LSM: why scanners suck: the real story

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KA1RBI

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This is an additional followup to an earlier posting of mine re: the LSM "simulcast distortion" problem with scanners

There is also a writeup here on RR concerning "simulcast digital distortion" but that is full of misinfo and disinfo. Its biggest problem is that it doesn't correctly describe the problem...
I've now put up a web page about LSM with further technical information including several images. The reasons for not putting the information directly in this thread are two:
1) The RR limits on attachments and sizes, and
2) I've taken the precaution of copyrighting the document.
However the page does not contain ads, there is no paywall, javascript, bugs, or other malware, we hope.

The page does contain a number of images and includes the raw sample data for those who might be interested in reproducing its results. As an exclusive for RR readers, below is a smal program that can be run against this sample data to produce the demodulated data that's shown in the graphs.

This is really the essence of "software radio" !

The page is located at LSM 101

and here is the program

Code:
#! /usr/bin/env python

import sys
import numpy as np

# will be complex baseband at zero IF
if_data = np.zeros(480, dtype=np.complex64)

# read data from stdin and populate if_data
for i in xrange(480):
        re, im = sys.stdin.readline().split()
        if_data[i] = float(re) + 1j*float(im)

# quadrature (fm) demodulate 
fm_demod = np.angle(if_data[1:] * np.conj(if_data[:-1]))

# magnitude squared (am demod)
am_demod = np.abs(if_data[:-1]) ** 2

# print
for val in fm_demod:    # <=== change here to print am_demod instead
        print val
There is much more at the above URL, but here's one sentence I've extracted from the Conclusions:
Thus we see that when we attempt to demodulate CQPSK as if it were C4FM our clock recovery loop is making decisions at the exact opposite of the correct times.
This author would be most grateful to receive feedback, especially concerning any errors, omissions, distortions, etc.

73

Max
 

jcardani

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Orlando, FL &amp; Ocean City, NJ
Hi Max,

I am thinking of getting the USRP1 or equiv. Do you use a receiver with USRP connected to IF output or do you use the RX board directly? Also what versions of GNU radio do I need to be compatible with Op25?

thanks,

Joe
 

KA1RBI

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Joined
Aug 15, 2008
Messages
493
Location
Portage Escarpment
Hi Max,

I am thinking of getting the USRP1 or equiv. Do you use a receiver with USRP connected to IF output or do you use the RX board directly?
Both. My USRP1 contains a WBX as well as an LFRX. I've used both methods and there are many subtle pros and cons either way. When using a tapped IF the USRP LFRX is tuned to 455 KHz.

Also what versions of GNU radio do I need to be compatible with Op25?

thanks,

Joe
The latest OP25 installs GNU Radio for you automatically - so it shouldn't be necessary to even worry about gr versions. Nonetheless, the minimum version of GNU Radio that we require is 3.7.

73

Max
 

W9BU

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Brownsburg, Indiana
I've now put up a web page about LSM with further technical information including several images.
Very interesting and enlightening info.

The reasons for not putting the information directly in this thread are two:
1) The RR limits on attachments and sizes, and
2) I've taken the precaution of copyrighting the document.
As RadioReference Wiki Manager, I invite you to add this analysis to the Wiki. The Wiki does not have the same limits on attachments that the Forums have. WRT your copyright, your intellectual property, once published in the Wiki, becomes RadioReference's IP, so I would respect your decision to not publish this in the Wiki should you make that decision.

If you would like to discuss this further, please PM me.
 

KA1RBI

Member
Joined
Aug 15, 2008
Messages
493
Location
Portage Escarpment
As RadioReference Wiki Manager, I invite you to add this analysis to the Wiki. The Wiki does not have the same limits on attachments that the Forums have. WRT your copyright, your intellectual property, once published in the Wiki, becomes RadioReference's IP, so I would respect your decision to not publish this in the Wiki should you make that decision.

If you would like to discuss this further, please PM me.
Why not place a link to this thread from the aforementioned wiki article ?

73

Max
 
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