New subject: How to get IQ samples from multiple rtl-sdr dongles in asynchronized manner?

30 Jan 2014

Hi everybody,

Let me report current progress of this thread.

Now my matlab program can detect and correct sampling and carrier (both
phase and frequency) error for dongles, and show the sampling phase
difference between two dongles (at 8X oversampling rate) after correction.

You can try it by yourself here :
https://github.com/JiaoXianjun/multi-rtl-sdr-calibration

Some runs results have also been attached here. (time location of FCCH SCH
and BCCH have also been shown there in different color)

In the attached pictures, you may find that from the GSM frame point of
view those FCCH, SCH and BCCH from two dongles seem almost in the same time
location. (this needs to be verified further by check/demodulate
information carried in SCH and BCCH, which is still under development). But
if we inspect the time location in a 8X oversampling scale (zoom-in), there
is a fixed sampling phase difference between two dongles (after correction
respectively).

Unfortunately, this fixed sampling phase difference may vary from run to
run (one rum means we restart rtl_tcp).
Fortunately, it remains fixed in a single run. This offers us a opportunity
to track continuous input I&Q (don't re-run rtl_tcp!) from multiple
dongles, and needn't worry about phase/sampling jumping/discontinuous
things.

Why do carrier and sampling error estimation and correction respectively?
Inspired by James Peroulas james(a)peroulas.com (author of Lte-Cell-Scanner),
for example, when there is a non-coherent up/down-converter used outside
dongle (like MMDS LNB used by Omri Iluz omri(a)iluz.net to detect 2.4GHz band
signal), there won't be strict relationship between sampling error (which
is in baseband) and carrier error (which may be located in inside dongle or
outside dongle mixer).

But for my in-hands dongles, sampling error and carrier seems have
relationship. This may be because they have common clock source inside
dongle.

Which I detected by my matlab program gsm_sync_demod.m:
dongle 1: sampling PPM 27.1739,     carrier PPM -28.513
dongle 2: sampling PPM 116.3043,   carrier PPM -118.4247

In my program a positive sampling PPM means sampling clock is slower than
standard/ideal source (in our case, this source is GSM base-station). If LO
of mixer uses the same clock as sampling, that may cause a lower LO in
mixer/down-converter. I guess the LO frequency of mixer in dongle is higher
than input antenna RF frequency, which means for mixer: output = LO -
input. Thus a lower LO generates a lower input into baseband, that's why I
get a negative carrier PPM (negative carrier PPM means detected carrier
frequency is lower than standard/ideal source).
I don't know if my guess is correct. Anyone is familiar with the hardware
architecture of the 820t dongle?

BR

Jiao Xianjun

On Wed, Jan 8, 2014 at 5:23 PM, jdow &lt;jdow(a)earthlink.net&gt; wrote:

...
  The last I looked rtl_tcp did not allow full control
over the dongle. But,
 then, to do what I really want has required bypassing rtlsdr to go
 directly to the usb library so I can access the underlying USB system
 directly for a specific feature. (A feed through feature in rtlsdr's
 shared library would be nice when trying to nail down the dongle's
 identity for restoring context on program startup.)

 (Building an SDR program without multi-threading is rather like trying
 to build a bicycle without chains or cables. It can be done. But it's
 a pathetic tool when you get done with it. Erm, but I do note gear
 driven bikes need less maintenance under adverse conditions. Been there
 done that - MANY decades ago.)

 {^_^}   Joanne/W6MKU

 On 2014/01/07 23:32, Jiao Xianjun wrote:

  Good idea.

 Thanks for your advice!

 But that seems we will do some repeating works.

 rtl_tcp already use mutiple buffers and multi-threading. So why not use it
 directly as a reliable relay.

 On Tue, Jan 7, 2014 at 1:51 AM, jdow &lt;jdow(a)earthlink.net
 <mailto:jdow@earthlink.net>> wrote:

     Have you tried using ping-pong buffers? Process on one buffer and
 receive
     to another. You may have to use multi-threading to make this work
 nicely.

     {^_^}   Joanne/W6MKU

     On 2014/01/06 01:34, Jiao Xianjun wrote:

         Hi,

         Today I did some experiments using CW signal which is generated
 by signal
         generator. The conclusion is a little bit sad.

         sync read and UDP lose samples/data high probably:
         1. If there are some other operations (which costs some time)
 between two
         successive sync reads, some samples will be lost.
         2. Some times, UDP packets are just lost.

         So, seems that I have two choices:
         1. struggle to use async read mode.
         2. use rtl_tcp utility directly, which is offered with rtl-sdr
 code. This
         program use async mode and TCP, which has avoided the two
 shortcomings I
         addressed.

         I will try the 2nd method, and try to move on with calibration.

