United States Air Force
Military Auxillary Radio System
"Providing the reins of command in emergencies"

MixW Calibtration Procedure

There is one very important step to perform before we begin the calibration procedure, setting the MixW sound card sample rate. This procedure is not included in the MixW instruction but is presented here as explained by Gary, AFA1CE:

Here is a brief explanation of what I did to calibrate the sound card in my laptop.
The so called "standard sample rate" is 11025; this means that hardware manufactures and software driver developers should make the audio cards and drivers compliant. Well guess what it does not always happen.
To verify your PC sample rate there is a program included with MIXW called "CheckSR.exe," it is located in the root MIXW directory. Run the CheckSR.exe file using 11025 as the sample rate. Let it run until the numbers stabilize; it can be from 5 minutes to several hours. This value is the actual sample rate of your sound card. You may have to average out the RX ant TX values.
Place this value in your MIXW sample rate configuration . That is; configure/sound device settings in place of the default value of 11025. Then run your calibration against WWV again.
I will be glad to further explain this procedure with anyone on the air or on the landline.


I. Receive Calibration

1. Tune your radio to 15000 kHz or other clearly audible WWV frequency.

2. Switch MixW to SSTV mode.

3. In the Mode window, select "WWV"

4. At the top of the SSTV window, tap "RX".

5. Soon, a vertical line will begin to appear in the SSTV window. Wait until you get an entire screen full of the vertical line. If your soundcard receive is perfectly calibrated, this line will be perfectly vertical. If not, it will slant left or right.
6. Use the slant keys to make the line vertical. Use the arrow keys to move the line closer to the screen edge. (It is much easier to correct the slant if you compare it to the vertical screen edge).

7. Once you have the line perfectly vertical, click "STOP" at the top of the SSTV window. Then click "MODE" at the top of the MixW window and then click "MODE SETTINGS".

8. If you made slant corrections in the previous steps, you should see a number in the "RX slant correction" window. Click the "GO!" button and the correction will be sent to Soundcard Settings as the permanent RX sample rate correction.

9. If you feel you made an error and wish to start over, click "Configure", then "Soundcard Settings". Change the number in the Clock adjustment, ppm: "RX" window to 0. Then start all over.

II. Transmit Calibration

Note: This test should only be done AFTER you have done a valid WWV slant test and entered the resulting RX clock adjustment in the MixW sound card settings. You should also do on-air testing of received MFSK images to confirm your RX clock adjustment before proceeding.

1. Turn off your radio
2. Under sound card settings, select "full duplex" box and check Sample Rate set to 11025.
3. On sound card Input settings, change from LINE IN to WAVE
4. Simultaneously press CNTL, SHIFT and T. A pop up window should appear.
5. You should see a strong "carrier" in the water fall. Adjust the power slider in the pop up window so that the signal sidebands are minimal and you have a nice red "carrier" signal (this is your sound card output "looping back" to MixW input)
6. Let this test go for a several minutes until the TX/RX OFFSET numbers at the top of the pop up window stabilize within a few PPM. This number is the offset to be applied to your RX clock adjust setting to arrive at the TX clock adjust setting. The offset may a positive or negative number. For example, if the TX/RX Offset is -200 and your WWV-tested RX clock adjustment is +85, your TX clock adjust should be set to 85 - 200 = -135. Or, if the offset is 50, the TX clock adjust would be 85 + 50 = 135
7. Hit ESC to stop the test, and don't forget to reset your Input back to LINE IN. (Leaving Full Duplex checked is OK)
8. Reboot MixW.

Note: It doesn't seem to matter if what numbers you have set in your TX and RX clock adjust when you do this test. The clock adjust in PPM (parts per million) is directly related to the Sample Rate according to the following formula: (1 divided by Sample Rate) x 1,000,000 = PPM per Hz Sample Rate. At the default sample rate of 11025, one Hz of sample rate is equal to 90.7 ppm of clock adjustment. Some people change their sample rate to correct for slant, but PPM allows much finer adjustment.


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Disclaimer. Products discussed on this page are presented for reference only. The author and MARS et al, have no affiliation with these products. As usual the user is soley responsible for their proper installation and use.