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Author Topic: SDR Knobs and Buttons in the future  (Read 98331 times)
K9IUQ
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Posts: 2078




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« Reply #45 on: September 13, 2013, 06:28:53 AM »


 not so successful at others, especially where hard realtime issues come into play (missing a buffer on output leads to CW tone coming out of radio on Tx; variable latency means using RF for CW sidetone is impractical); 

 the Windows environment is very tough to do hard real time in (and Linux or Mac OS are really no better).

And to get there requires an enormous amount of software, and hardware help to solve the real time problem.

I made the same point you just did concerning RTOS. I was told Real Time is a piece of cake on PC's or Macs. These comments are earlier in this thread.

Now we can not have it both ways. Someone here is wrong....

Stan K9IUQ
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M0HCN
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« Reply #46 on: September 13, 2013, 09:40:48 AM »

Realtime has an interesting definition in the software game, it does not (necessarily) mean fast, it means there is a deadline and it WILL be met, no matter what.

A classical example of a realtime system is payroll processing, that money HAS to move on the specified day or really bad things happen to a lot of people. The deadline is once a month, so this is not fast, but it is immovable.

Now for our purposes we are looking at short deadline hard realtime, which even more then the other sort is not trivial, if we want to keep the latency in the say 10ms range, and it takes 1ms to get the data to and from the modulator, and our DSP takes say 5ms to do whatever it does to a block of data, then any delay in executing the dsp thread greater then 4ms will result in a missed deadline, and all sorts of things can cause delays.

Higher priority interrupts (SMM Mode springs to mind), lock contention (Another thread has a lock on some required resource), memory exhaustion (This is why one should NEVER allocate or free memory from a RT thread, it can always block), cache lines pingponging between cores, even the memory subsystem can throttle a pc in response to temperature, and for a worst case analysis you have to add up all the possible delay causes and ensure that the total comes in below the limit required time to meet the deadline.

Typically most general purpose operating systems have an average latency in the sub ms region, but with large spikes caused by things like contended locks in task management structures and interactions between the memory and IO subsytems, a hard RTOS guarantees that these spikes will never exceed a defined length, no commodity os makes ANY guarantee about the maximum scheduling latency.

Further, latency and throughput are goals that tend to work in opposition, low latency systems tend to not be particularly high throughput, so folks optimizing for a compute heavy load will trade poor scheduling latency for more throughput every time.

Doing reliable realtime at latency in the single digit realm on a commodity PC is tough, to the point that the guys doing serious audio and video editing get **very** picky about what hardware they will buy for the purpose. Even things like video cards can sometimes be shockingly badly behaved, locking the bus for entire milliseconds to get a few extra frames per second on the 'benchmark' in some random gamer magazine.

Now all that said, reasonable reliability as opposed to hard realtime is much easier, at least if you are a hacker and not above getting your hands dirty, but even this in the ms latency region can cause severe bouts of headscratching when you move the work to a new pc, selling and supporting such products is a bit if a nightmare.

Making SDR economic at the cheap HF set level needs the tech to become sufficiently mature for someone to be willing to commit to an ASIC rather then doing the work in a commodity FPGA, and at that point there is a question about just how 'software defined' the set really is, it becomes more of a 'digital radio' then a software defined one.
Eventually someone will pull the trigger on this, but not until they figure they have a product with at least a 5 year market life at a few thousand units a year, and the technology is still moving too fast for that to be the case.


73 Dan.
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KF6QEX
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« Reply #47 on: September 15, 2013, 03:15:58 PM »

Quote
I made the same point you just did concerning RTOS. I was told Real Time is a piece of cake on PC's or Macs. These comments are earlier in this thread.

Now we can not have it both ways. Someone here is wrong....

