I am working on an electronics project and I need to store speech data in an extremely limited memory. Since the microcontroller is not equipped to dynamically decompress audio data, I am stuck with having to use uncompressed audio.
After some experimentation I figured out that 4 bit audio sampled at 4000 Hz is enough to convey speech without being annoying. It takes 2000 bytes to store a second of audio, which is manageable if not perfect.
The problem here is that no audio editor I know of can output any lower than 8 bit audio at 8000 Hz.
I have tried using Nyquist prompt to quantize sound data to 4 bits and downsampling audio to 4000 Hz in Audacity. It sort of works, but I can still only export to 8 bit 8000 Hz PCM RAW. And even then, I found that the RAW binary still has full 8-bit data. There are hex values like 0x86, 0x7f all over the place where I would have expected all samples to have 0 in their lower 4 bits. Further, the Nyquist prompt output is full of high frequency noise. Filtering this noise with an equalization filter does filter the noise, but it also "de-quantizes" the audio. This is only logical, I guess, because the track is in 8 bit PCM and there is no way to tell audacity not to mess with the quantization.
So, I am essentially looking for an audio editor that can work with 4 bits. It need not be as fully featured as the more famous ones, but as long as it can import 8-bit data and output 4-bit data after equalization, it will work. Also, it can be a command line tool, some abandoned open source project or anything as long as it does my job.
I don't have a digital audio background, so I might not have asked this question properly or followed proper terminology, and I apologise for that.
Any general advice pertaining to this question/alternate methods/pointers in the right direction are welcome.