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