I am trying to use the UDOO as an audio recording device from the MIC input. I have the out of a device plugged into the mic input. It all works good as I can hear the input on the speaker output clear. However when I record it with both Audacity and arecord the recording is static, almost like its overmodulated but worse. I have went through several how to related to Alsamixer and Pulse. I found no changes. I also saw in the alsa mixer if I lower the mic recording mixer, that I find it back in the red once I record. Wondered if anyone else has tried this or seen this and maybe has some ideas on how to overcome. Again it only happens on the recording the raw seems to sound good when you listen to it live. Any help would be great. Thanks
Are you trying to connect a "line level" output into the microphone jack? If so, that explains the clipping-- microphones put out a very weak signal, less than a tenth of line-level, and if feeding an input meant for this strength of signal with any other device, you'd be seriously overloading it. Try building a voltage divider circuit, to knock the level down by about 20 times to start, though you might need to go even lower. Something like this: (input signal from device) --> 220kohm resistor --> X --> 10kohm resistor --> GND and tap the signal at point "X" to feed to mic-in. If it's still too "hot", either raise the value of the 220k resistor, or lower the 10k, It's unfortunate that the UDOO doesn't include any true line-level inputs. The audio chip used (VT1613) has them, which is why LINE and AUX show up in alsamixer, but they weren't brought out to jacks.
"It's unfortunate that the UDOO doesn't include any true line-level inputs. The audio chip used (VT1613) has them, which is why LINE and AUX show up in alsamixer, but they weren't brought out to jacks." Does this imply that those line inputs would be accessible in software if only they could be brought out to a stereo jack via some mod? Or would it also require kernel mods? Thanks
I went ahead and did the mod for this, also patching the vt1613 analog sound driver to allow full use of the extra inputs-- see post9521.html Without that kernel patch, you can still pass audio through from LINE or AUX pins to the output, but no recording source control is exposed in the mixer, so the ADC remains stuck on the mic input. Its recording level wasn't adjustable either, and defaulted to a setting that was a bit too "hot" for the line-ins. For some reason this chip has an unusually sensitive ADC-- I had to use a 28:1 voltage divider to avoid clipping even at the lowest recording level, even though pass-through of much stronger signals (500mv P-P after a 2;1 divider) sounded OK. That may be a consequence of running the chip's analog section on 3.3V rather than the recommended 5V, or perhaps some input scaling register I haven't found, though I've combed through the full VT1613_R100.pdf datasheet.
