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Re: LF: VLF vorbis stream, Question

To: <[email protected]>
Subject: Re: LF: VLF vorbis stream, Question
From: "Markus Vester" <[email protected]>
Date: Sat, 20 Jun 2015 23:01:00 +0200
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Hi Stefan,
 
I don't really know the answer but here's a guess: If I remember right, your MF receiver gets the signal from the antenna through a mechanical filter. The passband is some 8 or 10 kHz wide, occupying only a part of the whole 24 kHz Nyquist range. Outside the passband there will still be some noise from the RX (and possibly also weak NDB signals) but at much lower level. I suspect that if you turn up the volume, a few of these weak channels may rise above a threshold where the Vorbis encoder decides to retain the information. If this is correct, the increased bitrate would mainly be from out-of-band information, and a lower level would not compromise the in-band performance.  
 
One way to check the hypothesis is to view the full band in a relatively fast spectrogram, with low contrast to show a large dynamic range. Attached is a spectrogram from the lovely song "Dreams" by the Cranberries, replayed from a 128 kbit/s mp3 file. The output from Mediaplayer was routed to Spectrogram software through the analog mixer. You can see many black patches, which correspond to temporary low levels in some audio channels. These have obviously been cut out by the encoder to reduce bitrate. It's interesting to note that although the nominal samplerate of this mp3 is 44.1 kHz, most of the high frequency content above 16 kHz has also been dropped, except for occasional louder segments.  
 
Best 73,
Markus (DF6NM)
 

From: DK7FC
Sent: Saturday, June 20, 2015 9:04 PM
Subject: LF: VLF vorbis stream, Question

Hi all,

There are vorbis streams available on VLF by Paul Nicholson and Wolf/DL4YHF and a few more known people from the scene. As far as i know these are in vorbis format, like my MF/VLF stream.
What i found yesterday, the data rate depends slightly on the volume level.

Now i'm asking myselfe if there is a 'quality' loss when using a low mic volume level ??

To become more precise: The usual method is to check where the noise level of the soundcard can be found, e.g. in Speclab it shows -120 dB for example. Then, if one connects the RX, it may rise to -118 dB (in what ever FFT bin width and Mic gain level). Then, connecting the antenna to the RX may let the (daytime!) noise level rise to -100 dB. Then one knows that the daytime band noise level is 18 dB above the noise level of soundcard+RX and everything is fine and the dynamic range is somewhere near 100 dB or maybe just 90 dB but at least it is high enough...
Can this method or thinking be applied when SpecLab is getting its data via a vorbis stream???
I can detect the noise level without an antenna connected and prove that it is about 20 dB lower as when the antenna is connected. So i assumed that everything is all right. But when playing with the mic gain level, i can see that the data rate rises about 10% when adding another 20 dB. So is there a loss of data, resulting in a lower SNR of incoming signals when using a low mic level, although it is still well enough above the soundcard+RX noise?? (Of course i want to keep the mic level as low as possible without a quality loss, to have a dynamic range as high as possible)
I just noticed that effect last night and now i'm aksing if there are unwanted losses.

73, Stefan

Attachment: dreams_mp3_128kbs_short.jpg
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