Support and General Use > Recording
Strange noises when recording to mp3
lee321987:
Answering Davide:
Yes, the offset is present on WAV's and MP3's.
Yes, the offset is occurring on both units, but it's much stronger on the c250.
I don't know about e2x0 units.
saratoga:
If you're encoding to mp3 anyway, lame can be told to remove the DC offset using its highpass filter option. However, since people cannot hear a DC offset, and because DACs should filter them out, it shouldn't impact quality.
lee321987:
But it does seem to be the problem (at least with the Rockbox mp3 encoder). I'm using the exact same version of Rockbox on both players (not even a different "../tools/configure" number), and the only difference I can find is that the c240 produces a much smaller offset (and doesn't produce this noise).
I'm not putting down Rockbox's encoder. I think the size of the offset produced by my DAP may be a good deal over average.
Anyway Rockbox still doubled the amount of recordings I can store internally by giving me WavPack (the OF only offered WAV), and multiplied MANY times my total capacity by allowing me to record onto the MicroSD. Oh yeah, AND gave me 22050 Hz sample rate (OF only offered 16000 Hz). Go Rockbox!
Multiplex:
But couldn't the offset be causing the noise?
From a PCM value of 0V to whatever the offset is may look like a sharp edge, the encoder probably can't cope with that and noise is the result of it trying to encode it.
Look at a JPG of this web page for example, the sharp transitions will cause visible noise when you zoom in.
dreamlayers:
I doubt the WAV is totally silent. There's probably some quiet sound from the microphone and maybe some noise which is like white noise and less noticeable and less objectionable. One could take a lossless recording, remove the offset, normalize the quiet section and see if there is some noise.
I don't think the edge is the problem, because the the problem is constant and not just at the start and end of the recording. I think the DC offset itself is a problem. Like I said earlier, a DC offset even messes up LAME, resulting in bass pulses. (As someone else said, with LAME it is possible to remove that with the high pass filter.)
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