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Filter Quality vs Panadapter Bandwidth

Alan - KA4B
Alan - KA4B Member ✭✭
edited June 2020 in SmartSDR for Windows
It is my understanding that with the Flex 5000 as you move from a 48 KHz to 192 Khz panadapter bandwidth the filters become less steep. Does the same type of thing happen with the Flex 6500 or does it no longer have that limitation?

Answers

  • Tim - W4TME
    Tim - W4TME Administrator, FlexRadio Employee admin
    edited March 2017
    No it does not because the sampling rate in the FLEX-6000 is fixed.
  • Alan - KA4B
    Alan - KA4B Member ✭✭
    edited July 2018
    How does the quality of the "Brick Wall" filters in the 6000 compare with the 5000 at 48 KHz bandwidth?
  • Tim - W4TME
    Tim - W4TME Administrator, FlexRadio Employee admin
    edited December 2016
    That is only half of the question. You also have to reference the DSP filter size too as they are inter-related. We have done a lot of work on the filters for SmartSDR v1.1; especially the narrower filters for digital and CW to make them more "brick wall". The 100 Hz filter in the FLEX-6000 as of SmartSDR 1.1 has twice the steepness of a 2048/48ksps filter in PowerSDR.
  • Alan - KA4B
    Alan - KA4B Member ✭✭
    edited February 2015
    I would presume then that the 2.4 KHz SSB filter would be at least as good as with the 5000?
  • Bill-W9OL
    Bill-W9OL Member
    edited October 2016
    Wow! As I thought the 5K was sharp.
  • Tim - W4TME
    Tim - W4TME Administrator, FlexRadio Employee admin
    edited December 2016
    They are sharp, but wide SSB filters do not require the same filter skirt steepness of narrow mode filters for modes like CW and PSK-31. There is a trade off between skirt sharpness and latency (signal path delay). The narrower the filter (more taps) the longer it takes for the signal to be processed through the DSP filter (greater latency). In SmartSDR we adjust the filter's taps to optimize the steepness of the filter skirts and latency vs their bandwidth.
  • Charles - K5UA
    Charles - K5UA Member ✭✭
    edited March 2015
    Tim, I figured there was a latency penalty for steep skirts after reading about digital filters design. I'm happy to hear that the skirts are independent of the panadapter bandwidth. In the PSDR manual, Flex published response curves of the narrow filters as a function of sampling rate. Are there response curves of the narrow SSDR CW filters that could be published somewhere on the Flex website? I understand that the Flex engineers are going to do their best to optimize the shape factors of the narrow filters (within the constraints imposed by other factors), but it sure would be helpful for those of us taking these filters into the war zone to know their response curves. Thanks in advance.
  • Tim - W4TME
    Tim - W4TME Administrator, FlexRadio Employee admin
    edited December 2016
    We will probably publish the response curves once we are happy that we have the filters coded properly. There will be a significant change in filter performance between SmartSDR 1.0 and 1.1. Their may be additional filter performance improvements as we work through the future 1.x versions.
  • Ken - NM9P
    Ken - NM9P Member ✭✭
    edited June 2020
    As Tim said, there is a big difference in PSDR between narrow CW filters when the DSP buffer is 2048 as compared to the buffer at 512. But the latency was unacceptably high on my 1500 with the internal keyer. What a joy it has been running the filters on the 6500. They we're laser sharp on 1.0 firmware. Seemed that they Suffered a bit in 1.0.24 but are still better than anything else I have ever had. And it has been reported that they are going to be better than ever in 1.1.... Amazing rig here!
  • Charles - K5UA
    Charles - K5UA Member ✭✭
    edited March 2015
    Great! Thanks for the heads up.
  • Steve-N5AC
    Steve-N5AC Community Manager admin
    edited July 2018
