rtl-sdr offset tuning and other questions

[Dimitri Stolnikov]

Hi David,

> 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.

>   - What exactly is offset tuning? How is offset tuning different from  
> tuning to an offset?

For E4000 tuners only this will shift thei 0-IF point outside of RTL  
sampling bandwidth which may help if you have a wideband (DECT or such)  
signal you want to sample but don't want it to be distorted by the usual  
LO-leakage peak in the middle. The drawback is that you might get hit by a  
image signal.

> 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?

For narrowband signals you are good to do offset tuning yourself.

> - regarding AGC, what is the difference between AGC and auto gain?

AGC is for RTL chip, auto gain is for the tuner (if implemented).

> RTLSDR_API int rtlsdr_set_tuner_gain_mode(rtlsdr_dev_t *dev, int manual)

Works for E4000 tuners only AFAIR.

Steve: do other tuners provide AGC functions as well?

> 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.

Yes.

> What are the benefits and costs of having either or both on?

It depends on the application. While in presence of strong interference  
the tuner auto gain seems to perform well on E4000 devices, the RTL gain  
 from my understanding only scales the signal to optimally use the dynamic  
range of 8 bits. Within controlled environment (narrowband  
antenna/LNA/filters) i usually let them off entirely and set the gains for  
best SNR manually.

Historically we have implemented them with some time in between so now  
both gains have their own setters.

>
> - 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.

Yes.

> 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.

You should spend as little time as possible inside a callback, this is a  
libusb requirement. You might copy some buffers to be passed to a worker  
thread later on and keep skipping buffers inside the callback until your  
worker thread is ready for the next shot.

> - 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.

Can't tell, never evaluated that myself. But again, this largely depends  
on the tuner interactions involved.

> smidge more documentation.  I'd be happy to submit a comments-only patch  
> if there's interest. :-)

This would be appreciated.


Best regards,
Dimitri