Rotary Phone Audio Guestbook

• Rotary Phone Audio Guestbook
  • Background
  • Post-Event
    • Future Work (Action Items)
  • Materials
  • Hardware
    • Wiring
      • Hook
      • Phone Cord
    • Microphone Replacement (Optional)
  • Software
    • Dev Environment
    • Dependencies
    • Config
    • AudioInterface Class
      • audioGuestBook systemctl service
    • Operation Mode 1: audioGuestBook
    • Operation Mode 2: audioGuestBookwithRotaryDialer

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

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. Alternatively, depending on your scenario, it might be preferable to attach directly to a 5V power supply.

Future Work (Action Items)

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.

I would also like to thread the audio playback so I can have a monitor/watchdog service terminate the thread upon hook callback so that the message doesn't continue playing once the user hangs up.

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 you don't want to solder anything ---	---	---
3.5mm Male to Screw Terminal Connector	Optional: can connect the handset cables directly to the USB audio interface via these screw terminals	2	$1.37
---	--- 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

Hardware

Wiring

Hook

• Use multimeter to do a continuity check to find out which pins control the hook:

On-hook --> Open circuit (Value == 1)	Off-hook --> Current flowing

• The B screw terminal on the rotary phone is connected to the black wire which is grounded to the rpi.

• The L2 screw terminal on the rotary phone is connected to the white wire which is connected to GPIO pin 22 on the rpi.

  

• Note: the green wire was used for the experimental rotary encoder feature identified in the future work section.

Rotary Phone Block Terminal	Top-down view

Phone Cord

• The wires from the handset cord need to be connected to the USB audio interface
  • I soldered it but you can alternatively use 2x 3.5mm Male to Screw Terminal Connector which plug directly into the rpi.
    • Note: The USB audio interface looks weird in the pics since I stripped the plastic shell off in order to solder directly to the mic/speaker leads

• Use this ALSA command from the command line to test if the mic is working on the rpi before you set up the rotary phone: aplay -l
  • You might have a different hardware mapping than I did, in which case you would change the alsa_hw_mapping in the config.yaml.
  • Here's a good reference to device selection.
  • You can also check this from Python.

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
• Prepare your lav mic
  • I pulled off the 3.5mm male headphone pin since it is usually coated and annoyingly difficult to solder directly on to.
  • Carefully separate the two wires from the lav mic and spiral up the surrounding copper. This will act as our ground signal.
• Extend the green wire from the phone cord clip to the ground point of the lav mic.
• Red to red, black to blue as per the following diagram:

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

Config

• This file allows you to customize your own set up (edit rpi pins, audio reduction, alsa mapping, etc), modify the yaml as necessary.

• For GPIO mapping, refer to the wiring diagram specific to your rpi:

AudioInterface Class

• Utilizes pydub and pyaudio extensively.
• Houses the main playback/record logic and has future #TODO expansion for postprocessing the audio. Would like to test on an rpi4 to see if it can handle it better for real-time applications.

audioGuestBook systemctl 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: audioGuestBook

• This is the main operation mode of the device.
• There are two callbacks in main which poll the gpio pins for the specified activity (hook depressed, hook released).
• Once triggered the appropriate function is called.
• On hook (depressed)
  • Nothing happens
• Off hook (released)
  • Plays back your own welcome message located in /sounds/voicemail.wav followed by the beep indicating ready to record.
  • Begins recording the guests voice message.
  • Guest hangs up, recording is stopped and stored to the /recordings/ directory.

Operation Mode 2: audioGuestBookwithRotaryDialer

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.
• The rotary dialer is a bit more complex to set up, you need a pull up resistor connected between the F screw terminal and 5V on the rpi and the other end on GPIO 23. #TODO: Diagram