RX888 MkI Recorded Frequency Discrepancies

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DR53
Posts: 15
Joined: Sat Oct 17, 2020 11:40 am

RX888 MkI Recorded Frequency Discrepancies

#1

Unread post by DR53 »

Hi Simon,

RX888 support appears to have migrated to the groups.io forum of late, but I'll be a good boy and post this here :)

I have both a MkI and a MkII RX888, and the latest SDRC releases have been superb for them both, but a few days ago I came across a problem with recordings made on the MkI, in that frequencies shown for signals are inceasingly inaccurate the further you go from the recording's centre frequency. Over a 2MHz wide recording, for instance, the error grows from zero at the centre to about 65Hz at 900kHz either side of the centre, the indicated frequencies being high at the lower end of the recording and low at the upper end. The screenshots in the attached zip file were taken during a test in which I sent a pair of signals at 9.1MHz and 10.9MHz from a GPS-locked signal generator and made a short 2MHz wide recording centred on 10MHz, showing the displayed frequencies live and from the recording. The RX888 was using 128Msps (new dll) and 4MHz B/W, but I first noticed the problem last week when I was comparing simultaneous recordings made on the MkI, MkII and Perseus to evaluate their NDB catching powers, and noticed that things were not as they should have been on the MkI recording. Three or four recordings made with the MkI since have all shown the same problem. The recordings were only 350kHz wide, and the errors were only up to 12Hz at the edges, but when you are used to working to 1Hz accuracy they tend to stand out quite easily!

Back in Apr 2019 I'd noticed a similar problem with the Perseus, which you sorted out after I'd made a load of sample rate data logs for you: if you need the same data this time just let me know and I'll start churning them out.
Edit: having looked at the log file I can see that this won't be possible, as the sample rate data isn't available.

As for the new dll you issued, it's working beautifully on my PC (Ryzen9 3900X CPU and GeForce 2700 GPU): last night I had both the MkI and MkII running simultaneously at 128Mbps, 16MHz B/W, and both waterfalls were silky smooth, with the CPU at <4% and the GPU at 63%. Increasing the MkI B/W to 32MHz, its waterfall was choppy for a few seconds and remained slightly stuttery afterwards, but the audio was still perfect. However, the CPU was then nearing 12% and the GPU was at 90%, so I thought better of pushing things any further! Thanks indeed for the work you've put in to get these radios working so well!!

Regards,
Dave

PS - the radio enumeration is also working very well, although I've found that the MkII always needs to be started first for some reason: if I start the MkI first, starting the MkII causes the MkI waterfall to slow down by half, then both radios crash when you try to change anysetting and need to be power cycled to get them recognised by the PC again (not a problem once you are aware of it). Edit: the crashes appear to have gone away in the latest software update, and both radios now start ok in any order.
RX888 MkI Recording Errors.zip
RX888 Screenshots
(2.83 MiB) Downloaded 3 times

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Simon G4ELI
Posts: 2048
Joined: Thu Aug 06, 2020 7:27 am
Location: Mawnan Smith
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Re: RX888 MkI Recorded Frequency Discrepancies

#2

Unread post by Simon G4ELI »

So,

To summarise the above: you believe that with a 2MHz wide recording stations are not on the correct frequency?

I have just made a 128Msps 2MHz recording centered on 10.8 MHz. WWV and RWM are perfectly on frequency.

Any frequency calibration will *not* be reflected in the recording.

Recording / playback is the same for all hardware.
Simon Brown :shock:
www.sdr-radio.com

Do not send me direct e-mail, thank-you!

DR53
Posts: 15
Joined: Sat Oct 17, 2020 11:40 am

Re: RX888 MkI Recorded Frequency Discrepancies

#3

Unread post by DR53 »

Hi Simon,

I've had another play with the MkI, and the problem has nothing to to do with the recordings, but it appears to be exactly the same sample rate discrepancy as discovered with the Perseus here:

https://sdr-radio.groups.io/g/main/topi ... 2C0&jump=1

Pics 1-3 attached show what happens with the MkI when a 10MHz signal is moved around in the waterfall display, ie the waterfall shows the signal apparently at 10MHz all the time, but the IF display shows a considerable error at either side of the display (obviously why I was seeing frequency errors in the recordings). The radio was running this time at 128Msps with 8MHz bandwidth, but the errors appear in all other settings I've tried.

In contrast, pics 4-6 show the same 10MHz signal on the MkII (same settings as for the MkI): this time the IF display correctly shows the frequency as 10MHz exactly, regardless of where the signal appears in the waterfall display, as would be expected. So, is the problem peculiar to my MkI, or is it just a software fix which is needed, as with the Perseus? I guess you'd need to add diagnostics to collect the real sample rate data from a few radios to find out?

(Had to zip the pic files as the forum won't let me attach more than five at a time.)

Regards,
Dave
MkI and MkII comparisons.zip
Screenshots - MkI and MkII.
(1.64 MiB) Not downloaded yet

jdow
Posts: 588
Joined: Mon Aug 10, 2020 8:17 pm

Re: RX888 MkI Recorded Frequency Discrepancies

#4

Unread post by jdow »

Are you VERY VERY sure that no samples are being dropped?
{^_^}

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