Way way out in left field

I am doing something that the limesdr is 100% not designed for. I have… periodic good and bad results. I don’t know that anyone here can help me, but it is worth a shot to ask.

There is an open source project that works with capturing the RAW RF signal from a Laserdisc player, and decoding it in software (software designed laserdisc player). This is not for piracy, but for preservation (google ld-decode & domesday86). The newest laserdisc player is probably 20 years old now.

There were previously 2 methods to capture the RF, one is an old TV card that a custom driver was made for (for SDR, ironically) that allowed raw capture (cxadc). The second, is a custom built device. The TV card has limited MSPS, and is PCI only, so harder and harder to find a good machine that has that slot. The TV card is also very noisy, and only 8bit. Not everyone can build a “domesday duplicater”. People with physical impairments.

Enter LimeSDR-USB. I have used the limesdr-usb to capture the RF signal from the laserdisc player, but the signal oscillates from “very good” to “quite poor” at roughly a 30 second interval. I don’t see this oscillation when using the TV card and everything else is the same in the chain. I tapped an RF test point on the player, installed a BNC connector on the back of the machine, and connect that to LimeSDR-USB LNAL-1. I did HF easy mod #1. I use an inline physical low pass filter at 15Mhz from crystek. The frequency spectrum of the LD RF video is ~1.5Mhz - 13.5Mhz.

Does anyone have any idea, why the signal would be influenced in this matter? What is significant about 30 seconds and the LimeSDR?

example S/N chart (this is a PAL disc, so 25frames/sec)

I have a couple of guesses you can check:

  1. is the DC offset of the card causing problems? Can you try shifting the real LO frequency of the LimeSDR out of the passband and then using a software mixer to bring the signal of interest back to baseband to put the DC bang out of band?
  2. Do you have an AGC problem? Maybe the gain of the LimeSDR is being changed during operation? Do you have it locked?
  3. Do you have a frequency error or drift on the Lime? Can you tune into a known frequency (e.g. an AM station, or WWV, or better still an actual frequency reference) and see if it is drifting?
    That error pattern looks an awful lot like a rectified sine wave - so I am wondering if the signal is walking across the DC bang.

Thanks for your suggestions, I will investigate those and see if i can get something better. They mostly make sense to me, I am quite new to SDR, and am just using pothos flow and gnu radio companion. Will have to do some research.


SDRAngel has a ATV decoder that is reported to support PAL. I have not tried that decoder beyond a screen of static as there are not too many analog stations (only low power). I used SDRAngel with my LimeSDR-A after following the “compilation instructions” link on this page… I don’t know if you can save the output but I would assume you can. SDRAngel is open source. On top of pothosware I had more stable results but less control.

laserdisc RF signal is FM modulated, i believe OTA is amplitude modulated.