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.
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.
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](#operation-mode-2-audioguestbookwithrotarydialer) 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.
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.
| [rotary phone](https://www.ebay.com/b/Rotary-Dial-Telephone/38038/bn_55192308) | 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](https://www.raspberrypi.com/products/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](https://www.adafruit.com/product/3252) | 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](https://a.co/d/1gb2zhC) | Any high capacity/throughput micro SD card that is rpi compatible | 1 | $8.99 |
| [USB Audio Adapter](https://www.adafruit.com/product/1475) | 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](https://www.adafruit.com/product/1099) | | 1 | $2.50 |
| --- | **--- If you don't want to solder anything ---** | --- | --- |
| [3.5mm Male to Screw Terminal Connector](https://www.parts-express.com/3.5mm-Male-to-Screw-Terminal-Connector-090-110?quantity=1&utm_source=google&utm_medium=cpc&utm_campaign=18395892906&utm_content=145242146127&gadid=623430178298&gclid=CjwKCAiAioifBhAXEiwApzCztl7aVb18WP4hDxnlQUCHsb62oIcnduFCSCbn9LFkZovYTQdr6omb3RoCD_gQAvD_BwE) | 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](https://www.adafruit.com/product/2011) | Optional: maximize capacity based on what will fit within your rotary enclosure. | 1 | $12.50 |
| [LiPo Shim](https://www.adafruit.com/product/3196) | 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](https://www.adafruit.com/product/1904) | Optional: for re-charging the LiPo. | 1 | $6.95 |
| --- | **--- If replacing the built-it microphone ---** | --- | --- |
| [LavMic](https://www.amazon.com/dp/B01N6P80OQ?ref=nb_sb_ss_w_as-reorder-t1_ypp_rep_k3_1_9&=&crid=15WZEWMZ17EM9&=&sprefix=saramonic) | 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 |
**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.
- 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](https://www.parts-express.com/3.5mm-Male-to-Screw-Terminal-Connector-090-110?quantity=1&utm_source=google&utm_medium=cpc&utm_campaign=18395892906&utm_content=145242146127&gadid=623430178298&gclid=CjwKCAiAioifBhAXEiwApzCztl7aVb18WP4hDxnlQUCHsb62oIcnduFCSCbn9LFkZovYTQdr6omb3RoCD_gQAvD_BwE) which plug directly into the rpi.
- 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](config.yaml).
- [Here's](https://superuser.com/questions/53957/what-do-alsa-devices-like-hw0-0-mean-how-do-i-figure-out-which-to-use) a good reference to device selection.
- You can also check [this](https://stackoverflow.com/questions/32838279/getting-list-of-audio-input-devices-in-python) from Python.
I found the sound quality of the built-in [carbon microphone](https://en.wikipedia.org/wiki/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](https://www.circuits-diy.com/mic-converter-circuit/).
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
[Here's](https://jayproulx.medium.com/headless-raspberry-pi-zero-w-setup-with-ssh-and-wi-fi-8ddd8c4d2742) a great guide to get the rpi setup headless w/ SSH & WiFi dialed in.
The provided service ensures the Python script starts on boot. The installer script will place this service file in the `/etc/systemd/system`` directory and modify paths according to your project directory.
To manually control the service:
```sh
sudo systemctl enable audioGuestBook.service
sudo systemctl start audioGuestBook.service
```
This service ensures smooth operation without manual intervention every time your Raspberry Pi boots up.
- 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.
- 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.
- 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
If you find that the behaviors for hanging up and lifting the phone are reversed, it's likely that the `hook_type` in `config.yaml` is incorrectly set. Ensure that it matches your phone's hook switch type (NC or NO).
