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Topic: 12KHz issue (cause: byte shifting in I2S) (Read 4043 times)
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DJ0RG
Neuling
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Posts: 19
Noch neu
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Re:[not solved] Receiving strong sigs 12.KHZ
« Reply #1 on: 04. September 2018, 15:27:40 »
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There was a bug in the audio chain, formerly it was like this:
* light low pass filter with only 4 taps @48ksps sample rate
* downsampling-by-4
* heavy Hilbert lowpass filter with +45 and -45 degrees phase shift for I & Q respectively @12ksps sample rate, 199 taps
I changed it to:
* Hilbert lowpass filter with +45 and -45 degrees phase shift for I & Q respectively @48ksps sample rate, 121 taps
* light low pass filter with 4 taps
* downsampling-by-4
The Hilbert lowpass filter with 121 taps prevents the aliasing and suppresses the unwanted signals. Thats DSP standard, so its amazing that it remained unnoticed for so long. Probably my fault :-). |
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| The "twin peaks syndrom" pops up and disappear nearly by chance. Some firmware versions have a very high probility that it appears, some have a very low one. The differences in two firmwares with very high and very low probility has definitely nothing to do with the cause. it is a slightly different timing (and nobody knows where it is ctitical) that makes the difference. I remember the time I fought against this ghost. Many times I was happy because I thought I have fixed it - but a few versions later it was back again. |
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Would it make sense to implement some configuration options for filter (pre-)settings? I think this would give some flexibility to users of different hardware and device usage. There appear to be at least two known working settings now, which could be the first two entries in a list of up to 256 filter setting sets. "If you experience strange behavior with your device, try a different filter setting".
Or is the config rack full and / or unchangeable?
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S53DZ
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Posts: 58
Ich liebe dieses Forum!
 
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Re:[not solved] Receiving strong sigs 12.KHZ
« Reply #3 on: 04. September 2018, 15:43:19 »
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Hi all,
I am attaching the PDF with my recent measurements running the latest FW.
Please, consider these as informative.
I have used the input level as high as not to cause (too much) noticable IMD.
73 Bojan
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F4HTX
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Scotty, energize...
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Re:[not solved] Receiving strong sigs 12.KHZ
« Reply #4 on: 04. September 2018, 16:05:12 »
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Hi Andreas,
This fix removed your very strong birdies which you could remove by resetting codec many times?
Yes, many meaning randomly many.
There is obviously something still lurking, but at least the receiver is usable. I'll be happy for now 
About WM8731 :
Here is what the scope see on the MCLK, SCLK and SDIN pins of my WM8731.
Rise time is quite slow, I didn't had time to check in the datasheet timing diagram if it's an issue or not. Anyway, these are the measurements.
on SDIN : R21 to 50Ω doesn't change anything, removing R6 dramatically increase the rise time.
Vdd on my MCU is 3.24V
73's
François
Edit : Rise Time is 50ns/div !
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DB4PLE
positron
Urgestein
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Posts: 1278
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Re:[not solved] Receiving strong sigs 12.KHZ
« Reply #5 on: 04. September 2018, 16:17:54 »
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Hello
Would it make sense to implement some configuration options for filter (pre-)settings? I think this would give some flexibility to users of different hardware and device usage. There appear to be at least two known working settings now, which could be the first two entries in a list of up to 256 filter setting sets. "If you experience strange behavior with your device, try a different filter setting".
Or is the config rack full and / or unchangeable?
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I have to admit I have no idea what you mean with the above statement.
What we fixed was the wrong order of certain filtering actions. It was (I have to trust Frank here, which I do without problems) that how we did this was simply wrong and nothing more than a bug. So there is here more or less only black or white here. We either do it right, or we do it wrong. And if we do it right, there is not too much need to configure the filters in question for the normal user.
Do not forget: For experimental purpose, anyone can replace the current filters, signal processing etc. by changing on the code and if it is better than before, by asking for a merge of these changes. This is part of the open source deal, not every wish can and must be realized by the core developers.
And finally, there is work in progress to change the whole audio processing to a much more flexible and powerful approach.
73
Danilo
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DJ0RG
Neuling
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Re:[not solved] Receiving strong sigs 12.KHZ
« Reply #6 on: 04. September 2018, 16:30:16 »
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Understood and all good, I thought there would be several valid configs. My mistake!
BTW: Marvelous jobs done here, chapeau!
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DB4PLE
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Urgestein
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Re:[not solved] Receiving strong sigs 12.KHZ
« Reply #8 on: 04. September 2018, 17:20:49 »
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Hi Andreas, Francois,
according to my investigations AFAIR, the twin peaks issue is an internal issue IN the codec, nothing related to the externally visible I2S communication. So investigating the data lines is an interesting experience but most likely a dead end.
And I would suggest to move this discussion to a different thread anyway, since it is not related to the 12khz issue.
And it may confuse others as much I was confused since this thread is very long already.
73
Danilo
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DD4WH
positron
alter Hase
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Ich liebe dieses Forum!
