RE: How to get IQ samples from multiple rtl-sdr dongles in asynchronized manner?
Hi, I don't use the method of hardware modification. That can do synchronization promisingly, sure. I know this scheme couple of months ago, and good to know it works pretty well from you. My thought is doing the compensation by software according to a common source over the air instead of over the hardware. Do you think it is doable? And what would be the bottleneck according to your experience? Any possibility to tune the hardware by software after we estimate the synchronization error in frequency and timing? BR Jiao xianjun -----Original Message----- From: "Sdr Guru" <sdrguru1@gmail.com> Sent: 2013/12/30 19:01 To: "Jiao Xianjun" <putaoshu@gmail.com> Cc: "osmocom-sdr@lists.osmocom.org" <osmocom-sdr@lists.osmocom.org> Subject: Re: How to get IQ samples from multiple rtl-sdr dongles in asynchronized manner? Hi Are you using a common clock? http://kaira.sgo.fi/2013/09/16-dual-channel-coherent-digital.html I've modified some of the RTL dongles, played with GNURadio and Octave. The results are promising, sample level correlation (2.4M/10, FM radio signal). On Mon, Dec 30, 2013 at 9:38 AM, Jiao Xianjun <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 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. Cheap (8USD+PP), simple, computer-controlled and legal FM band "marker" http://blog.palosaari.fi/2013/08/naked-hardware-12-usb-audio-transmitter.htm... 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. Testing it and I'll let you know. Currently I just test the demo in Ubuntu-Linux. BR Jiao Xianjun On Mon, Sep 2, 2013 at 8:23 PM, Sylvain AZARIAN <sylvain.azarian@gmail.com> wrote: Thanks. sylvain 2013/9/2 Sdr Guru <sdrguru1@gmail.com> The second way, use MLAT enabled dump1090 https://github.com/antirez/dump1090/pull/23 http://www.satsignal.eu/raspberry-pi/dump1090.html On Sun, Aug 25, 2013 at 4:13 PM, Jiao Xianjun <putaoshu@gmail.com> wrote: Hi, I want to use multiple rtl-sdr dongles to do some multi-antenna experiments. Is it possible to read IQ samples from multiple rtl-sdr dongles in a synchronized manner? I already have a glance at dump1090 codes, which is a project using rtl-sdr to decode aircraft broadcasting ADS-B messages in 1090MHz. Seems that I should use rtlsdr_read_async() instead of rtlsdr_read_sync(), because that if rtlsdr_read_sync() is used, I have to call it multiple times sequentially. That looks not synchronized. But rtlsdr_read_async() function only accept one rtl-sdr device as input parameter, and it will be blocked after it is called. So seems that it also can't be used for my purpose directly. Also welcome any opinion on how to improve rtl-sdr lib/driver to support this feature. Thank you. BR Jiao Xianjun
Hello Jiao,
My thought is doing the compensation by software according to a common source over the air instead of over the hardware.
Do you think it is doable?
Yes. This is most certainly possible. The same thing was done long ago in radio astronomy. Two big telescopes on different continents made recordings aiming at a coomon direction in the sky. By evaluating the recordings they could make interferometry. http://en.wikipedia.org/wiki/Very-long-baseline_interferometry This is a very interesting approach for the future. With several dongles that have some signals in common it will be possible to synchronize in software. One of the common signals could be a GPS diciplined frequency standard that the software could use to provide extreme frequency stability on the processed signals from all the dongles. Once syncronization is arranged one could use the multi-channel antenna for interference suppression and to improve S/N of any desired signal.
And what would be the bottleneck according to your experience? There is a lot of code that has to be written, but I do not think there is any bottleneck to worry about in hardware
Any possibility to tune the hardware by software after we estimate the synchronization error in frequency and timing? The entire passband of the dongles will be coherent. You can tune to any frequency within the passband and also evaluate several frequencies at the same time.
In case one wants to change the center frequency one would loose synchronization and one would have to restart the synchronization procedure. SDR has just begun. Most of the interesting things have not yet been done:-) Regards Leif / SM5BSZ
Good to know that. Actually I also notice that radio astronomy is attracting more and more signla processing researchers to work on it. Some conference, such as ICASSP, envn setup a dedicate session on radio astronomy. There is a radio astronomy called "DOME" are carrying on, which involves computation on massive data and signal processing algorithm. You have notice the problem that re-tuning will casue lossing synchronization. That's also the problem I considered. Because we always want the rtl-sdr work in the band we are interested, but unfortunatelly in that band there maybe no any pre-known reference signal for us to do synchronization. So a possible solution maybe make a beacon, which can generate ultra-narrow-band or ultra-low-power signal in the target band to help us do the on-line calibration. Any better ideas to avoid this dedicate beacon? On Mon, Dec 30, 2013 at 11:09 PM, Leif Asbrink <leif@sm5bsz.com> wrote:
Hello Jiao,
My thought is doing the compensation by software according to a common source over the air instead of over the hardware.
Do you think it is doable?
Yes. This is most certainly possible. The same thing was done long ago in radio astronomy. Two big telescopes on different continents made recordings aiming at a coomon direction in the sky. By evaluating the recordings they could make interferometry.
http://en.wikipedia.org/wiki/Very-long-baseline_interferometry
This is a very interesting approach for the future. With several dongles that have some signals in common it will be possible to synchronize in software.
One of the common signals could be a GPS diciplined frequency standard that the software could use to provide extreme frequency stability on the processed signals from all the dongles.
Once syncronization is arranged one could use the multi-channel antenna for interference suppression and to improve S/N of any desired signal.
And what would be the bottleneck according to your experience? There is a lot of code that has to be written, but I do not think there is any bottleneck to worry about in hardware
Any possibility to tune the hardware by software after we estimate the synchronization error in frequency and timing? The entire passband of the dongles will be coherent. You can tune to any frequency within the passband and also evaluate several frequencies at the same time.
In case one wants to change the center frequency one would loose synchronization and one would have to restart the synchronization procedure.
SDR has just begun. Most of the interesting things have not yet been done:-)
Regards
Leif / SM5BSZ
On Thu, 2 Jan 2014 20:09:56 +0800 Jiao Xianjun <putaoshu@gmail.com> wrote:
Because we always want the rtl-sdr work in the band we are interested, but unfortunatelly in that band there maybe no any pre-known reference signal for us to do synchronization. So a possible solution maybe make a beacon, which can generate ultra-narrow-band or ultra-low-power signal in the target band to help us do the on-line calibration. Any better ideas to avoid this dedicate beacon? To add a beacon is a very good idea. It can be placed near the band edge where S/N is not so good due to aliasing so it would not disturb signals of interest. By having very good frequency accuracy on the beacon one can synchronize all the dongles to get very high stability and accuracy:-)
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