osmocom-sdr

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906 discussions

rtl-sdr offset tuning and other questions
by David Jacobowitz 19 Sep '13

19 Sep '13

Hi guys, I'm new to this list (and to radio) so I hope you will please indulge me if I ask something that is a FAQ. Also, some of these questions are about the dongle, not the library. I am working on a personal project to use SDR techniques to decode aviation navigation signals (VOR). I've got the signal processing mostly working from recorded signals, but am now trying to integrate my SW with the radio in real time. I have a few questions: - What exactly is offset tuning? How is offset tuning different from tuning to an offset? Is this a feature that mostly benefits people who are not going to put their IF through another mixer? In my application I am already tuning to an offset, and pulling down a wide enough IF that actually holds many channels of interest. (VOR channels have 50kHz spacing). I then use a software mixer/channelizer to choose the channel I want. Am I correct in assuming that offset tuning is of no use to me? - regarding AGC, what is the difference between AGC and auto gain? That is, the library API has RTLSDR_API int rtlsdr_set_tuner_gain_mode(rtlsdr_dev_t *dev, int manual) RTLSDR_API int rtlsdr_set_agc_mode(rtlsdr_dev_t *dev, int on); I'm guessing that these affect different AGCs. One for the tuner and one for the RTL device. What are the benefits and costs of having either or both on? - regarding rtlsdr_read_async(...) and related functions. I take it that the library is setting up a ring buffer and calling me back when it has a new buffer of data for me. How long to I have to work with this buffer? Obviously, if I want to work in real-time I need to keep up with the sample rate. But my application can afford to throw away buffers since it can decode a few ms of data from one station and then revisit it much later. However, I'd like to know how long I have until the buffer gets clobbered. I'm presuming it's stable until all the n-1 other buffers have been hit. - generally how fast can the RTL devices tune? I know, this is not an rtlsdr question per se, but I'm curious. I noticed that when I tune, I get a delay. This is a great library and I'm so glad it's out there! I was not looking forward to plumbing the depths of USB drivers to understand how to pull data from the RTL dongle! I think rtl-sdr.h could use perhaps a smidge more documentation. I'd be happy to submit a comments-only patch if there's interest. :-) Regards, Dave Jacobowitz

