[Lf] Spectran beta 4 is released]

[Andre' Kesteloot]

Alberto di Bene wrote:

> SPECTRAN beta 4, build 110 has been just released.
>
> Beta 4 is a major enhancement with respect to beta 3.
> Full duplex, real time audio processing has been added,
> with denoising, bandpassing and CW peaking functions.
>
> Here are some of the new features :
>
> - Display of the frequency where the mouse cursor is hovering on.
>
> - Possibility to switch off the visual AGC (not the audio AGC, but
>   that AGC that in beta 3 was always applied when computing the
>   color of the spectrogram). It has been brought to our attention that
>   with strong QRN this AGC had the side effect of 'chopping' otherwise
>   quite readable QRSS signals, making difficult to discriminate
>   between dots and lines.
>
> - More consistent timing when computing the overlap factor, leading
>   to a more constant speed of the spectrogram.
>
> - Semiautomatic procedure for the setting of the Windows mixer.
>   The Windows mixer is the most non-standard feature of Win9x,
>   a real nightmare (thanks Bill...). Lacking tight specifications,
>   every soundcard manufacturer has felt free to implement the mixer
>   functions in the way they pleased them more. Hopefully the procedure
>   we have devised to set the mixer should take care of this.
>
> - Denoiser : the denoiser is of the Widrow-Hoff type, i.e. a FIR filter
>   whose h coefficients are adjusted in real time with the LMS (Least Mean
>   Square) algorithm, making the filter to converge towards a dynamic
>   solution that minimizes the so-called AWGN noise. We have devised a
>   variant of the standard method, where the leaking factor applied to the
>   h coefficients has been made proportional to the mean amplitude of the
>   audio signal.
>
> - Bandpass : from the low-cut and high-cut limits set with the mouse a
>   rectangular windows is defined, its IFFT is computed, then a Hamming
>   window is applied to it, and finally a forward FFT is computed, leading
>   to a 'well-shaped' filtering window that minimizes the Gibbs phenomenon.
>   This window is applied to the FFT of the audio signal, which is then
>   brought again in the time domain with an IFFT, ready for further processing.
>   You can see a simulation done with Matlab of the final result at this
>   URL : http://www.weaksignals.com/images/bandpass.gif
>   The irregularities seen on the stop band are due to the simulation of
>   rounding the coefficients to 5 significant digits.
>
> - CW Peaking : this function is simply an IIR resonator, with two zeros
>   on the real axis at -1 and 1, and two complex conjugate poles at a radius
>   of 0.995. The angle of the poles is computed in real time, when the frequency
>   of the desired peak is set with the mouse. This kind of resonator has been
>   choosen by personal judgement, listening to its effect on CW signals. We
>   feel that its shape is much more adequate for CW than a 'brick-wall' filter,
>   even if the stop-band rejection could be better. But you can always use the
>   CW peak filter and the bandpass filter in cascade, if you so prefer. For those
>   wanting to see the interesting shape of this filter, there is another
>   Matlab simulation here : http://www.weaksignals.com/images/cwpeak.gif
>   A word of warning : the bandpass of this filter is roughly 30 Hz, so if used
>   with very fast CW it could smooth a bit too much the keying component.
>
> These more or less are the major enhancements of beta 4. Only a few final words
> on how to use them. For the mixer setting, Spectran will recognize the first
> execution of beta 4 on a given machine, and will automatically start the setup
> procedure. Just follow the simple instructions on the screen, and you will not
> be bothered again.
> To set the filters, a filter definition panel has been added. If you briefly tap
> on either Shift key, the panel will appear. Another tap on the Shift key, and it
> will disappear. When it is visible, the left mouse button will set the lower limit
> of the bandpass, the right mouse button the upper limit. To set the CW peak frequency,
> use the combination Ctrl-Left Mouse Button.
> To actually switch the filters on and off the audio processing chain, use the red leds
> on the left side of the main Spectran panel. If you want to use them all
> simultaneously, you need at least a 166 MHz Pentium if you limit the sampling
> frequency to 11025 Hz. For 22050 Hz a 266 MHz Pentium should do.
> The 'PassThru' led, when choosen, simply activates the rerouting of the input
> signal, unprocessed, to the output. In this mode the signal does not incur in the
> delay of about one second between input and output, due to the buffering and
> processing. It is intended to make tuning easier (which it isn't with the delay...)
>
> This should be all. Sorry for the long message, but a bit of explanations were in
> order. One last thing : we need your feedback. We 'think' our mixer setting
> procedure should take care of all the possible variations around (it did in our
> few test cases), but there is always a case you did not anticipate... so please
> tell us how it works on your computer with your soundcard. Thanks.
>
> Spectran beta 4 is freely available here : http:/www.weaksignals.com
>
> 73
>
> Alberto  I2PHD
> Vittorio  IK2CZL