Hi,
I recorded the contest as MPEG 3 mono 8 Khz that created 1 hour file of more
or less 6.5Mb.
The only problem I encountered was that after 15 hours of recording it just
stopped and I couldn' restart it anymore, unless I re-boot the machine, I
think.
In condition I was I prefer to keep WL running due to a tremendous 48 hours
Pile-up. When you re-boot system you never know if it starts to work fine as
before!!!
Everything else worked out fine (network was just great!!!)
the only problem was on the 10 minutes rule window.
73's
Luca IK2NCJ
(one of D44TD)
----- Original Message -----
From: "Eric Scace K3NA" <eric@k3na.org>
To: "WriteLog" <writelog@contesting.com>
Sent: Friday, November 29, 2002 4:48 AM
Subject: [WriteLog] testing audio compression systems
> Hello --
>
> I recorded the WW CW contest using CCITT A-law and was not surprised to
find the results were less than perfect.
>
> I am curious to find out if anyone has done any systematic tests of
various compression choices available to us for our specific
> application. Below I've jotted down some thoughts about the problem...
but I haven't tested anything and don't really have the
> apparatus available to construct a proper test.
>
> Maybe someone on the reflector has the skills and laboratory access to
construct some tests for evaluation. I'd be interested to
> hear more on this subject.
>
> -- Eric K3NA
>
> ===============
>
> Random thoughts about audio recording requirements (recognizing that my
technical knowledge is limited):
>
> 1. Dynamic range:
>
> 1.1 We expect our receivers to have much better than 100 dB dynamic
range. For an audio record to replicate what the receiver
> presents during the contest, both the sound board and the compression
technology need to support a similar dynamic range.
>
> 1.2 Most of the time in the contest we don't need the full dynamic
range; i.e., we are listening to a pretty loud signal. But
> occasionally we are pulling out a weak, just-above-the-noise signal from
between some loud adjacent channel contesters. This later
> situation appears to be the most demanding; i.e., can the compressed
recording recover the weak signal when very loud signals are
> all around?
>
> 1.3 I tentatively conclude that the compression algorithm should
provide at least 100 dB of dynamic range. That seems to point
> toward CD-quality if one wants the audio record to faithfully recover what
was heard by the operator.
>
> 1.4 A 16-bit PCM technique provides 64,000 linear quantizing levels.
That is about 35 dB of range. I have no idea what dynamic
> range can be handled by MPEG, etc.
>
> 2. Audio bandwidth:
>
> 2.1 While we talk of a nominal "2.4 kHz" receiver bandwidth on SSB, in
fact we are hearing signals far above this bandwidth.
> That is because the filters are not perfectly cutting off at 2.4 kHz.
Therefore, it would be an error to assume that we could limit
> the sampling rate to something like 5,000 samples/s (observing the Nyquist
limits). An 8 kiloample/second rate has a theoretical
> upper limit of 4 kHz. To avoid aliasing problems on playback (that could
generate new QRM on top of the desired signals), the audio
> being recording would have to pass through a 4 kHz low pass filter first.
Is it fair to assume that such a low pass filter is built
> into the encoders?
>
> 3. Disk space:
>
> 3.1 WriteLog can be configured to send the recorded audio to a
different computer over the Ethernet, if there is not adequate
> space on the logging computer's disk.
>
> 3.2 A 1m12s "CD quality" file on my computer is 1.570 MB in size.
That translates into 21.8 kB/s.... or 1.308 MB/mn... or
> 78.480 MB/hr... or 3.767 GB/48-hour contest for stereo.
>
> 3.3 It's not difficult to have several GB of spare capacity on a disk
to record a contest. After the contest, the files can be
> archived on CD (several, if CD-quality recording was used) or DVD.
DVD-writing drives are starting to become affordable.
>
> 3.4 Disk space would be larger for multi-op stations. W3LPL could use
as much as 12 channels or 23 GB (if both listening posts
> on 160m and 10m were left on 'record' during the entire contest period),
for instance.
>
> 3.5 How much space exists on a DVD? Can a DVD-write drive keep up
with the data rate during the contest? If so, one could
> record straight to the DVD drive.
>
> 3.6 With inexpensive CDs available today, maybe it's just not worth
attempting to compress below "CD quality" audio.
>
> ======== end =======
>
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