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LF: Cleaning up DCF77 junk around 74.55 kHz

To: <[email protected]>, <[email protected]>
Subject: LF: Cleaning up DCF77 junk around 74.55 kHz
From: "Markus Vester" <[email protected]>
Date: Fri, 6 Dec 2013 00:57:47 +0100
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I think I have found a way to eliminate interference from the DCF77 pseudonoise modulation in 74.55 kHz spectrograms.
As explained before, the problem is that the phase-modulating PRN sequence carries the timecode of DCF77, sending an inverted sequence with every "1" bit. Thus what would otherwise be a comb spectrum of discrete 1 Hz lines is being spread out, contaminating the space between the lines with noise-like patterns.
The idea is now to unspread the lines, based on the known time bits, which are also communicated by prolonged second gaps (0.2 s for a "1" bit). Multiplying the 74.5 kHz stream by the 1, 1, -1... data will fold all the interference energy back to discrete 1 Hz multiples. These can then be notched out. Finally the despeading has to be undone with the same data, restoring the desired spectral features (amateur signals or Loran lines), but without the interference. 
Example results and zipped MathCad and data files are in 
and a comparison of unprocessed and processed spectrograms from the night November 7 / 8:  (spectrogram as received),  (spectrogram with DCF77 sidebands removed).
Unfortunately there was no signal from Bob or Dex during the night of the recording. All you can see are a few Loran lines, eg. on 74556.6076, 74550.5868, 74549.9400 Hz. I want to try this again, but not today as the big antenna which is needed for 74 kHz is currently retracted due to heavy winds.
For anyone interested in the postprocessing, here are the steps in detail:
1. Using SpecLab, VAC and SndInput, I took a narrowband IQ recording, with decimation of 768 down to 15.625 Hz samplerate. There are two audio channels, one centered on the desired band around 74.550 kHz, and the other on 77.5 kHz for reference. In the example, the overnight recording started 13-11-07 16:38, and ended next morning 6:24. The 6 MB .tmp file was preceded by a dummy .bmp header to be able to read it with the old MathCad version 6.0. 
2. In the MathCad script, the second dips were identified and the bits extracted from therir duration. The first 15 bits which carry encrypted weather information are not included in the phase modulation, and have to be substituted by a fixed 111111111100000 sequence. In principle, the time telegrams are completely predictable (except possible leap seconds), so with accurate time information one could do without the 77.5 kHz channel and the bit decoding.
3. The time domain data is split into overlapping 2048 sample chunks, allowing to generate spectrograms at 7.63 mHz resolution. For each chunk, a windowed FFT is generated, leading to the unprocessed spectrogram (sp_2e.png)
4. In time domain, the samples during each second with a preceding "1" bit are multiplied by -1, despreading the interference.
5. These data chunks are also Fourier transformed, producing spectra with discrete 1 Hz peaks.
6. The bins on and near integer Hz frequencies are nulled, notching out the interference.
7. The data is brought back to time domain with an inverse FFT.
8. The polarity inversion of the "1" periods is undone, applying the same procedure as in step 4 again.
9. Finally data is Fourier transformed again, producing the clean spectrogram result (spr_2e.png).
Best 73,
Markus (DF6NM)

Sent: Tuesday, October 01, 2013 7:35 AM
Subject: Re: LF: 74.550kHz Sep 29/30

Here are clearer shots of the DCF77 sidebands from the morning:
- the RX antenna is resonant around 75 kHz, which emphasizes the PRN sidebands below the DCF77 carrier. The fifth and sixth lobe are still visible. In reality, the upper sidebands are slightly stronger. This is probably due to an offset (or a 75 kHz notch) in the transmitter antenna matching, which happens to help us now.
- the Swiss time signal HBG on 75 kHz is no longer on air.
- there is an RTTY signal at 73.6 kHz which could be CFH. 
- the 1 Hz lines are surrounded by a somewhat regular fine structure, consisting of 16.6 mHz spaced sub-lines. This is probably due to parts of the BCD timecode and weather information data which are repeating or similar in consecutive minutes.
Best 73,
Markus (DF6NM)

Sent: Monday, September 30, 2013 11:26 PM
Subject: Re: LF: 74.550kHz Sep 29/30

Hi Bob, LF,
last night my improvised grabber indeed showed weak and slightly fuzzy traces on both your QRG's, and on all other integer Hz frequencies as well.
These are presumably artifacts from DCF77 which is only about 160 km from here. In addition to the well known AM timecode, it also carries  pseudorandom phase modulation, which has been proposed in the 80ies to provide higher resolution timing (albeit orders of magnitude worse than Loran or GPS). The resulting sidebands extend a couple of kHz on either side of the carrier, with pronounced minima around multiples of the chip rate 77500/120 = 645.833 Hz, see (page 358). The same code sequence is repeated every second, so in theory the spectrum would consist of sharp 1 Hz spaced lines. However, additionally the sign of the sequence is alternated with the disseminated timecode bits, producing some widening or "fuzzyness" of the lines.
Attached is a spectrogram which was taken tonight on the resonant antenna. Between statics, you can still see the fourth sideband lobe which is centered near 74.6 kHz. The spectral gaps are on 
 74916.666 Hz,
 74270.833 Hz,
 73625.000 Hz,
with small and sharp central lines, presumably caused by slight inbalances or nonlinearities in the transmitter.
By these criteria, if you have the choice I would recommend to operate somewhere near these gaps, but not exactly in their middle, and also preferably not exactly on integer Hz frequencies ;-)
Best 73,
Markus (DF6NM)

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