trunk call

a line for a real telephone

12 HP

Plug an old rotary phone into the socket in the panel and the module is its line. Its carbon mic comes out of MIC, whatever you patch into EAR is in its ear, and lifting the handset and dialling come out as gates and a CV.

The TRUNK CALL panel: black, a rotary telephone front-on in gold dots, its handset across the top under the name and its dial behind BIAS, the big knob on the spine. Under BIAS a window in the panel shows the socket's face: three narrow slots and a square hole in a row, down the spine. EAR and BIAS's CV in at the top, GAIN and SIDETONE under them, MIC and HOOK beside the socket, PULSE and DIGIT at the foot. trunk call at the top, daylight noise at the bottom.

Turn a knob: drag it, roll the wheel over it or use the arrow keys; a double click puts it back. The phone is plugged in: click its handset to lift it, put a finger in the dial and drag it round to the stop, then let go (or type the digits on it); lifted, drag the handset up and down. The patches step on by themselves until you touch anything.

What you hear is MIC and the rack round it. The handset hears her voice, or yours.

The handset hears your own microphone, from when you press it; what it hears stays in your browser.

example patches

The hook and the dial here are TRUNK CALL's firmware, running in your browser; the line, the carbon mic and the hybrid are a model of its circuit. Her voice, the music on hold and the rest of the rack are made in the page.

The phone

An old rotary phone's own cord plugs into the 610 socket on the spine. The module feeds its loop with a steady current from both rails, hears the handset lifted, and counts the dial's pulses as it runs back. Nothing on the line is ever more than the rails, and it doesn't ring.

The carbon mic

An old handset's microphone is a cup of carbon granules, and its voice is the line's current changing as they shake. So the current is its whole character.

BIAS: the loop current. 10 mA at zero is thin and frying, the voice breaking up and dropping out; 25 at noon is a phone call; 40 at full is loud, dark and packed flat.
BIAS CV: 3 mA a volt, added to the knob and held to 8 to 45 mA. Fast, it's a tremolo that crackles in every trough.
GAIN: the mic's preamp, from −21 to +21 dB, into a soft clip. Past three o'clock it fries; at full it's a wall you shout through.

The earpiece

EAR: anything patched in is in the earpiece, on the same two wires as the mic.
SIDETONE: at zero, EAR is nulled out of MIC, as far as the phone's line lets it. At noon there's no null, and MIC is the whole line both ways. Past noon MIC is fed back to the earpiece, and from two o'clock the handset hears itself and howls; move it, cup it or lay it down and the howl moves. With BIAS up it howls sooner.

What comes out

MIC: the handset's voice, after GAIN.
HOOK: 5 V while the handset's lifted. Dial pulses don't drop it; it falls 400 ms after you hang up.
PULSE: a trigger for every pulse the dial makes, ten a second. Dial 5 and it's five triggers; dial 0 and it's a roll of ten.
DIGIT: the last digit dialled, half a volt a pulse, from 0.5 V for a 1 to 5 V for a 0. It steps once the dial's back, and holds.

Inside

One board sits 10 mm behind the panel with everything on its front, the USB-C in a notch in its top edge; only the power header, the debug header and the two flashing buttons are on its back. The socket's contacts are a wall socket's own springs, held in a black printed carrier that stands on the board round a hole cut through it, its nose up in the panel's window, so a measured plug changes a print, not the board. An RP2350 counts the dial; the sound never goes near it.

The TRUNK CALL panel with the phone's plug in its socket and its knobs on.

Drag to turn it; click it to take it apart.

Specs

Format

Width: 12 HP
Depth: 21 mm behind the panel, plus the power cable’s plug; the phone’s plug stands about 37 mm out in front (estimated)
+12 V: about 66 mA with the handset down, 91 mA lifted with BIAS at noon, 115 mA at the most (estimated)
−12 V: about 30 mA with the handset down, 53 mA lifted with BIAS at noon, 77 mA at the most (estimated)
+5 V: not used
Panel: 1.6 mm FR4, gold through black mask; the socket in a window on the spine
Controls: 3 knobs (BIAS's big), a 610 socket

I/O

EAR: audio, ±5 V, to the earpiece
BIAS: CV, ±5 V, 3 mA a volt added to the knob
MIC: audio, up to ±5.2 V through its soft clip
HOOK: gate, 0 or 5 V
PULSE: trigger, 5 V for 5 ms a pulse
DIGIT: 0.5 V a pulse, 0.5 to 5 V; 0 V until a digit's dialled

Engine

Loop: a steady current, 10 to 40 mA on the knob, 8 to 45 mA with the CV
Phones: rotary phones on the 600 series plug; others through an adaptor. Tone-dial phones work as a handset, but their digits aren't counted
Dialling: 7 to 13 pulses a second
Ringing: none
Brain: RP2350, for the hook and the dial; the sound is all analogue
Firmware: over USB-C, or the debug header on the back