         BR

         Jiao Xianjun

         On Sat, Jan 4, 2014 at 8:34 PM, Jiao Xianjun &lt;putaoshu(a)gmail.com
         <mailto:putaoshu@gmail.com>
         <mailto:putaoshu@gmail.com <mailto:putaoshu@gmail.com>>> wrote:

              Hi,

              I have questions on usage of rtlsdr_read_sync(dev, buffer,
         out_block_size,
              &n_read):

              For example, if sampling rate is 1Msps, and out_block_size
 is 1000000,
              question is:

              1. the rtlsdr_read_sync() will cost 1s exactly? Or is there
 any
         lower level
              device/driver buffer, which perhaps feed rtlsdr_read_sync()
 with
         history
              data quickly and makes rtlsdr_read_sync() return in a time
 shorter
         than 1s?

              2. in this infinite processing loop:
              while(1)
              {
              rtlsdr_read_sync(dev, buffer, out_block_size, &n_read);
              processing_function(buffer); // let's assume that this cost
 0.001s
              }
              Those samples during the 0.001s processing period will be
 losted or
         not? Is
              there any method to not lost them?

              Thanks very much!

              BR

              Jiao Xianjun

              On Thu, Jan 2, 2014 at 9:25 PM, Jiao Xianjun <
 putaoshu(a)gmail.com
         <mailto:putaoshu@gmail.com>
              <mailto:putaoshu@gmail.com <mailto:putaoshu@gmail.com>>>
 wrote:

                  I see some UDP packet performance issues in my laptop
 (but not
         in my
                  desktop computer). Will the common (interleave multiple
         receives) UDP
                  link helps?

                  The issue is that fread for the second dongle in matlab
 get
         less data
                  than expectation sometimes. Seem that fread for the first
         dongle works well.

         ------------------------------__----------------------------
 --__--------------------

                  From: Sdr Guru <mailto:sdrguru1@gmail.com
         <mailto:sdrguru1@gmail.com>>
                  Sent: 2014/1/2 5:39

                  To: osmocom-sdr(a)lists.osmocom.org
         <mailto:osmocom-sdr@lists.osmocom.org>
         <mailto:osmocom-sdr@lists.__osmocom.org

         <mailto:osmocom-sdr@lists.osmocom.org>>

                  Subject: Re: How to get IQ samples from multiple rtl-sdr
 dongles in
                  asynchronized manner?

                  Hi

                  rtl-sdr-relay
                  Some of the recommendations.
                  Please add PPM error calculation, exactly like new
 rtl_test -p
                  but multiple receivers simultaneously.
                  It provides immediate information if something is wrong
 with
         USB or dongles.
         https://github.com/keenerd/__rtl-sdr/commit/__
 b5f89dcf40463130e717b6c9bb3a39__a3c8b9535f
         <https://github.com/keenerd/rtl-sdr/commit/
 b5f89dcf40463130e717b6c9bb3a39a3c8b9535f>
         https://github.com/keenerd/__rtl-sdr/blob/master/src/rtl___test.c

         <https://github.com/keenerd/rtl-sdr/blob/master/src/rtl_test.c>

                  Please add automatic eeprom PPM calibration
         https://github.com/keenerd/__rtl-sdr/commit/__
 ecf267737ca52f5005b7a12a352307__e8cd763ed6

         <https://github.com/keenerd/rtl-sdr/commit/
 ecf267737ca52f5005b7a12a352307e8cd763ed6>

                  default sample rate 2.4M (28.8/12) or 1.2M (28.8/24),
 probably
         lower jitter
                  MAX_NUM_DEV 4->16 :)

                  Some nice to have features.
                  ip binding
                  multicast support
                  one common (interleaved) stream of all the receivers
                  timestamped stream

                  I'm trying to convert MATLAB script to Ocatve.

                  SG

                  On Mon, Dec 30, 2013 at 9:38 AM, Jiao Xianjun
         &lt;putaoshu(a)gmail.com <mailto:putaoshu@gmail.com>
                  <mailto:putaoshu@gmail.com <mailto:putaoshu@gmail.com>>>
 wrote:

                      Hi guys,

                      For the multiple dongles synchronization in signal
 level
         instead of
                      bits/packets level, I setup a working repo in
 github, and
         write a
                      initial demo framework. See below:

         https://github.com/__JiaoXianjun/multi-rtl-sdr-udp-__relay.git

         <https://github.com/JiaoXianjun/multi-rtl-sdr-udp-relay.git>

                      You may find information and instruction of demo
 quickly by
         reading
                      the README.

                      My initial purpose is performing in-fly calibration
 for
         multiple
                      dongles according to some pre-known signal (GSM,
 ADS-B?) to
         let them
                      work together coherently.

                      An ideal scheme may be that we should generate a very
         narrow band
                      and very week signal in (or just located at the edge
 of) target
                      working band of dongles, and perform the software
 in-fly
         calibration
                      in background (or driver level). This would be user
 friendly.

                      I know it is far from final state currently, and many
         things are not
                      clear yet (See TODO). But please join me if you also
 think
         this is a
                      good idea. Just check out the demo and run it to
 have a look.

                      Currently I just test the demo in Ubuntu-Linux.

                      BR

                      Jiao Xianjun

Re: How to get IQ samples from multiple rtl-sdr dongles in asynchronized manner?