[Top][All Lists]
[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index]
Re: [Discuss-gnuradio] Re: Mode S and ADS-B
From: |
Martin Dvh |
Subject: |
Re: [Discuss-gnuradio] Re: Mode S and ADS-B |
Date: |
Mon, 16 Apr 2007 20:24:46 +0200 |
User-agent: |
Debian Thunderbird 1.0.2 (X11/20070113) |
Hew How Chee wrote:
> Hi Martin,
>
>
>
>>To have the most fun, you also really want a tuner
>>with a 8 Mhz wide IF at the right IF-freq.
>>
>>For now I get the best results when using an
>>external microtune tuner with a 5.75 Mhz IF.
>>
>
>
> Is the Microtune tuner still available in the market ?
> Or are there any other alternatives (tuner with low IF
> output)
They are very obsolete. I know Matt has been able to get a few, but he is also
running out of them.
You don't need a low-IF output as long as the frequency of the IF is not near a
multiple of the niquist freq (half the sampling freq)
What you DO need is a large signal of the IF output. 1 Volt pp would be ideal.
I have been looking at some Philips alternatives (I don't know the types/names
out of my head.) which look promising.
>
>
>
>>I have been using the code with an external tuner
>>frontend and did some experiments with fm-receiving,
>>which just works fine.
>
>
> Interested to know what is the speed of the CPU that
> you are using and how many percent of the CPU power
> has been used up.
I have a dual-core AMD athlon 64 X2 3800+ (2x2 Ghz)
I think only one CPU is used by gnuradio.
I haven't looked at the processor percentages in detail.
I did have to relax the channelfilter-parameters to get it to run in realtime.
(Ideally the filter should be a 100 kHz low-pass which results in a 200 kHz
wide channel)
When using 27 MSPS sampling rate and you want a 100 kHz lowpass you get an
enourmous amount of filtertaps.
By widening the filter and relaxing the max-ripple you get away with less
filtertaps in exchange for a too wide filter.
This results in less channel selectivity.
This is what you want:
chan_filt_coeffs = optfir.low_pass (1, # gain
cxadc_rate, # sampling rate
80e3, # passband cutoff
115e3, # stopband cutoff
0.1, # passband ripple
60) # stopband
attenuation
This is the best my processor can do:
chan_filt_coeffs = optfir.low_pass (1, # gain
cxadc_rate, # sampling rate
80e3, # passband cutoff
1.5*115e3, # stopband cutoff
10*0.1, # passband ripple
60) # stopband
attenuation
What also works for me is:
chan_filt_coeffs = gr.firdes.low_pass ( 1,
cxadc_rate,
125.0e3, #cutoff (you would want 100.0e3)
50.0e3 #transition wide, you would want 20.0e3 to 40.0e3 here
)
I know that a few years back when I was experimenting with a bt878 as capture
source (also 27 MSPS) I also ran into this.
Then I had a AMD duron 800 Mhz
For that processor I had to widen the filter to 1.6 Mhz to get it to run in
realtime.
This is when I started my attemt to build a GPGPU decimating FIR-filter to run
the filter on my videocard.
I never submitted the code because the results were a bit disappointing.
This is mainly because FIR-filters on a GPU are very much bandwidth limited and
my videocard only has a memory bandwidth of 6 Mbyte/sec.
You also have the AGP bottleneck of getting data onto the videocard.
With current videocards this could be a viable option again because current
videocards have much higher memory bandwith and PCI-express is much
better at transporting data to and from the videocard (readback is much better).
So I think a TV-capture card with a GPGPU decimating freq-translating
fir-filter and for the rest the normal gnuradio distribution would run in
realtime on a lot of machines if they have a fast videocard.
Greetings,
Martin
> Trying to gauge whether with direct
> ADC sampling, are there still enough CPU power to run
> FM-stereo and RDS decoder in real time.
>
> Regards,
> How Chee
>
>
>
> __________________________________________________
> Do You Yahoo!?
> Tired of spam? Yahoo! Mail has the best spam protection around
> http://mail.yahoo.com
>
- [Discuss-gnuradio] Re: Mode S and ADS-B, Eric A. Cottrell, 2007/04/13
- Re: [Discuss-gnuradio] Re: Mode S and ADS-B, Martin Dvh, 2007/04/14
- Re: [Discuss-gnuradio] Re: Mode S and ADS-B, Cory Papenfuss, 2007/04/15
- Re: [Discuss-gnuradio] Re: Mode S and ADS-B, Martin Dvh, 2007/04/15
- Re: [Discuss-gnuradio] Re: Mode S and ADS-B, Eric A. Cottrell, 2007/04/15
- Re: [Discuss-gnuradio] Re: Mode S and ADS-B, Martin Dvh, 2007/04/15
- Re: [Discuss-gnuradio] Re: Mode S and ADS-B, Eric A. Cottrell, 2007/04/29
- Re: [Discuss-gnuradio] Re: Mode S and ADS-B, Martin Dvh, 2007/04/29
- Re: [Discuss-gnuradio] Re: Mode S and ADS-B,
Martin Dvh <=