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Re:[not solved] Receiving strong sigs 12.KHZ
« Reply #9 on: 04. September 2018, 17:21:22 »
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@Bojan: thanks a lot for your measurement pdf! That confirms that the alias frequencies are well suppressed now. The unwanted signal at 10124kHz is the mirror that remains at 51 to 54dB below the wanted signal in your case. That specific unwanted signal is due to IQ amplitude/phase imbalance (probably you had -12kHz translate activated?). Did you have the automatic IQ activated? If not, try to adjust the IQ corrections manually, you can reach about 60-70dB rejection of the mirror.
One thing remains to be tested with the new filter setup: opposite sideband suppression
If anybody has an interest in this, you could test the following:
* inject an RF carrier at 10100.000kHz
* tune in at 10101.000kHz with LSB, listen and note the Signal level on the dBm-display
* switch to USB, listen and note the signal level again
* test with different filter bandwidths
* test with different distances to the injected carrier [eg. tune in at 10100.500kHz and switch sideband, or use 10100.250kHz or use 10102.000kHz]
-> the higher the distance of Rx freq to the carrier freq, the better the sideband suppression should be
73 Frank
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-----------------------------------------
Teensy Convolution SDR
https://github.com/DD4WH/Teensy-ConvolutionSDR
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DD4WH
positron
alter Hase
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Posts: 462
Ich liebe dieses Forum!
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Re:[not solved] Receiving strong sigs 12.KHZ
« Reply #10 on: 04. September 2018, 17:31:25 »
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just to add to the comment by Danilo (and to add to the confusion ;-)), I can see that we have four causes of birdies (and only the first one was causing the initial problem):
* aliasing because of insufficient lowpass filtering before the decimation --> software bug, which is fixed now
* twinpeak issue --> hardware cause in the codec
* IQ signal amplitude and phase imbalance --> this can be cured by switching the automatic IQ correction ON or properly adjusting phase and amplitude manually
* Hilbert filters performance at low audio frequencies --> this determines opposite sideband suppression
Not to mention the birdies caused by all the ocillators in/at the MCU.
Because we have at least these four causes, the search for the reasons for birdies is not really easy ;-)
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-----------------------------------------
Teensy Convolution SDR
https://github.com/DD4WH/Teensy-ConvolutionSDR
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F4HTX
schon länger dabei
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Posts: 66
Scotty, energize...
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Re:12KHz issue (cause: byte shifting in I2S)
« Reply #12 on: 06. September 2018, 08:46:11 »
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Hello Andreas and team,
Me again I post in this thread, but it’s ovi40 & uhsdr related. So, I’m not sure, may be it might be moved to uhsdr zone the forum.
Short film is sometime more explicit than long emails : here are two videos with Twin Peaks on my OVI40. The two videos are with 2.9.51 firmware.
https://we.tl/t-B2tEN3u6PK
1. On the first one, I performed multiple codec reset without success. Offline I did hundreds of codec resets, hoping to find one doing something I could hear, no success so far.
2. The second one exhibits something weird I noticed : the codec reset behaviour is different with AGC on or off.
2.1. With AGC off, reseting the codec does something you can hear, so it’s actually working, but not affecting the twin peaks.
2.2. And weirdo : in the beginning of the shot, you will notice the audio change when invoking the menu screen : some RF coupling between MCU and LCD ? I plan to do a ribbon cable to remote the LCD, unfortunately I do not have the right connector in stock.
More vids : if you are interested, I also uploaded the ovi40 usual frequency scan around 10.100MHz with 2.9.50 & 2.9.51 (+12kHz and RX IQ on) :
https://we.tl/t-JXuVCWXKOL
Best Regards,
François
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| « Last Edit: 06. September 2018, 08:46:35 by F4HTX » |
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F4HTX
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Scotty, energize...
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Re:12KHz issue (cause: byte shifting in I2S)
« Reply #14 on: 06. September 2018, 09:31:16 »
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Hello Andreas
François: Is there another fw version where you reliable do *not* have TPS? If we have a firmware version where it occurs reliable and another where it occurs reliable not this would be a hint that it is not only by chance. There are (actually unknown) influences (timing) we cannot access directly (e.g. compiler optimizations).
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Good question : the point is I really do not know how to give a usefull answer to that. This is what I could try :
1. F1F3F5 reset of the ovi40 in 2.9.51
2. check if TPS is there with eg. Xlate -12kHz, RxIQ on, AGC on
3. downgrade and goto 1.
Any other parameter to care for ?
EDIT:
Do you have Auto-IQ on? Same effect is when IQ is wrong or you do have hardware problem in IQ chain.
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Yes RxIQ is on.
73's
François
EDIT : no change with 2.9.52 (not surprising)
EDIT 2 : I just noticed that the twin peaks are not stable, they are moving apart (a few kHz/mn)
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| « Last Edit: 06. September 2018, 10:09:05 by F4HTX » |
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10M1.pdf