4 5

[PATCH] rtl_sdr: Direct sampling mode + RTL AGC
by psb hlw 15 Sep '13

15 Sep '13

Hi, I didn't find rtl_sdr with implemented direct sampling mode, so I made the patch. Also it has an option to set RTL AGC on. This feature allows to use tuners as cheap data loggers without changing the schematic of the tuners. You can just connect wires directly to pins 1-2 of RTL2832 (I channel, diff. input) or pins 3-4 (Q channel, diff. input). When direct sampling mode is on - tuner's chip outputs are disabled and doesn't affect external signal. For example, this command will write 8 bit samples to file 'mydata' at 2.4 MHz sampling rate with automatic gain: rtl_sdr.exe -f 0 -s 2400000 -i -G mydata Hope this will be useful for somebody as it was for me. If you will accept the patch - please, update the windows binaries. Thanks, psb. ------------------------------------------------------------------------------- diff --git a/src/rtl_sdr.c b/src/rtl_sdr.c index eeb6dba..3564329 100644 --- a/src/rtl_sdr.c +++ b/src/rtl_sdr.c @@ -42,6 +42,7 @@ static int do_exit = 0; static uint32_t bytes_to_read = 0; static rtlsdr_dev_t *dev = NULL; +static int samp_mode = 0; void usage(void) { @@ -54,6 +55,9 @@ void usage(void) "\t[-b output_block_size (default: 16 * 16384)]\n" "\t[-n number of samples to read (default: 0, infinite)]\n" "\t[-S force sync output (default: async)]\n" + "\t[-i set direct sampling mode (I)]\n" + "\t[-q set direct sampling mode (Q)]\n" + "\t[-G use RTL automatic gain]\n" "\tfilename (a '-' dumps samples to stdout)\n\n"); exit(1); } @@ -81,6 +85,8 @@ static void sighandler(int signum) static void rtlsdr_callback(unsigned char *buf, uint32_t len, void *ctx) { + uint32_t i, r; + unsigned char *p1, *p2; if (ctx) { if (do_exit) return; @@ -91,7 +97,16 @@ static void rtlsdr_callback(unsigned char *buf, uint32_t len, void *ctx) rtlsdr_cancel_async(dev); } - if (fwrite(buf, 1, len, (FILE*)ctx) != len) { + if (samp_mode) { + // For direct sampling mode we throw out each 2nd value, + // i.e., save only one channel data - I or Q. + for (i=0, p1=buf, p2=buf; i<len; i+=2, p2++) + *p1++ = *p2++; + r = (fwrite(buf, 1, len/2, (FILE*)ctx) == len/2); + } else + r = (fwrite(buf, 1, len, (FILE*)ctx) == len); + + if (!r) { fprintf(stderr, "Short write, samples lost, exiting!\n"); rtlsdr_cancel_async(dev); } @@ -119,8 +134,10 @@ int main(int argc, char **argv) uint32_t out_block_size = DEFAULT_BUF_LENGTH; int device_count; char vendor[256], product[256], serial[256]; + uint32_t use_rtlagc = 0; + unsigned char *p1, *p2; - while ((opt = getopt(argc, argv, "d:f:g:s:b:n:S::")) != -1) { + while ((opt = getopt(argc, argv, "d:f:g:s:b:n:S::Gqi")) != -1) { switch (opt) { case 'd': dev_index = atoi(optarg); @@ -143,6 +160,15 @@ int main(int argc, char **argv) case 'S': sync_mode = 1; break; + case 'q': + samp_mode = 2; + break; + case 'i': + samp_mode = 1; + break; + case 'G': + use_rtlagc = 1; + break; default: usage(); break; @@ -205,6 +231,15 @@ int main(int argc, char **argv) if (r < 0) fprintf(stderr, "WARNING: Failed to set sample rate.\n"); + /* Set direct sampling */ + if (samp_mode) { + r = rtlsdr_set_direct_sampling(dev, samp_mode); + if (r < 0) + fprintf(stderr, "WARNING: Failed to set direct sampling mode.\n"); + else + fprintf(stderr, "Tuner set to direct sampling mode (%c).\n", (samp_mode==1)?'I':'Q'); + } + /* Set the frequency */ r = rtlsdr_set_center_freq(dev, frequency); if (r < 0) @@ -231,6 +266,21 @@ int main(int argc, char **argv) fprintf(stderr, "Tuner gain set to %f dB.\n", gain/10.0); } + /* Set RTL automatic gain */ + if (1 == use_rtlagc) { + /* Enable automatic RTL gain */ + r = rtlsdr_set_agc_mode(dev, 1); + if (r < 0) + fprintf(stderr, "WARNING: Failed to enable RTL automatic gain.\n"); + else + fprintf(stderr, "RTL automatic gain enabled.\n"); + } else { + /* Disable automatic RTL gain */ + r = rtlsdr_set_agc_mode(dev, 0); + if (r < 0) + fprintf(stderr, "WARNING: Failed to disable RTL automatic gain.\n"); + } + if(strcmp(filename, "-") == 0) { /* Write samples to stdout */ file = stdout; #ifdef _WIN32 @@ -263,7 +313,16 @@ int main(int argc, char **argv) do_exit = 1; } - if (fwrite(buffer, 1, n_read, file) != (size_t)n_read) { + if (samp_mode) { + // For direct sampling mode we throw out each 2nd value, + // i.e., save only one channel data - I or Q. + for (i=0, p1=buffer, p2=buffer; i<n_read; i+=2, p2++) + *p1++ = *p2++; + r = (fwrite(buffer, 1, n_read/2, file)== n_read/2); + } else + r = (fwrite(buffer, 1, n_read, file) == n_read); + + if (!r) { fprintf(stderr, "Short write, samples lost, exiting!\n"); break; }

1 0

rtl-sdr on Nokia n900 phones
by Fritz Wuehler 13 Sep '13

13 Sep '13

I heard someone recently made rtl-sdr work on an n900. Is it just working drivers, or is it actually usable? I ask, because the processor may not be up to the job.

