Another device has been tested for the project, LCTech’s ISD1820-powered ‘voice module’.
This is a fairly small board centred around a single-chip recording and playback solution. From a little googling, it was found that the chips can be purchased for about £0.56 a piece.
The downside of them is that by default, they can only record up to 10 seconds of audio, and the quality is awful. They record at 6.4KHz, which is far below the Nyquist frequency for human-audible sound of about 44Khz (see CD-quality audio for examples).
This can be adjusted up to 20 seconds of audio (at an even lower sampling rate of 3.2KHz), or down to 8 seconds of audio, at 8Khz.
The quality is not helped by the microphone being equally cheap and soldered directly to the board – it’s very hard to get it close enough to the sounds you want to capture.
Despite this, the interface to the chips is very well-suited to the project- they run off 3V sources, like 2 AA batteries, or most LiPo cells, and all of their functions are controlled by simple IO pins. There’s no user interface, and all responses are immediate.
This lends itself well to having a simple ‘hold to record’ function, and ‘pull to playback’. The chip will even cut off playback halfway through if you press record, and write over the sound!
The other downside is that there’s no way of digitally copying the recorded sounds from the chip, as with the MP3 player, or any of the devices using SD cards. This means there’s no way of saving, editing, or otherwise altering the sounds via computer.
Although the chips can only record a single sound at a time, they’re so cheap and simple it may be worth creating a ‘master’ device which controls multiple chips, and adding some pre- and post-processing to the audio, like a preamp stage for the microphone, and an amplifier for the speaker to increase the volume and fidelity.
It also conveniently fits into the test case, even on its original board!