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

[Pander <pa...@us...>]

On 05/14/2015 07:52 AM, fmiser wrote:
>> Pander wrote:
>>
>> I use the same microphone (USB 24 bit 48kHz) to do the
>> following recordings:
>>
>> rec -q -r 48000 -c 1 -b 24 -e gsm-full-rate --buffer 16384
>> -C 10 test.wav trim 0 360
> 
> It seems odd that you are specifying a compression quality
> for a .wav file.

That is an omission. When not using this, it gives the same result.

> 
>> rec -q -r 48000 -c 1 -b 24 -e gsm-full-rate --buffer 16384
>> -C 10 test.ogg trim 0 360
>>
>> But the result in spectrograms is are very different, see:
>> https://i.imgur.com/fTMvDuK.png WAV
>> https://i.imgur.com/z6YUcxF.png OGG
> 
> A spectrogram is a "picture" of the sound.  If the two
> recordings sound different, the spectrograms will also be
> different - by as much as the sound is different.
> 

They are indeed recordings of different moment but under the same quiet
conditions.

>>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.
> 

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

> The .ogg file is quieter, and there is a steady tone from 80
> to 180 seconds - but at a different frequency than the one
> visible in the .wav.
> 
> I'm not sure what your goal is for the spectrograms, but if
> you are wanting to see the difference between encodings, or
> other processing, it usually works better to start with an
> audio file rather than a microphone recording so there won't
> be a difference in the source - only in the process.

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.

> 
>> How do I get rid of the veritcal lines when recoding WAV?
> 
> By getting rid of the sound that created them?
>  

Probably, but they appear even when I don't hear extremely loud noises.

>> How can I get the OGG recording improve use of range?
> 
> Maybe by normalizing?

I use the recordings with a calibrated system so I am not allowed to do
normalisations that alter the sound level.

> 
> 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

> 
> I'm doing a lot of guessing and speculating!  *smiles*

In short, I used to record with rec with an identical command on a
Raspberry Pi and got the spectrogram with more bright colors so to say.

On a new Raspberry Pi with also Raspbian, I get now a spectrogram with
darker colors. I am trying to figure out what is happening. Hence, I
tried using explicit encoding and also OGG format without compression
for comparison.

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?

> 
> -- f
> 
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