3 3

rtl_fm, rtl_power, mingw, r820t
by keenerd 13 Sep '13

13 Sep '13

Hey all. I've been shouting it all over /r/rtlsdr and ##rtlsdr, but just for you fine folks that are only on the ML, here[1] is what I've been up to the past week: Overhauled scanning in rtl_fm. This fixed a number of issues, where either scanning did not work at all and some brokenness on OpenBSD. It also meant giving up on the async API entirely. Released a whole new application, rtl_power. This is something I'd had bouncing around on my hard drive for many months, half-unwritten due to over-complexity. It'll generate CSV files of arbitrarily slow/wide/detailed FFTs. Also a little visualization script[2] that won't choke when you give it 50,000 columns of data. In the process of testing rtl_power, two more things came up. First was windows builds. I've got a half-finished mingw32 build script[3] that'll let me produce windows binaries[4] on a linux host. These have been confirmed to actually run and probably work. Only W32 for now, but just because I was too lazy to build the W64 toolchain. Second was the discovery[5] that scanning had gotten a whole lot slower since my original experiments with rtl_fm. 85mS to retune is unacceptably slow. Since the r820t driver is the slowest, I started looking into figuring out what is going on. Still figuring that out but I have done basic cleanups to the code (removing redundancy, particularly around register writes) and the driver is 15% shorter with no modifications to code behavior. I would not mind talking to Steve or one of the other devs on IRC about some of the more ambiguous aspects of the driver. (Like, why some reg writes are backed up to R828_Arry[] and others aren't. And is that array thread safe at all? If not I'll have to do some very large changes to support multiple dongles in rtl_power.) I would really like to get these changes merged, particularly the rtl_fm stuff because the version in the official repo is rather bit-rotten. -Kyle http://kmkeen.com [1] https://github.com/keenerd/rtl-sdr [2] http://www.reddit.com/r/RTLSDR/comments/1ktzwi/ [3] http://kmkeen.com/tmp/mingw32.sh.txt [4] http://kmkeen.com/tmp/rtl-sdr-mingw32-2013-08-28.zip [5] http://www.reddit.com/r/RTLSDR/comments/1l2vlx/

3 2

[PATCH] rtl_adsb: Fix invalid memory access
by Will Glynn 13 Sep '13

13 Sep '13

single_manchester() considers both i and i+1, but the loop only tests that i is in bounds. This causes undefined behavior, including but not limited to a SIGBUS-related crash on Mac OS X. --- src/rtl_adsb.c | 3 ++- 1 file changed, 2 insertions(+), 1 deletion(-) diff --git a/src/rtl_adsb.c b/src/rtl_adsb.c index 44b62e2..3c353a0 100644 --- a/src/rtl_adsb.c +++ b/src/rtl_adsb.c @@ -258,6 +258,7 @@ void manchester(uint16_t *buf, int len) uint16_t a=0, b=0; uint16_t bit; int i, i2, start, errors; + int maximum_i = len - 1; // len-1 since we look at i and i+1 // todo, allow wrap across buffers i = 0; while (i < len) { @@ -275,7 +276,7 @@ void manchester(uint16_t *buf, int len) i2 = start = i; errors = 0; /* mark bits until encoding breaks */ - for ( ; i < len; i+=2, i2++) { + for ( ; i < maximum_i; i+=2, i2++) { bit = single_manchester(a, b, buf[i], buf[i+1]); a = buf[i]; b = buf[i+1]; -- 1.8.3.4

