I'm trying to make a kind of sound application repurposing an old ADSL Router. At this point I already have compiled a kernel with Sound Supporting and Alsa drivers and also compiled alsa-lib and portaudio libraries.
Now I'm making some tests with c++ to know what kind of things I can do in the system (I dream with basic DSP, but the principal wish is to make something to record and play the recorded files). The thing is that I have troubles with something very basic like playing a sine wave. I'm using the paex_sine.c example from http://portaudio.com/docs/v19-doxydocs/group__examples__src.html but when played it has some drops like jitter.
Of course, the first thing that I made was trying to tune the device settings (Buffer size, Bits depth, Sample rate and Device Latency). I'm doing the tests with 2 different sound cards. One is a cheap usb card (the cheapest card of the market), but also did the tests with a Zoom H1 Recorderd, that has USB Sound card functionality, Class Compliant. In both cases I have the same results.
I also tested playing a .wav file with this example https://github.com/hosackm/wavplayer/blob/master/src/wavplay.c but same result. I attach the original and the jittered one.
The hardware is an old Huawei ADSL router. It use an RT63365e 500MHz (surprising with 4 cores) but is working at 420MHz. It also has a 32MB RAM. I'm pretty sure that there is not a resources issue because the CPU is used at max 11% by the other processes/system. Also have 5MB of RAM free.
So I don't know what could be the source of the issue. It's my first time compiling the Linux Kernel and modding an Embedded System. Do you think in any point that I could research for trying to fix the problem? Maybe I'm missing some kernel configuration at building stage. I read al menuconfig options and I didn't find anything like audio priority or something like that.
Original audio: https://vocaroo.com/1i0Rhyrmyhzf Audio with jitter: https://vocaroo.com/15l3p2wL5AhH
It's an old question, but since you haven't got any answer yet, I'm not sure your problem was solved or not.
My guess is, your issue lies with alsa-lib. I had a similar issue on an embedded environment (in my case, it was caused by the signals alsa-lib based on, because signals aren't thread-safe).
Maybe give a try to https://gitlab.com/bztsrc/nanoalsa
It does not need alsa-lib nor pulseaudio, it is as simple as it gets. This is a dependency-free, C/C++ stb-style single header solution. Wav player examples are provided in the repo (both with the simple
alsa_write()and the asyncalsa_start()/alsa_stop()API).If you need more features and plugins too, then I recommend to take a look at the https://github.com/tinyalsa/tinyalsa library, which is another alsa-lib replacement. It's more complex than nanoalsa, but still much simpler and less buggy than alsa-lib.