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Date: Mon, 22 Jun 2015 18:41:02 +0200
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Subject: Re: LF: VLF vorbis stream, Question
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Hello Paul,

Thanks, that could be most useful here. Interesting to see the little 
loss at q=0.5 / q=0.8 but the much lower data rate. I'm also running VBR 
but q=1 so far, and got 215 kbps. I'm going to play with these settings.

Best 73,
Stefan

Am 22.06.2015 10:45, schrieb Paul Nicholson:
>
> I tested weak signal detection.  Measuring carrier S/N
> after a range of compression types and quality settings.
>
>  Test carrier: 400 Hz;
>  Noise: Additive white Gaussian;
>  Bandwidth: 16.666 mHz (60 seconds);
>  Sample rate: 48k/sec;
>  Sample width: 16 bits;
>  Channels: 1 (mono);
>
> Results:
>
>   Format           Bit rate        S/N
> --------------------------------------------
>  Uncompressed      768 kbps       8.66 dB
>  FLAC              749 kbps       8.66 dB  (note 1)
>  Vorbis VBR q=0.8  171 kbps       8.68 dB  (note 2)
>  Vorbis VBR q=0.5   85.9 kbps     8.60 dB
>  Vorbis VBR q=0.3   68.8 kbps     8.64 dB
>  Vorbis VBR q=0.2   65.2 kbps     8.65 dB
>  Vorbis VBR q=0.1   61.0 kbps     8.60 dB
>  Vorbis VBR q=-0.1  35.4 kbps     8.72 dB
>  Vorbis CBR 80kbps  80.8 kbps     8.64 dB  (note 3)
>  Vorbis CBR 60kbps  60.8 kbps     8.64 dB
>  Vorbis CBR 40kbps  40.4 kbps     8.71 dB
>  Vorbis CBR 32kbps  32.5 kbps     8.48 dB  (note 4)
>
> Note 1: The test stream is virtually all random noise so FLAC
> doesn't reduce the size much.   When the band contains lots of
> 'foreground' signals, it does better.  Eg with baseband VLF
> containing the usual sferics and MSK signals, the FLAC bit rate
> is about 50% of the raw bit rate.
>
> Note 2: VBR = variable bit rate mode, compression level set by
> a quality factor between -0.1 and 1.0
>
> Note 3: CBR = constant bit rate mode, compression set by
> specifying a target average bit rate.
>
> Note 4: 32 kbps is the minimum allowed with 48k samples/sec mono.
>
> Tests were run using vlfrx-tools with command (VBR)
>
>  vtgen -s f=400,a=0.001 -n a=0.3 -r48000 -T2015-06-22,+600 |
>     vtcat -- - -,i2 |     # Restrict to 16 bit samples
>     vtvorbis -ep -q0.3 |  # Vorbis encode without timestamps
>     tee /tmp/test.vb |    # Save vorbis to measure bps
>     vtvorbis -dp  |       # Vorbis decode
>     vtnspec -f400 -r16.666e-3 -w10 -v -aN 10 > /tmp/spec.dat
>
> For CBR, the vorbis encode command is replaced with
>
>     vtvorbis -ep -b32 |  # Vorbis encode without timestamps
>
> This runs a 10 minute test, non-coherent averaging.
>
> We see negligible effect on the S/N right down to the lowest
> quality setting.  In this test we have pure white noise and a
> single weak carrier buried in the noise.
>
> What if the band also contains some foreground signals?
> I repeated some of the tests but this time adding in three strong
> carriers at other frequencies.
>
>   Format           Bit rate        S/N
> --------------------------------------------
>  FLAC              752 kbps       8.66 dB
>  Vorbis VBR q=0.5   88.5 kbps     8.58 dB
>  Vorbis VBR q=-0.1  37.8 kbps     8.63 dB
>  Vorbis CBR 32kbps  32.5 kbps     8.48 dB
>
> Again, no significant degradation of weak signal detection, even
> with the strongest compression.
>
> Now, what if the measurement bandwidth is wider. I change to 1Hz
> bandwidth with increased amplitude of the test carrier to
> compensate, still with three strong carriers mixed in:
>
>   Format           Bit rate        S/N
> --------------------------------------------
>  Uncompressed      768 kbps       6.27 dB
>  Vorbis VBR q=-0.1  47.6 kbps     6.32 dB
>  Vorbis CBR 32kbps  36.4 kbps     6.30 dB
>
> Vorbis works by separating (in the frequency domain) those bins
> which it considers to contain background noise and encoding them
> with fewer bits.   This doesn't affect S/N noticeably until you
> get down to less than about 2 bits resolution.  Radio astronomers
> (eg SETI) exploit this and routinely use 1 or 2 bit samples.
>
> -- 
> Paul Nicholson
> -- 
>
>
>