2 2

R820T vs. E4000
by Benedikt Heinz 09 Sep '13

09 Sep '13

Hi everyone, although there are some comparisons between the R820T and the E4000 already [1, 2], I also did some tests with another use-case in mind. I'm working on a thing similar to RTLSDR-Scanner [3]. I want to monitor a large part of the spectrum continuously. So I compared the R820T with the E4000 using RTLSDR-Scanner w/ and w/o an antenna. My results are here: https://docs.google.com/folder/d/0ByDAKwyEiyx_XzZ5ZnpRV1VZWDQ/edit?usp=shar… There's much more spurs with the E4000 than w/ the R820T. According to [1, 2] one also would expect a better overall sensivity compared to the E4000. However, the GSM900 signals for example seem to be way better with the E4000 according to the RTLSDR-Scanner. Tuning to a certain channel w/ OsmoSDR Source in GNUradio gives about the same SNR - contrary to the RTLSDR-Scanner output. Can anyone explain this? Also, the DVB-T channel at 502MHz is quite weak in the R820T RTLSDR-Scanner output when compared to the E4000. I had a closer look at the lower limit of the channel in gnuradio. This can be seen in the 502MHz_*.png pictures. The E4000 produces a nice +20dB step while one can hardly see the channel in the R820T spectrum. I don't understand this as well. Is this AGC-related? Manually setting a fixed gain didn't really help though... Any explanations? Thank you! Best regards, Hunz [1] http://steve-m.de/projects/rtl-sdr/tuner_comparison/ [2] http://www.hamradioscience.com/rtl2832u-r820t-vs-rtl2832u-e4000/#more-1852 [3] https://github.com/EarToEarOak/RTLSDR-Scanner

5 5

rtl-sdr on Nokia n900 phones
by Anonymous 08 Sep '13

08 Sep '13

I heard someone recently made rtl-sdr work on an n900. Is it just working drivers, or is it actually usable? I ask, because the processor may not be up to the job.

1 0

Reset after hot insertion of USB tuner
by Richard Ranson 02 Sep '13

02 Sep '13

I have had great help before from this list, so many thanks for that. I am using an ezcap tuner with a raspberry pi running a Debian based linux OS called Raspbian. I have built the drivers from the git hub and they work fine. As an aside, I found the installation much easier than when I tried it directly on my PC. My question is that I find that if I plug the USB device in when linux is running, it causes a re-boot of the whole OS. (btw, disconnection has no effect). Is this the case with full blown linux systems, or just some peculiarity of the more limited raspberry pi? Also, it is a bit of a nuisance, so does anyone know of a way to stop this? many thanks Richard

2 1

Calibration / PPM
by John 30 Aug '13

30 Aug '13

I am trying to get my head around calibration of my RTL dongle. I used gqrx with a known frequency (467.6375Mhz). Based on this, the dongle needs offset of -16Khz. In gqrx, this offset can be entered directly in the user interface. With rtl_test -p, the PPM number varies. I have gotten positive and negative numbers. When using the RTL block in gnuradio companion, I need to enter PPM. I have seen various formulas to calculate PPM from frequencies but for me they do not yield sane results. I could be just misusing these formulas. Any assistance would be appreciated. Thanks, John

