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Minor additions to MSK144 phase eq docs

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git-svn-id: svn+ssh://svn.code.sf.net/p/wsjt/wsjt/branches/wsjtx@8117 ab8295b8-cf94-4d9e-aec4-7959e3be5d79
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Bill Somerville 2017-09-23 22:02:24 +00:00
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doc/user_guide/en/measurement_tools.adoc
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@ -76,64 +76,71 @@ Wait for about a minute and then hit the *Stop* button. A file named
=== Phase Response and Equalization === Phase Response and Equalization
*Measure phase response* under the *Tools* menu is for advanced *Measure phase response* under the *Tools* menu is for advanced MSK144
MSK144 users. Phase equalization is used to compensate for group-delay users. Phase equalization is used to compensate for group-delay
variation across the passband of receiver filters. Careful application variation across the passband of receiver filters. Careful application
of this facility can reduce intersymbol interference, resulting in of this facility can reduce intersymbol interference, resulting in
improved decoding sensitivity. improved decoding sensitivity. If you use a software-defined receiver
If you use a software-defined receiver with linear-phase with linear-phase filters there is no need to apply phase
filters there is no need to apply phase equalization. equalization.
After a received frame is decoded *Measure phase response* generates After a received frame is decoded *Measure phase response* generates
an undistorted waveform whose Fourier transform is used as a an undistorted waveform whose Fourier transform is used as a
frequency-dependent phase reference to compare with the phase of the frequency-dependent phase reference to compare with the phase of the
received frame's Fourier coefficients. received frame's Fourier coefficients. Phase differences between the
Phase differences between the reference reference and the received waveform include contributions from the
and the received waveform include contributions from the originating station's transmit filter, the propagation channel, and
originating station's transmit filter, the propagation channel, and filters filters in the receiver. If the received frame originates from a
in the receiver. If the received frame originates from a station known station known to transmit signals having little phase distortion (say,
to transmit signals having little phase distortion (say, a station known a station known to use a properly adjusted
to use a properly adjusted software-defined-transceiver) and if the software-defined-transceiver) and if the received signal is relatively
received signal is relatively free from multipath distortion so that the free from multipath distortion so that the channel phase is close to
channel phase is close to linear, the measured phase differences will be linear, the measured phase differences will be representative of the
representative of the local receiver's phase response. local receiver's phase response.
Complete the following steps to generate a phase equalization curve: Complete the following steps to generate a phase equalization curve:
- Record a number of wav files that contain decodable signals from your chosen - Record a number of wav files that contain decodable signals from
reference station. Best results will be obtained when the SNR of the your chosen reference station. Best results will be obtained when the
reference signals is at least 9 dB. SNR of the reference signals is at least 9 dB.
- Enter the callsign of the reference station in the DX Call box. - Enter the callsign of the reference station in the DX Call box.
- Select *Measure phase response* from the *Tools* menu, and process - Select *Measure phase response* from the *Tools* menu, and process
the wav files. The mode character will change from `&` to `^` while _WSJT-X_ the wav files. The mode character will change from `&` to `^` while
is measuring the phase response and it will change back to `&` after the _WSJT-X_ is measuring the phase response and it will change back to
measurement is completed. The program needs to average a number `&` after the measurement is completed. The program needs to average a
of high-SNR frames to accurately estimate the phase, so it may be necessary number of high-SNR frames to accurately estimate the phase, so it may
to process several wav files. The measurement can be aborted at any time be necessary to process several wav files. The measurement can be
by selecting *Measure phase response* again to toggle the phase measurement aborted at any time by selecting *Measure phase response* again to
off. toggle the phase measurement off.
When the measurement is complete _WSJT-X_ will save the measured phase response
in the *Log directory*, in a file with suffix ".pcoeff". The filename will +
contain the callsign of the reference station and a timestamp.
For example: K0TPP_170923_112027.pcoeff When the measurement is complete _WSJT-X_ will save the measured
phase response in the *Log directory*, in a file with suffix
".pcoeff". The filename will contain the callsign of the reference
station and a timestamp. For example: K0TPP_170923_112027.pcoeff
- Select *Equalization tools ...* under the *Tools* menu and click the - Select *Equalization tools ...* under the *Tools* menu and click the
*Phase ...* button to view the contents of the *Log directory*. Select *Phase ...* button to view the contents of the *Log directory*. Select
the desired pcoeff file. The measured phase values will be plotted as discrete the desired pcoeff file. The measured phase values will be plotted as
circles along with a fitted curve labeled "Proposed". This is the discrete circles along with a fitted curve labeled "Proposed". This is
proposed phase equalization curve. It's a good idea to repeat the the proposed phase equalization curve. It's a good idea to repeat the
phase measurement several times, using different wav files for each phase measurement several times, using different wav files for each
measurement, to ensure that your measurements are measurement, to ensure that your measurements are repeatable.
repeatable.
- Once you are satisfied with a fitted curve, push the *Apply* button - Once you are satisfied with a fitted curve, push the *Apply* button
to save the proposed response. The red curve will be replaced with a to save the proposed response. The red curve will be replaced with a
light green curve labeled "Current" to indicate that the phase equalization light green curve labeled "Current" to indicate that the phase
curve is now being applied to the received data. Another curve labeled equalization curve is now being applied to the received data. Another
"Group Delay" will appear. The "Group Delay" curve shows the group delay curve labeled "Group Delay" will appear. The "Group Delay" curve shows
variation across the passband, in ms. the group delay variation across the passband, in ms. Push the
*Discard* button to remove the captured data, leaving only the applied
phase equalization curve and corresponding group delay curve.
- To revert to no phase equalization, push the *Restore Defaults*
button followed by the *Apply* button.
The three numbers that are printed at the end of each MSK144 decode line The three numbers that are printed at the end of each MSK144 decode line
can be used to assess the improvement provided by equalization. These numbers can be used to assess the improvement provided by equalization. These numbers