[Lf] Re: lf digest, Vol 1 #146 - 1 msg

Frank Gentges fgentges@mindspring.com
Mon, 26 Jun 2000 08:24:26 -0400

Frank Gentges wrote:
> Vittorio et al,
> I am hoping there is a free lunch out there in the compression schemes

Snip snip

> My current thinking is to use a 750 Hz BFO frequency.  Then filter the
> audio data through a 600-900 Hz FIR bandpass filter to cut energy way
> down, (60 db?) below 500 and above 1000 Hz.
> Initially sample this audio at 2000 samples per second to go into the
> digital FIR filter.  This is based on there being no energy above 1000
> Hz to avoid that alias.
> Then we have a 2000 samples per second rate of the output data stream of
> the FIR. We take every other FIR output sample to achieve a 1000 sample
> per second rate and throw the rest on the floor as they have no new
> information.  We would have a mirror image alias on the other side of
> 500 Hz that would just be ignored or filtered out with a high pass
> filter.
> That gets us 16000 bits per second if we transmit 16 bit samples.  The
> FIR filter may be a bit too much computation for a PC but I don't have
> experience with that yet.

Snip Snip.

Sorry to follow up on my previous post but a good nights sleep and some
analog filter analysis....

Instead, use an analog active bandpass filter with a 600-900 passband
and a 0-500 and a 1000-up 60 db stopband.  Put the 300 Hz bandwidth CW
output of the RX320 through this filter and use the 750 Hz BFO
frequency.  Sample the filter output at 1 kHz.  This will probably
require us to use a higher rate that is a multiple of 1 kHz and only
select nth samples to make up 1 kHz.  The analog active filter can be a
6 pole butterworth which is pretty simple.

Given the relative simplicity of this filter, a switched capacitor
filter may not make sense given that the clock will be in the low end of
the LF band.  We may want to try it just to see how bad the problem is.

Enough for now, still looking for comments.


Frank Gentges 
Check out our LF web page at <http://amrad.org/projects/lf>