Hi Tobias, Stefan and all,
Indeed, but despite what Tobias said the Vorbis encoder / decoder leaves
extremely weak signal intact, which are *many* dBs below the human
hearing threshold.
At least with moderate compression / "quality" settings as used for the
VLF streams.
As Stefan noted there seems to be some dependence on the overall input
level. Here (for the "VLF6" channel on Paul's server, labelled
Bielefeld), the statics and worldwide sferics background provide so much
"dithering noise" that the netto bitrate (sent from this end) is almost
constant. It's about 100 kBit/second.
Maybe Paul has made some more comparisons between the uncompressed and
compressed/decompressed SNR readings on the VLF experiments some time ago.
All the best,
Wolf DL4YHF
(almost heading for southern Germany... possibly no email in the very
remote valley (Lautertal) in the Schwäbische Alb, over the next week)
Am 21.06.2015 14:58, schrieb Tobias DG3LV:
Hi Stefan, LF !
For the suitability of "(ogg) vorbis" format as a transport for
antenna (or IF) signals please consider reading the articles at
english or german Wikipedia about the technical details of this
lossy(!) audio codec :
https://en.wikipedia.org/wiki/Vorbis
https://de.wikipedia.org/wiki/Vorbis
The output of vorbis is completely "synthetic" (as "MP3" is) and
targeted to the human ear and brain as a "receiver", but not for an RF
receiver and subsequent DigitalSignalProcessing.
But there is a (free, too) alternative : a lossless(!) audio codec "FLAC"
https://en.wikipedia.org/wiki/FLAC
https://de.wikipedia.org/wiki/Free_Lossless_Audio_Codec
Both codecs are implemented in the free "FFmpeg" suite, available for
Windows and Linux (Raspi ?) and other OSes.
https://en.wikipedia.org/wiki/FFmpeg
https://en.wikipedia.org/wiki/List_of_hardware_and_software_that_supports_FLAC
73 de dg3lv Tobias
Am 20.06.2015 um 21:04 schrieb DK7FC:
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
|
