Audio guestbook using an old rotary phone and a raspberry pi zero
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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.).

image

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
image image
  • 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.

    image

  • 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
image image

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

image

  • 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:

image

image

image

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

Config

  • This file allows you to customize your own set up (edit rpi pins, audio reduction, alsa mapping, etc), modify the yaml as necessary.
  • Ensure the sample rate is supported by your audio interface (default = 44100 Hz (decimal not required))
  • For GPIO mapping, refer to the wiring diagram specific to your rpi: image

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

Troubleshooting

Verify default audio interface

A few users had issues where audio I/O was defaulting to HDMI. To alleviate this, check the following:

Check the Sound Card Configuration:

Verify the available sound devices using the following command:

aplay -l

Ensure that your USB audio interface is listed and note the card and device numbers.

Set the Default Sound Card:

If you want to route audio through your USB audio interface, you'll need to make it the default sound card. Edit the ALSA configuration file (usually located at /etc/asound.conf or ~/.asoundrc) and add the following:

defaults.pcm.card X
defaults.ctl.card X

Replace X with the card number of your USB audio interface obtained from the previous step.

Restart ALSA

sudo /etc/init.d/alsa-utils restart

Support

If this code helped you or if you have some feedback, I'd be thrilled to hear about it! Feel like saying thanks? You can buy me a coffee .