Re: [SoX-users] Differences recording to OGG and WAV and how to fix this

[fmiser <fm...@gm...>]

> > fmiser wrote:
> >
> > From what I can see of the spectrograms, they don't look
> > to me like they are the same audio.  The .wav file has a
> > higher level, and has broadband pulses.  Maybe something
> > like a hand clap, or wind noise at the mic.

> Pander wrote:
> 
> Nope. I have a sound measuring setup and these broadband
> verticalines show up many times without a clear reason.

When you playback the files, can you hear the difference?  That is, can you hear what we see in the spectrogram?

> I make many recordings under the same conditions and these
> differences are huge. I am looking for the reason. Perhaps
> it is the file format.

Maybe.  GSM is certainly not what I would use as a format for
sound analysis.

> > Another oddity I see is you are recording with a sampling
> > rate of 48 kHz - but the spectrograms display 0 Hz to 180
> > Hz.  So apparently there is some other processing beyond
> > what you have listed.  Could the difference in signal
> > level be a consequence of this other processing?  Or
> > could this processing have an artifact that is creating
> > broadband pulses?
> 
> I look only at a part of the spectrum with
> 
> sox test.wav -n remix - rate 375 spectrogram

> What is the default encoding that is used when none is
> specified and you record to WAV? Is that signed-integer or
> gsm-full-rate
> 
> In other words, for SPL usage and spectrum analysis (no
> compression/reduction required), what is the best format to
> use?

Probably not GSM!  I don't know much about it, but it looks
like GSM is one of the lossy formats.

Lossless formats would be any of the PCM files - like .wav,
.aiff and loss-less encoding like flac.

Excerpts from SoX man page:

Accuracy 
       Many file formats that compress audio discard some of
       the audio signal information whilst doing
       so. Converting to such a format and then converting
       back again will not produce an exact copy of the
       original audio.  This is the case for many formats used
       in telephony (e.g. A-law, GSM) where low signal
       bandwidth is more important than high audio fidelity,
       and for many formats used in portable music players
       (e.g.  MP3, Vorbis) where adequate fidelity can be
       retained even with the large compression ratios that
       are needed to make portable players practical.

gsm-full-rate
       GSM is currently used for the vast majority of the
       world's digital wireless telephone calls.  It utilises
       several audio formats with different bit-rates and
       associated speech quality.  SoX has support for GSM's
       original 13kbps `Full Rate' audio format.  It is
       usually CPU-intensive to work with GSM audio.

Another tool I like using for sound analysis is baudline, a
linux-only free-to-use program that does FFT and spectrum
display realtime or from a file.