How do the new New Digital Mode Low Latency Filters work ?

Juan TG9AJR

I just updated my 6600 to v3.4.21 all seem to work OK.

But notice with the new the New Digital Mode Low Latency Filters that on FT8 (DIGU 3.0K) the waterfall on the JTDX apparently is black on the edges (starts around 200 and ends around 2800) and the rest now seem brighter than before (did NOT changed any settings on JTDX).

On the Radio Setup Settings I noticed the new Filters tab and noticed at the bottom the checked marked Use Low Latency Filters for Digital Modes. If I unchecked it the JTDX waterfall display back as it was before the update.

So I will like to know how this new filters work ?

Mike-VA3MW

We had a talk about this in a support meeting this morning.

The low latency digital mode option was developed and added for ARQ modes (Automatic Repeat Query) which require a very fast turnaround from RX to TX.

The roll-off at the edges is to be expected with the softer but faster filters. With these filters active, the latency is about 34ms. With the traditional filters in Auto mode, it is about 120-130ms.

If you are an FT8 or FT4 user, you want to option to decode around the edges at times, so NOT using the low latency filters is the selection you want to use. In DIGU (or DIGL) our audio filters are FLAT which is a big advantage over the traditional mechanical filters your find in SuperHet radios.

FT8 has almost a 5 second receiving window (+/- 2.5 seconds) and FT4 has a 2 second (+/- 1 second). As you can see, a 100ms difference will have little impact.

We did raise your findings to engineering though. :)

I hope that helps explain a few things.

73

Mike-VA3MW

I opened a ticket for you. 73

Juan TG9AJR

Thanks Mike, 73

Neil Rogers

I've also reproduced on flex 6600 the 4.3.1 bandwidth clipping 200<->2800Hz with the low latency filter enabled.

Unclicked the bandwidth returns to 0<->3kHz

Neil G0GGG

Juan TG9AJR

Hi Neil, yes the same here.

Let's see what we learn from this new Low Latency Filter.

Cheers de Juan TG9AJR

Neil Rogers

Further information the clipping does not seem to be present with the latest release of wsjtx.

But is present when using JTDX or MSHV.

This 6600 is used only on 6m there's currently no signals to test further at the moment.

Neil G0GGG

Juan TG9AJR

I also operate a lot of 6m FT8 and 99.999% is basically only with JTDX, and no plans to replace it because...

Juan TG9AJR

KD0RC

Thanks for the explanation Mike.

Juan TG9AJR

Thank you Mike for the explanation. So I will opt to turn off this filter option for FT8 (most of my operating case). Maybe some day in the future this filter can be flatten completely and take advantage for FT8/FT4 as well, but in the meantime it is working great, thanks.

73 de Juan TG9AJR

Mike-VA3MW

A bit more. I heard from Anna, who worked on this.

The filters will be a different shape. The "low latency" filters are IIR filters while the regular ones are FIR.

So, what is the difference between the 2?

Good question. I will defer to the InterWeb.

https://www.advsolned.com/difference-between-iir-and-fir-filters-a-practical-design-guide/

I'm sure we have some hams here who can explain it a bit better for us (not me) . :)

Juan TG9AJR

Thanks so read and learning to do, but as you said perhaps someone can explain us more.

I heard from Tim that ARQ mode is like Pactor mode, so not sure how much people still utilizing Pactor as much as FT8 FT4 RTTY CW SSB. So I hope some day this Low Latency Filter or a new filter (I know current Flex filters for FT8 are amazing), but so much put into for an ARQ (Pactor) filter perhaps even improve more the current one or something new. But off course I'm just saying if I understood all this correctly, perhaps I'm wrong.

Anyway despite this filter so far this new version on my 6600 installed and is working so far so good, thanks all the good work.

Erik Carling EI4KF

I can decode at the waterfall edges using JTDX with low latency filters active, in fact in the FT8 pile up today for FT8WW I decoded stations above 3000Hz. So does that mean that my filters are not working?

You can see in the picture the new filters are active, a station clear and being decoded at 200Hz. It is the same at the high end with no problem at 3000Hz.

So what to do for the best in FT8. Have the new filter on or off?

Erik.

Neil Rogers

Hi Erik,

I did a quick test on 10m ft8 on a constant signal strength signal using JTDX testing in the 2800-3000 zone then moving the signal to the centre of the bandwidth the jtdx reported signal was the same.

I'm using filter off so I can view the signal at the bandwidth edge for my TX df selection

Neil g0ggg

Erik Carling EI4KF

Hi Neil. Yes but with the filter on I do not get the problem reported by yourself and Juan.

I wanted to know

a) does that mean the latency filters are really working in my case?

b) for FT8 is it better to have them on or off (in other words I understand the benefit for ARQ modes as explained by Mike but do they make any difference on FT8?)

However, no direct answers. Even though I have no issue in JTDX with them being on, I think I will assume that they should be off as you have them. I do not use ARQ modes so I won't need to remember to set them to on at any time.

Erik G0CGL