I mean in order to sect an appropriate PC hardware requirements. (sorry if it is an obvious question) .No, that's a very valid question. Generally, the computational load of a filter is basically (n_taps x sampling rate).
So, the computational load for the *same* filter scales linearly with rate. BUT:
The sharper, measured in 1/sampling rate, a filter is, the more taps it needs.
There are approximate formulas for filters where the passband ripple
gets
designed to be equal to the stoppband ripple; I've learned that with
, the
stoppband attenuation and the relative transition with 
(
) the
length N of the filter amounts to: 
That approximation is "pretty good" in my experience.
Now, since N is proportional to the sampling rate, and since the computational load is both proportional to sampling rate and N, you get quadratic dependency on sampling rate for a fixed transition width.
Back to your original question: The USRP doesn't really care about the sampling rate. It can, without a problem produce the 25MS/s (or even 50MS/s with 8bit samples), the only limit here is the bandwidth of gigabit ethernet. Everything that can go wrong happens on your PC. Generally, "D" is the worst form of failure, because it means that even your Operating System didn't have enough time to process all the samples.
So considering requirements for your PC: With your 8000 real taps filter, you probably won't be able to find a computer that can deal with that like it is. Without taking advantage of polyphase technologies, that would be 16000 real multiplications and 16000 additions per sample -- that's a whopping 16000 FMAC/S * 25MS/s = 400GFMAC/s.
GNU Radio has pretty good optimization for filters, so you might get considerable speed up. It's pretty hard hence to say what you need, because it depends so much on what you want to do.
Exactly, I don't need to process the full 25 MHz of bandwidth but the problem is that my different subbands are into this spectrum. For example:Yeah, but how much bandwidth do you **really** need at once?
Now, the question is: multiplex USRP those 125 channel and will be the new rate 200 Ksps?Sorry, I don't really understand your question.
Best regards,
Marcus
On 06/24/2015 07:41 PM, Luis Grajales
wrote:
LuisThank you so much for all your help,Hi Marcus,Thank you! I wrote the answers according to your last email.
<1><I guess the idea was......If that's the case, your PC can't keep up to both the calculations needed for the filters and receiving the data from your USRP.>
you are right. For this reason I checked my CPU behavior by each option (scenario 1, 2, and 3) to do my filter bank selection. However, I had a doubt between USRP and my PC.
I know that there is a direct relationship between BW to be processing and consumed PC resources, but I'm wondering, how will be this relationship between the whole set (USRP + GNU Radio), I mean in order to sect an appropriate PC hardware requirements. (sorry if it is an obvious question) .
<2><I think your polyphase filter bank approach .....I'd say: start gr_filter_design, and design a filter there, playing around with the parameters until things look better.>
I think the same, even though at the end I will need to put many channels to the null sink because this polyphase filter bank has equal channel bandwidths and my subbands have not equal bandwidth but I think that FFT will be a perfect solution after the filtering process.
I know that there is a work related with unequal channel bandwidths (Fred Harris • Elettra Venosa • Xiaofei Chen) but I couldn't find some examples about it or projects related with GNU Radio.
By the way, thank you so much for your comment. I'm gonna check the gr_filter_design tool and playing around the parameters.
<3><Generally, you don't even closely seem to need the full 25MS/s ........>
Exactly, I don't need to process the full 25 MHz of bandwidth but the problem is that my different subbands are into this spectrum. For example:
Subband 1: 2,3 MHz to 2,510 MHz (BW: 210 KHz)
----
Subband 14: 25,600 MHz to 26,100 MHz (BW: 490 Khz)
The lowest bandwidth is 120 KHz and the biggest is 800 Khz. In that case, I was doing some approximation and channels of 200 Khz will be ok, that means, 100MHz/(500) = 200 Khz and I will have 125 channels with the decimation factor 500. Please correct me if I am wrong.
If I can do that, it will be an excellent option as well.
Now, the question is: multiplex USRP those 125 channel and will be the new rate 200 Ksps?
