Rockbox

As far as I know, the analog audio from the FM radio is passed directly to the analog mixer when *not* recording (bypass mode). When recording, the audio from the FM radio is passed into the A→D converter, through the D→A converter and into the analog mixer so you’re hearing the sampled and quantized audio signal. There will always be some quality loss due to the sampling and quantizing,

Can you provide an example of the quality loss, so we can determine if it’s just quantizing / sampling or something else?

Marek Salaba could record the sound your Sansa Fuze makes with your computer and upload it here? The file has to be smaller than 2mb. Thanks

I will try to make a record.

BTW this description is exactly what I am noticing on my e200v2. The problem doesn’t occur on e200v1.

I notice the recording screen disables EQ, crossfeed etc, maybe this is the “degradation” reported?

Besides, samples on the AMS v2 are only 14 bits and the real sample rate is 1/2 what is shown in the screen since the ADC runs at 1/2 the clock rate of the DAC on all AMS. None of this is corrected for, meaning that samples are repeated once and the volume is 12dB lower than what it should be.

On e200v1, samples are 16 bits but the proper sample rate is enforced by discarding every other duplicated sample. Even so, actual recorded quality on e200v1 is a bit better than what monitoring shows because monitoring has the full sample rate.

Very interesting, Michael. It looks like an explanation for issues reported here http://www.rockbox.org/tracker/task/11631. I hope you and saratoga will be able to find a work-around eventually. :)

I did some fiddling around to revert to not using DMA channels for recording so that the samples can be properly processed during recording for bit depth, frequency and copying to the right channel for the mic source on as3525v1. Not all is complete here that I would like quite yet but it does work. Sample rates from 8kHz through 48kHz are enabled for recording per the divide-by-2 of the ADC.

   as3525-soft-dma-01.patch (19.2 KiB)

I’m happy with this one.

It takes care of some issue with hardware that has different settings for a given recording samplerate and the default recording rate doesn’t result in the default playback rate (for hardware reset).

Implements monitoring in both channels for all devices for mono and stereo.

Implements full-duplex audio on AMS (record and play simultaneously and as independently as possible).

   as3525-soft-dma-02.patch (26.8 KiB)

Patch committed in r31180. I await word on what happened.

FM recording quality is very clean now after your patch. Great work! If you feel like implementing DC offset correction for my Fuze v2 it’s detected as positive 2.16 by CEP.

I had thought about it. It really wouldn’t do anything that “Center Wave” wouldn’t accomplish in CEP since the DC offset is probably before the ADC, thus digitized before anything can be done.

Reporting cause you mentioned such possibility in the other task but I agree with your conclusion.