Rotary Phone Audio Guestbook

This project transforms a rotary phone into a voice recorder for use at special events (i.e. wedding audio guestbook, etc.).

• Rotary Phone Audio Guestbook
  • Background
  • Post-Event
    • Future Potential
  • Materials
  • Setup
    • Hardware
      • Wiring
      • Microphone Replacement (Optional)
    • Software
      • Dev Environment
      • Dependencies
      • AudioInterface Class
      • rotaryGuestBook.service
      • Operation Mode 1: rotaryGuestBook
      • Operation Mode 2: rotaryGuestBookwithRotaryDialer

Background

I was inspired by my own upcoming wedding to put together a DIY solution for an audio guestbook using a rotary phone. Most online rentals were charging $600 for an experience that didn't even offer the ability to add a custom voice mail and took about 4-6 weeks of turn around time to process the audio after the event. I tried to use as many parts that I had laying around to keep costs down. It worked out quite well and we were able to gather some very special voice messages.

Below you will find a parts list and detailed setup guide. Please feel free to reach out to me with any questions.

Post-Event

Since this was a trial by fire type of scenario there ended up being a few gotchas at the real event which I've since accounted for. Namely setting a time limit on the recording length as we had some youngsters leaving 5+ minute messages repeatedly and this ended up draining the battery.

Future Potential

A few weeks before the wedding I had the code registering dialed numbers from the rotary encoder with the goal of playing back special messages for certain guests who dialed a certain combination (i.e. dial an area code to hear a special message to my old roomates). The details of this operation mode are described in Mode 2 below. In order to activate this mode I had to wait for input when the phone was off the hook. This required an extra step of dialing zero before leaving a normal voice message. In the end we decided to keep it simple and I've thus migrated this code to the dev branch along with the code to run through post-porcessing the audio in a separate process. If any one is interested in expanding this please feel free.

Materials

Part	Notes	Quantity	Cost
rotary phone	Estate/garage/yard sales are probably the best places to find once of these. Ideally one with a phone jack since we will be using these four wires extensively.	1	$0.00-$60.00
raspberry pi zero	I didn't realize how hard these are to find these days. You can use any rpi or arduino style single-board computer but be aware of size constraints (i.e. must fit inside the rotary phone enclosure)	1	$9.99
raspberry pi zero case	Optional: added for protection. One of the cases on Amazon has a heat-sink cutout which might be nice for better heat dissapation since it will all be enclosed in the end.	1	$4.95
micro SD card	Any high capacity/throughput micro SD card that is rpi compatible	1	$8.99
USB Audio Adapter	Note: I removed the external plastic shell and directly soldered the wires instead of using the female 3.5mm receptacle.	1	$4.95
USB OTG Host Cable - MicroB OTG male to A female		1	$2.50
---	--- If running off a battery ---	---	---
LiPo Battery	Optional: maximize capacity based on what will fit within your rotary enclosure.	1	$12.50
LiPo Shim	Optional: if you plan to run this off a LiPo I would recommend something like this to interface with the rpi zero.	1	$9.95
LiPo Charger	Optional: for re-charging the LiPo.	1	$6.95
---	--- If replacing the built-it microphone ---	---	---
LavMic	Optional: if you'd like to replace the carbon microphone. This is an omnidirectional lavalier mic and outputs via a 3.5mm TRS	1	$24.95

Setup

Hardware

Wiring

Microphone Replacement (Optional)

I found the sound quality of the built-in carbon microphone on the rotary phone to be quite lacking in terms of amplitude, dynamic range and overall vocal quality. I tried boosting the gain from the digital (ALSA driver) side but this introduced an incredible amount of noise as expected. I then approached this from the analog domain and tried alternative circuitry to boost the sound quality based off this carbon-to-dynamic converter.

Might be worth a further investigation in the future since it retains the integrity of the original rotary phone.

My final attempt involved the introduction of some post-proceesing (see dev branch) to bandpass some of the freqs outside the speech domain and add some normalization. The processing was costly in terms of processing and power consumption/rendering time and I ultimately decided it was worth acquiring something that yielded a better capture right out the gate. Crap in, crap out - as they say in the sound recording industry.

To replace:

• Unscrew mouthpiece and remove the carbon mic
• Pop out the plastic terminal housing with the two metal leads
• Unscrew red and black wires from terminal

Software

Dev Environment

• rpi image: Rasbian w/ SSH enabled
• rpi on same network as development machine
• Desktop IDE: vscode w/ SSH FS extension

Here's a great guide to get the rpi setup headless w/ SSH & WiFi dialed in.

Dependencies

• pip3 install -r requirements.txt or pip install each manually:
  • GPIOZero
  • Pydub
  • PyAudio
  • PyYAML

AudioInterface Class

rotaryGuestBook.service

This service starts the python script on boot. Place it in the /etc/systemd/system directory.

systemctl enable audioGuestBook.service systemctl start audioGuestBook.service

Operation Mode 1: rotaryGuestBook

Operation Mode 2: rotaryGuestBookwithRotaryDialer

Note: Untested - decided not to go this route for my own wedding

• This mode is a special modification of the normal operation and requires a slightly different wiring connection since it accepts input from the rotary dialer.
• The idea was to playback special messages when particular users dial a certain number combination (i.e. 909 would play back a message for certain guests who lived with the groom in that area code).
• In this mode of operation the users will need to dial 0 on the rotary dialer in order to initiate the voicemail.