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
  • Materials
  • Setup
    • Prepare Your Rotary Phone
    • Download and Install the Custom Image
    • Configuration and Customization
  • Hardware
    • Wiring
      • Hook
      • Phone Cord
    • Optional: Microphone Replacement
  • Software
    • audioInterface
    • audioGuestBook
  • Support

Background

Inspired by my own upcoming wedding, I created a DIY solution for an audio guestbook using a rotary phone. With most online rentals charging exorbitant fees without offering custom voicemail options, I sought a more affordable and customizable solution. Here, I've detailed a guide on creating your own audio guestbook. If you have questions, don't hesitate to reach out.

Materials

Parts List

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

Setup

Prepare Your Rotary Phone

• Follow the Hardware section for detailed instructions on wiring your rotary phone to the Raspberry Pi.

Note: This is a crucial first step before using the software.

Download and Install the Custom Image

With your hardware ready, download the custom Raspberry Pi image provided, which has all necessary software pre-installed and pre-configured.

• Download the latest release: v1.0.0

• Extract the .gz file, i.e.: gunzip rpizero_rotary_phone_audio_guestbook_v<latest>_imagebackup.img.gz

• Flash the Image to an SD Card: Use a tool like the Raspberry Pi Imager or BalenaEtcher to flash the custom image onto your SD card:

  • 
  • Customize the config to specify the following username/password:

    username: admin
    password: password

  • and set up your wifi network accordingly

• Insert and Boot: Place the SD card into your Raspberry Pi and power it on to boot into the Rotary Phone Audio Guestbook.

Configuration and Customization

Once your system is up and running, you will want to make adjustments to suit your specific setup, especially if your hardware differs or you have personal preferences for how the guestbook operates.

• Replace the voicemail.wav with your own custom recording

• Check your ALSA HW mapping

  Depending on your audio interface's configuration, you may need to adjust the ALSA settings further. If after running aplay -l you find that the ALSA hardware mapping differs from what is expected or if you're experiencing audio issues, consider modifying .asoundrc to ensure your device correctly identifies and uses your audio interface. For example, if your USB audio interface is listed as card 1, device 0, you might add or modify .asoundrc to include:

  pcm.!default {
      type hw
      card 1
  }
  
  ctl.!default {
      type hw
      card 1
  }

• Adjust the config.yaml

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

  • alsa_hw_mapping: The ALSA hardware mapping for your audio interface. Use aplay --help for format guidance.
  • format: Set the audio format (default is cd). Refer to aplay --help for options.
  • file_type: The type of file to save recordings as (default is wav).
  • channels: Number of audio channels (default is 2 for stereo).
  • hook_gpio: The GPIO pin connected to the phone's hook switch.
    • For GPIO mapping, refer to the wiring diagram specific to your rpi:
    • 
  • hook_type: Set to NC (Normally Closed) or NO (Normally Open), depending on your phone's hook switch hardware setup.
  • recording_limit: The maximum length for a recording in seconds (default is 300).
  • sample_rate: The sample rate for recordings (default is 44100 Hz).

  Note: Adjust these settings as needed based on your specific hardware setup and preferences.

• Test audio playback/recording

To ensure your settings are correctly applied, you can test audio playback and recording after making changes. For playback, you can use a sample WAV file and the aplay command. For recording, arecord can be used followed by aplay to play back the recorded audio.

• Check audioGuestBook systemctl service

This service ensures smooth operation without manual intervention every time your Raspberry Pi boots up. The service file is sym linked to the /etc/systemd/system directory. Manual control of the service is possible as it operates as any other .service entity. You can quickly check the status with journalctl -u audioGuestBook.service

Hardware

Wiring

Hook

Understanding Hook Types: Depending on your rotary phone model, the hook switch may be Normally Closed (NC) or Normally Open (NO). When the phone is on the hook:

• NC: The circuit is closed (current flows).
• NO: The circuit is open (no current).

To accommodate either type, you'll need to update the config.yaml with the appropriate hook type setting.

• 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.

Optional: Microphone Replacement

For improved sound quality, consider replacing the built-in carbon microphone.

I found the sound quality of the built-in mic 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

audioInterface

• Utilizes ALSAs native aplay/arecord via subprocess calls.
• Houses the main playback/record logic.

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).
• In the code, depending on the hook_type set in the config.yaml, the software will adapt its behavior. For NC types, hanging up the phone will trigger the on_hook behavior, and lifting the phone will trigger the off_hook behavior. The opposite will be true for NO types.
• Once triggered the appropriate function is called.
• On hook (depressed)
  • Nothing happens
• Off hook (released)
  • Plays back your own added welcome message located in /sounds/voicemail.wav followed by the beep indicating the start of recording.
  • Begins recording the guests voice message.
  • Guest hangs up, recording is stopped and stored to the /recordings/ directory.
  • If the guest exceeds the recording_limit specified in the config.yaml, play the warning time_exceeded.wav sound and stop recording.

Support

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