    The filter quality is a function of the time value of samples that are passed through the filter. The filter that consumes 10ms worth of samples is twice as good as the filter that consumes 5ms worth of samples, as a general rule. So in PowerSDR, if you quadruple the sampling rate and leave the buffer size the same, you have cut the time value of the samples in the filter by a factor of four and consequentially, your filter is not as steep. In PowerSDR, you have two knobs to turn, the buffer size and the sampling rate. To see how much time data spends in the filter, just take the buffer size and divide by the sampling rate (this is a slight oversimplification because of the overlap buffer, but it will do for this discussion). So at 192,000Hz and a buffer size of 512, the filter will get 2.67ms of data. This will not be a great filter, but it will be low latency. Contrast this to a 2048 sample buffer at 48kHz and you get 42.7ms. These two knobs that you can turn in PowerSDR are settable per session -- so you can change them, but you end up restarting PowerSDR when you reset them. It's not quite as simple as moving a slider while you are operating to select a new filter. In SmartSDR, the sampling rate is fixed at 24kHz for a slice receiver. But the buffer size is variable starting with v1.1. We dynamically adjust the buffer size (number of filter taps) for the kind of operating. So when you are in sideband and latency is not an issue, the filters are set to "brick wall." However in CW, as you reduce the width of the filter, we also reduce the number of filter taps to reduce latency. This lets us make a 100Hz filter really 100Hz, but also reduce latency for the CW op that needs lower latency and selects a 400+ Hz filter. The steepest or best filters in SmartSDR consume around 85.3ms of samples to calculate the filter. This makes the filter more computationally expensive, but results in a better filter. Finally, all of the filters in SmartSDR today are FIR filters. They are computationally more expensive than their brother, the IIR filter, but they have a couple of key advantages: 1. They are linear phase so as not to distort phase data which is very important for many digital modes 2. There is no feedback (output to input) in a FIR filter so they will not ring. Ringing occurs in IIR filters when they are hit with impulses -- after the input stimulus has gone away they can continue to output samples reflective of the original signal. In PowerSDR, the panadapter spectrum is computed using a Fast Fourier Transform of the data present based on the sampling rate. If the sampling rate is 192ksps, the spectrum displayed will be 192kHz (they are equal for I/Q data). Again, in PowerSDR, you can zoom in but the sampling rate is set at the time that the program is setup. In SmartSDR, we can dynamically look at different sampling rates. We have written the software to dynamically select the best match of sampling rate and then grab only the bins that are of interest based on how you have the panadapter zoomed. You can see the effects of altering the sampling rate, though, because as you zoom out you will see a sudden rise in the noise floor. This is where a sampling rate change has occurred. All of this is done "under the covers" to give you a seamless view of up to 14.4MHz of spectrum per panadapter. The sampling rate of each panadapter is adjustable and independent (they can all be different). This allows you to look at the entire HF spectrum by using panadapters to select different 14MHz swaths of the bands -- one could show 0-14MHz, the next 14-28MHz, etc.
  • Alan - KA4B
    Alan - KA4B Member ✭✭
    edited February 2015
    That is what I was looking for. Thanks Steve, I'm sure others will appreciate it as well.
  • Charles - K5UA
    Charles - K5UA Member ✭✭
    edited April 2015
    Thank you Steve for the best explanation I've heard of the relationships and trade-offs that determine the shape factor of a digital filter. Everytime you weigh-in on a technical issue here in the flex community, we get more than a technical explanation, we also get a peek behind the curtain as to why certain decisions were made about the implementation of the issue. I really appreciate this, as I'm sure a lot of readers do. In spite of the challenges of making a steep skirted narrow CW filter, your explanation gives me confidence that your engineers are on top of this and will eventually deliver the best CW filters in the business.
  • rfoust
    rfoust Member ✭✭
    edited December 2016
    Agreed, love the technical explanations. Even if I don't completely understand, I get the idea and can appreciate the amount of effort that went into the design.
  • Ken - NM9P
    Ken - NM9P Member ✭✭
    edited June 2020
    Thanks, Steve. Good explanation. If it wasn't somewhat over our heads, we would not learn anything! But you do a good job of "bringing it down" just enough to keep our interest. At least mine, anyway!
  • Mike K5UX
    Mike K5UX Member ✭✭
    edited June 2014
    Really looking forward to 1.1 and beyond! Thanks
  • Mike K5UX
    Mike K5UX Member ✭✭
    edited June 2014
    Steve, thanks for the explanation. You have a way of explaining the technical stuff that makes it easy for many of us to understand.

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