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

Jiao Xianjun putaoshu at gmail.com
Sat Jan 4 12:34:18 UTC 2014


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 at 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 <sdrguru1 at gmail.com>
> Sent: 2014/1/2 5:39
> To: osmocom-sdr at 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/b5f89dcf40463130e717b6c9bb3a39a3c8b9535f
> 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/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 at 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.
>>
>> 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
>>
>>
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