unknown
1970-01-01 00:00:00 UTC
Hello Malcolm Robb,
currently I'm not actively working with dump1090 for lack of time (but
I fixed some network issues lately, that I think you already merged),
however what I was "about" to do when I was actively developing it,
was to compensate for the out of phase issue using the preamble levels
in order to estimate the phase shift and apply a correction. Because
the preamble is known, you can check what is the percentage of signal
that is mixed in the next sample, and because the preamble at some
point contains a large "silence" space, you can also estimate the
noise level more or less. All this together should allow you to
improve considerably the reception of out of phase signals.
Soon or later I'll try to do this if nobody does it before me, time permitt=
ing.
Cheers,
Salvatore
On Thu, Sep 11, 2014 at 10:33 AM, Malcolm Robb
r sampling frequency results in a wider bandwidth so would pull in more sig=
nals.
nasty for not a lot of improvement. The cheap and easy solution would be to=
change the crystal to a faster one, if the RTL chip can stand the over clo=
cking. If the data loss is at the PC end, then this is unlikely to help. Ho=
wever, if the data loss is at the dongle end, then overclocking should line=
arly increase all the 2.4Mhz parameters without data loss, again assuming t=
he RTL chip can take the overclocking.
t either 4Mhz, or better still 8Mhz. I understand that theoretically I can =
set the [RTL2832] dongle to sample at 3.2Mhz, but that data loss is virtual=
ly inevitable.
*plus or minus* 1 MHz, or 2 MHz in total.
Salvatore 'antirez' Sanfilippo
open source developer - GoPivotal
http://invece.org
"One would never undertake such a thing if one were not driven on by
some demon whom one can neither resist nor understand."
=E2=80=94 George Orwell
currently I'm not actively working with dump1090 for lack of time (but
I fixed some network issues lately, that I think you already merged),
however what I was "about" to do when I was actively developing it,
was to compensate for the out of phase issue using the preamble levels
in order to estimate the phase shift and apply a correction. Because
the preamble is known, you can check what is the percentage of signal
that is mixed in the next sample, and because the preamble at some
point contains a large "silence" space, you can also estimate the
noise level more or less. All this together should allow you to
improve considerably the reception of out of phase signals.
Soon or later I'll try to do this if nobody does it before me, time permitt=
ing.
Cheers,
Salvatore
On Thu, Sep 11, 2014 at 10:33 AM, Malcolm Robb
It's ADSB/ModeS. I'm partly responsible for dump1090. The 'bit rate' is 2=
Mhz, and the RF center is 1090Mhz.There are several issues with the current sampling at 2Mhz, the main ones=
being....1) The Bandwidth is +/- 1Mhz, but some aircraft are a long way off the ce=
nter 1090 Mhz frequency so are missed because they are out of band. A highe=r sampling frequency results in a wider bandwidth so would pull in more sig=
nals.
2) Nyquist means that you get nasty beating effects as signals drift in a=
nd out of phase with the sampling clock.I had considered sampling at 3Mhz which would help both the above, but th=
e loss of samples would be the deal breaker. At 2.4Mhz, the maths is a bit =nasty for not a lot of improvement. The cheap and easy solution would be to=
change the crystal to a faster one, if the RTL chip can stand the over clo=
cking. If the data loss is at the PC end, then this is unlikely to help. Ho=
wever, if the data loss is at the dongle end, then overclocking should line=
arly increase all the 2.4Mhz parameters without data loss, again assuming t=
he RTL chip can take the overclocking.
Cheers
Malcolm
________________________________________
Sent: 11 September 2014 00:26
To: Malcolm Robb
Subject: Re: Maximum sampling frequency questions.
I'm suffering quite badly from Nyquist issues. Ideally I'd like to sample a=Malcolm
________________________________________
Sent: 11 September 2014 00:26
To: Malcolm Robb
Subject: Re: Maximum sampling frequency questions.
I'm currently sampling a 2Mhz digital data stream at 2MHz, so obviously =
t either 4Mhz, or better still 8Mhz. I understand that theoretically I can =
set the [RTL2832] dongle to sample at 3.2Mhz, but that data loss is virtual=
ly inevitable.
The RTL2832 gives you *complex* (I/Q) samples. This does not exactly chan=
ge the Nyquist limit of 1 MHz (for a 2 MHz sample rate), but it makes it be=*plus or minus* 1 MHz, or 2 MHz in total.
If you have a digital radio signal which occupies 2 MHz of bandwidth, you=
- the signal is strong enough
- you are using the 2.4 MHz sample rate, not 2.0 (filters aren't perfect,=
especially not these ones, so you need some margin above Nyquist)- the signal is strong enough
- you are using the 2.4 MHz sample rate, not 2.0 (filters aren't perfect,=
- there is no DC offset (i.e. not E4000 tuner), or you have corrected it,=
or it doesn't matter for the modulation in questionPerhaps you could tell us more about what signal you're trying to receive=
, and what software you're currently using?(I don't have any answers for the other questions in your message.)
--
Kevin Reid <http://switchb.org/kpreid/>
--=20--
Kevin Reid <http://switchb.org/kpreid/>
Salvatore 'antirez' Sanfilippo
open source developer - GoPivotal
http://invece.org
"One would never undertake such a thing if one were not driven on by
some demon whom one can neither resist nor understand."
=E2=80=94 George Orwell