It is,depending on your definition or "real time" what kind of cake and the size of the piece of cake.
"pretty fast" and "almost instantly" "barely noticable" and "negligible delay" don't count as "real time".
THe easiest test to perform is this: I stumbled on it accidentally when each kid turned on the TVs really loud so I could hear both "at the same time".
Two different model satellite TV receivers tuned to the same tv program, create an echo when you listen. One of the two lags behind just enough to create the echo effect.
I expect with a "conventional radio and a SDR tuned to the same station side by side, the SDR will have a noticable delay varying depending on the model/type of SDR.
I also know that any number of "conventional" radios tuned to the same station have no audible delay
As for music production, USB microphones for example, include a 3.5mm jack for  monitoring  in "real time" before the signal enters the digital chain.
ie: http://www.musiciansfriend.com/pro-audio/rode-microphones-podcaster-usb-microphone has one and this one actually mentions the delay /latency issue : http://www.musiciansfriend.com/pro-audio/blue-yeti-usb-microphone

Quote
...The Yeti utilizes a high quality analog-to-digital converter to send incredible audio fidelity directly into your computer, a built-in headphone amplifier for zero-latency monitoring, and simple controls for headphone volume, ... 

I call all this phenomenon "asychnronous real time". Of course when you have two realities that are out of step with each other, although they make pills for that for humans, they don't have a pill of radios yet Smiley
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M0HCN
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« Reply #48 on: September 15, 2013, 03:40:08 PM »

Ah, you mean low latency, not realtime, definitions matter when discussing this stuff....

Odds are that that headphone jack on the usb mic is actually a digital mixer behind the ADC feeding a DAC and exists mostly so that the buffering in the PC does not cause comb filtering due to delay relative to the bone conduction path in the singers head.
The reason for putting a DAC in the mic is that attempting to use an ADC on one clock with a DAC on another is a can of worms, so best avoided to avoid customer support headaches.

The ADC and DAC will introduce about a ms of latency, due to the decimation and interpolation FIR networks, but the buffering to go from sample by sample processing (as you would do with a real DSP or in a gate array) to blocks of hundreds of samples as used in typical PC processing can add 10s of ms to the latency numbers, this moves the combing down from Khz to a few hundred Hz and makes it become very objectionable. 

In the context of a radio, for SSB latency of the up to say 50ms or so level is clearly a non issue, same for most data modes, as long as it is compensated for some of the EME modes.

CW is probably the only place it matters, and there are well known ways to solve this or at least to give the operator a choice of trade offs (Linear phase filters will ALWAYS introduce more group delay then non linear phase ones for example).

Low latency audio is a very tough problem on PCs, it can be done, but man what a pain.

73 Dan.
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K5TED
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Posts: 819




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« Reply #49 on: September 15, 2013, 06:16:14 PM »

Latency with SDR is a non-issue except for CW.

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AB2YC
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Posts: 53




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« Reply #50 on: September 18, 2013, 06:18:19 AM »

To the OP congratulations on getting your License.

I also have been in the IT field for over 20 years and realize PCs will be with us for some time.

It was SDR that drew me back into ham radio, it truly is the future.
You will always find those who resist change, just side step them and move on.

One point where SDR shines is the ability to quickly change the entire interface by just changing the computer controlling it.
I foresee a point where the manufactures will offer a control head that will basically be the interface replacing the PC.
and an example of this is the K3 by Elecraft, my understanding is that it is actually a SDR based radio with a hard interface.

Being and IT guy myself I don't have a real issue with the PC interface and find it a whole lot easier to do all the inter-connections
"Virtually" (serial, audio, etc), need another port just add another "Virtual" one (I don't need to open the case or run a cable).

I would suspect that most IT guys or technology guys would not have an issue with SDR, we already understand that you need a
decent PC with a clean OS. I'm more of a Linux guy when it comes to apps requiring computer control of hardware (I also run CNC).
I was a bit reluctant at first to use MS Windows to run a radio, but it (my Flex-1500) has been running fine for me so that reluctance is gone.

Again congratulations on getting your license, and welcome to the world of SDR.





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N4OI
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Posts: 214




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« Reply #51 on: September 18, 2013, 07:00:22 PM »

Latency with SDR is a non-issue except for CW.