2015-06-24 9:38 GMT+02:00 Marcus Müller <address@hidden>:
Hi Luis,
I guess the idea was to look whether your CPU was really your bottleneck; filter banks are one of the most CPU-hungry signal processing steps one can think of, and hence, your "D"ropped packets could be caused by that. If I understand correctly, with the null sink (and/or the file_sink) you don't get that behaviour?
If that's the case, your PC can't keep up to both the calculations needed for the filters and receiving the data from your USRP.
I think your polyphase filter bank approach has the brightest future (basically because I think polyphase filter banks are cool, and because they are really quite efficient), but your filter parameters are frightening: A transition bandwidth of 10kHz at a sampling rate of 25MHz is 0.04% transition width, which leads to a filter of more than 8000 taps. Relax that transition width!
I'd say: start gr_filter_design, and design a filter there, playing around with the parameters until things look better.
Generally, you don't even closely seem to need the full 25MS/s you get from the USRP in your flow graphs, but you select different subbands in each flow graph (and in your original mail). What is the real span you need from lower edge of your lowest channel to upper edge of your highest channel? You should let the USRP do as much decimation for you as possible. It's its job!
Best regards,
Marcus
On 06/24/2015 08:34 AM, Luis Grajales wrote:
LuisHi Martin,Thanks again,
Thank you for your message.
I was looking into the null sink blocks documentation and I couldn't find any method or function where I can get an intermediate value into this block. Then, I just verified data at the input of this block, which is the confirmation of the streaming at the output of each channel. For this case I used a file sink block.
Please let me know if I need to do something else, or what is your general idea of the streaming verification into a null sink blocks.
2015-06-23 18:20 GMT+02:00 Martin Braun <address@hidden>:
Luis,
can you please confirm/deny that streaming e.g. into a null sink works?
Thanks,
Martin
On 23.06.2015 09:16, Luis Grajales wrote:
> Hi,
>
>
> I'mtrying to filter 14 frequency bands (channels), which are
> dispersingwithinspectrum of 25 MHz (from 2 MHz - 27 MHz to be more
> _______________________________________________> specific) and each band has a different bandwidth. For example: 1th
> channel with 198 KHz (bandwidth), 2nd channel with 360 kHz, 3. channel
> with 650 kHz, and so on.
>
>
>
> Until now, I have tried 3 scenarios on GRC in order to get these 14
> channels and verify that the first step of my project (filtering) is
> working. For testing, I used just 4 of the 14 channels to see its
> functionality and CPU requirement.
>
>
>
> General configuration:
>
>
> USRP N200 ----------- channelizer --- [ch1] -----GUI FFT Sink
>
> ----[ch2 - ch4] ----
> Null Sink
>
>
> USRP setup -> samp_rate= 25 MHz, and Center Freq = 14,2 MHz.
>
>
> Scenario 1: channelizer = 4 band pass filters.
>
> Scenario 2: channelizer = 4 Xlating FIR filters.
>
> Scenario 3: channelizer = Polyphase channelizer, 5 equal channels of 5 MHz.
>
>
>
> For the 3 configurations I got massive “DDDDD..” at the console output.
> Moreover:
>
> Scenario 1: CPU 100% and stop the application.
>
> Scenario 2: CPU between 75 % and 95 %.
>
> Scenario 3: CPU between 53 % and 95 %
>
>
>
> Therefore, I really appreciate if you could give me some tips to avoid
> this massive “DDD” messages or alternatives to optimize the resource
> consumption for the filter bank, due to the results showed that the band
> pass filter (scenario 1) will not be a good option for me, or maybe I
> did something wrong.
>
>
>
> In the attached files you will find the 3 different GRC configurations.
>
>
> Thanks in advance,
>
>
> Luis Grajales
>
>
>
> Discuss-gnuradio mailing list
> address@hidden
> https://lists.gnu.org/mailman/listinfo/discuss-gnuradio
>
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