30 Jan
2014
30 Jan
'14
3:14 a.m.
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 <jdow(a)earthlink.net> 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 <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 <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
<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
