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Date: Sat, 12 May 2012 13:22:03 +0100
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From: Andy Talbot <andy.g4jnt@gmail.com>
To: rsgb_lf_group@blacksheep.org
Subject: Re: LF: Re: Demonstrating audibility of weak signals - utilty to play with.
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Clipping is where the generated samples of signal plus noise exceed
the 16 bit range of the .WAV file.   All values are calculated as
floating point, scaled to keep close to the maximum.  ie the tone at
0dB amplitude has a peak of 28282 (RMS = 20000)

Noise is calculated based on an RMS at 0dB of 20000 in the wav file
calculated for teh full sampling rate bandwidth, the same as the tone
0dB level, in a truely statistical manner.   So you can see that for
noise levels much above -20dB some spikes will occasionally exceed the
alllowed maximum.   If noise were 0dB at full bandwidth, and output
scale factor were also 0dB it would clip something like 63% of the
time as the RMS is defined as the one-signma probability.  The need to
adjust noise amplitude for defined signal bandwidth also adds to the
confusion

Noise is scaled depending on the requested S/N ratio, and the ratio of
defined signal bandwidth ro sampling rate and added to the tone
waveform.  The resulting sample generated, are converted to integers
by multiplying by the scaling factor (0dB = 1)   Any that exceed 32767
/ -32767 are clipped to those values to prevent overflow, and a count
of the number of occurances made    If just the odd few are present,
the amount of clipping is shown,, but  if the number of clipped
samples exceeds 1% of the total for the file, this is labelled
"Severe"   Users can then adjust the output scaling appropriately

Andy
www.g4jnt.com


On 12 May 2012 11:45, Clemens  Paul <cpaul@gmx.net> wrote:
> Hi Andy,
>
> thanks for the nice little program.
> I've been playing a bit with the utility and wonder under which
> circumstances clipping occurs.
> See attached screenshot.
> With the parameters used in this example there is clipping.
> I guess the level in 1Hz BW, here 33dB is the crucial parameter?
> Woud you mind to explain what output scaling means?
>
> 73
> Clemens
> DL4RAJ
>
>