1 1

Re: About 'internal AGC'
by Leif Asbrink 28 Aug '13

28 Aug '13

Hi Charlie, The controls for AGC do not affect the peculiar wideband LNA AGC in the R820T chip. The RTL1090 uses the same rtlsdr.dll as is used by SDR#. I do not know how to evaluate noise levels and signal levels in RTL1090, but with SDR# one can see funny things directly on the waterfall. The LNA is followed by a VERY sensitive power detector that is somehow followed by filters and amplifiers. It is arranged in a way to not react on narrowband signals, but already a 3 dB increase of the noise floor causes a loss of gain through AGC action. Most striking is this experiment: Connect a combiner to the input of the dongle and use it to combine a signal generator and a noise source. The noise source needs a filter that assures that it does not add any noise on the frequency of the desired signal. It is OK to use a T-connector if you do not have a wideband combiner. I used a 100 MHz low pass filter connected to a vacuum diode noise source cabable of delivering 17 dB excess noise combined with a signal generator on 144 MHz by use of a T connector. I used SDR# to look at the spectrum around 144 MHz. Without "RTL AGC" and without "Tuner AGC" the noise floor does not change when the noise source is switched on or off. That is expected because the noise source can not send any noise through the 100 MHz low pass filter. That is true at modest gain settings, but if the gain is set at maximum (49.6 dB) the noise floor increases by 3 dB when the noise source is turned on. A small but unexpected effect. The signal however is attenuated by 23 dB for a total loss at max gain of S/N of 26 dB!!!!! Please note that the true S/N is not affected at all. There is no noise added at 144 MHz. If I switch on "RTL AGC" or "Tuner AGC" or both, S/N still changes the same way. Depending on the signal level of the 144 MHz signal one can see the signal go down or the noise go up. Or both. The way sensitivity is lost due to out-of-band noise is invisible to the user. There is no warning about overload. The noise power from 0 to 100 MHz is -174+17+80 = -77 dBm ( -174 dBm/Hz = room temp) ( +17 dB is excess noise) ( +80dB is 100/MHz/1Hz) There is some filter loss and the dongle presumably has a high pass filter so one can assume that the noise power is -80 dBm RMS. I have tried to activate the LNA AGC by use of narrowband signals in the 50 to 100 MHz range. Even two signals at -30 dBm each do not have any effect regardless of the frequency spacing. It seems the "intelligent" power detector of the LNA AGC can reject narrowband signals even if they are much stronger than the noise floor. The wideband LNA AGC in the R820T may cause problems when an up-converter is used in front of the dongle. The noise floor of the up-converted HF spectrum may cause unexpected loss of sensitivity in the upper part of the HF spectrum where the noise floor is low. Adding a filter for the desired HF band. With some gain to ensure that the noise floor is higher in the desired frequency range than elsewhere could perhaps make the R820T dongles behave much better. 73 Leif / SM5BSZ > I use a R820T chipped dongle for receiving aircraft transponder signals > through a specialised application called RTL1090 from jetvision, the control > panel of which allows the tuner and device AGCs to be independently toggled > on or off. I have not however used this with SDR-Radio in the past so cannot > comment on the optimum settings. > > > > 73 > > Charlie > > www.G4EST.me.uk > > > > From: sdr-radio-com(a)yahoogroups.com [mailto:sdr-radio-com@yahoogroups.com] > On Behalf Of Leif Asbrink > Sent: 10 August 2013 22:30 > To: sdr-radio-com(a)yahoogroups.com > Subject: [sdr-radio-com] Re: About 'internal AGC' > > > > > > Hello Patrick and All, > > The R820T chip has an advanced AGC function that I do not > think can be disabled. The chip detects the noise floor > in a wide bandwidth and adjusts the gain to keep the noise floor > constant. > > The wideband AGC has surprising effects. If one tries to measure > the noise figure with a noise source that is manually switched > on and off one finds a really bad NF. That result is false however, > if one measures S/N of a weak signal one finds the true NF which > is quite good. > > To verify the finding one can inject a weak signal together > with the signal from a noise source. What happens when the > noise is turned on is that the signal becomes weaker while > the noise floor does not change. > > A 500 kHz wide filter in front of the R820T chip converts the > noise from the noise source to a narrowband signal which will > not affect the wideband AGC. > > I made some effort to switch this feature off but failed. > > The behaviour is probably quite clever for reception of digital TV > but I find it very disturbing in a general purpose SDR. I did not > take notes and I did not investigate in detail what types of signals > will affect the AGC and what types will not. That would be > a big investigation and I see no reason to do it because there > are other chips. > > The R820T gives good signals many times but I do not like the feeling > of not knowing what I am doing..... > > The "internal AGC" option is another thing as far as I understand. > The chip has RF AGC as well as IF AGC. > > 73 > > Leif / SM5BSZ > > > Hello Group, > > > > When using a DVB-T type dongle (mine is a R820T), do you tick the > "internal AGC" option or not ? > > > > I tried both "internal AGC" desactivated (with more gain) and AGC > activated (less gain to avoid spurs from my local FM TX) ... I can't tell > which one is better. Even on weakish sigs it's about the same. > > > > What about your experience ? > > > > Regards, > > Patrick > > > > >

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