

Detection Requirements
----------------------

Any pulse detection circuit must:

- Observe rapid transitions (10–15 pulses per second)
- Count accurately, even with contact bounce
- Distinguish digits via pause detection
- Remain isolated from -48V DC line voltage

Detection Options
-----------------

**Option 1: Software-Based Detection (Modern Embedded Style)**

- Monitor loop status via GPIO or interrupt
- Measure transitions and timing in firmware
- Debounce and group pulses into digits

✅ Simple and flexible  
✅ No extra hardware  
❌ Requires reliable software timing

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**Option 2: Dual-Path Detection**

- Split the signal from the off-hook circuit:
  - One path with filtering for steady-state hook detection
  - One fast path for raw edge detection (via Schmitt trigger or comparator)

✅ Preserves clean edges  
✅ Reduces false detection  
❌ Slightly more complex circuit

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**Option 3: Dedicated Pulse Decoder Circuit (Historical/Nostalgic)**

- Use analog building blocks:
  - Monostable 555 timers
  - RC differentiators and comparators
  - Custom relay-based pulse counters

✅ Vintage authenticity  
❌ Bulky and obsolete  
❌ Less flexible than software

Summary
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- Rotary dialing encodes digits as timed loop break pulses
- Reliable detection requires clean edge sensing, debouncing, and pause tracking
- Options include pure software, hybrid circuits, or dedicated analog logic
- Pulse dialing remains an essential compatibility feature in vintage phone systems