For his A-level electronic course work, [Andrew] decided to build a digital sound recorder that doesn’t use a microcontroller.
[Andrew]’s build captures audio from an on board microphone at 8000 samples/second. The audio is digitized into 8-bit sound data and sent to an SRAM. The recording and playback functions are controlled entirely by 4000-series logic chips. He admits the sound quality is pretty poor; this is mostly due to the 8kHz sample rate. In some circles, though, a terrible sample rate is seen as being pretty cool so we’re not going to say [Andrew]’s build is useless.
There’s some pretty smart design choices in [Andrew]’s build, like a cut off filter on the microphone set at 4000 Hz (the Nyquist frequency of his system). For the recording medium, he used an SRAM that can hold about half a megabyte of data. At 8000 samples/second, [Andrew]’s build can store a little more than 60 seconds of audio. The build may not be a logic chip computer, but there’s not any question in our mind that [Andrew] learned something. Check out [Andrew]’s 66-page coursework report here (PDF warning).