For some of us, this statement effectively reads, "Latency with SDR is a non-issue except for [ham radio]."

73
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K5TED
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Posts: 819




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« Reply #52 on: September 18, 2013, 09:46:50 PM »

Latency with SDR is a non-issue except for CW.

For some of us, this statement effectively reads, "Latency with SDR is a non-issue except for [ham radio]."

73

Low ground clearance of the Corvette is a non-issue except for off-roading.
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N4OI
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Posts: 214




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« Reply #53 on: September 19, 2013, 04:16:49 AM »

Latency with SDR is a non-issue except for CW.

For some of us, this statement effectively reads, "Latency with SDR is a non-issue except for [ham radio]."

73

Low ground clearance of the Corvette is a non-issue except for off-roading.

And you will not find off-roading anywhere in the Corvette's published features or specs -- perhaps SDR vendors should re-label their CW buttons to "Amp Tune" ...   Grin

73
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W1JKA
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« Reply #54 on: September 19, 2013, 04:50:25 AM »

  SDR newbie here, what kind of performance could I expect if I put an SDR mobile in my corvette and limited myself to secondary roads only? I plan to use a screwdriver for an antenna. Thanks
« Last Edit: September 19, 2013, 05:00:49 AM by W1JKA » Logged
K9IUQ
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Posts: 2078




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« Reply #55 on: September 19, 2013, 07:10:34 AM »

perhaps SDR vendors should re-label their CW buttons to "Amp Tune" ...   Grin

Perhaps it would be EVEN better if they eliminated the CW button,FSK button,and Digi buttons..

SDRs are for SSBers who like to brag and tell everyone how great their radio/audio works.. All other hams need not apply.

Stan K9IUQ
« Last Edit: September 19, 2013, 07:27:32 AM by K9IUQ » Logged
K5TED
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Posts: 819




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« Reply #56 on: September 20, 2013, 06:26:53 PM »

Perhaps those who can't operate SDR's shouldn't purchase them. We'll still talk to them. Some of us may use CW when we "talk" to them. We will undoubtedly be using much superior SDR/Digi setups than a typical knob rig/patch cord/external modem/isolation transformer concoction user. We won't need a patch cord. It'll just flow, trouble-free, digitally from PC to SDR.

Nice.

It's really the only way to go unless you insist on touching just two wires together to communicate. Nothing wrong with that, but it's a bit beneath the technological advantages of an SDR.
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K5TED
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Posts: 819




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« Reply #57 on: September 20, 2013, 06:32:24 PM »

Latency with SDR is a non-issue except for CW.

For some of us, this statement effectively reads, "Latency with SDR is a non-issue except for [ham radio]."

73


Low ground clearance of the Corvette is a non-issue except for off-roading.

And you will not find off-roading anywhere in the Corvette's published features or specs -- perhaps SDR vendors should re-label their CW buttons to "Amp Tune" ...   Grin

73

True. What you will find is a gearshifter that has the same designations as a Jeep gearshifter. Totally different vehicles. Same "D" on the shifter. (or 1, 2, 3, 4, etc.)

Perhaps Chevrolet should relabel the "D" to "Go Really Fast" and Jeep should relabel to "Go Really Slow Up A Telephone Pole"
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N4OI
Member

Posts: 214




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« Reply #58 on: September 20, 2013, 08:17:29 PM »

Latency with SDR is a non-issue except for CW.

For some of us, this statement effectively reads, "Latency with SDR is a non-issue except for [ham radio]."

73


Low ground clearance of the Corvette is a non-issue except for off-roading.

And you will not find off-roading anywhere in the Corvette's published features or specs -- perhaps SDR vendors should re-label their CW buttons to "Amp Tune" ...   Grin

73

True. What you will find is a gearshifter that has the same designations as a Jeep gearshifter. Totally different vehicles. Same "D" on the shifter. (or 1, 2, 3, 4, etc.)

Perhaps Chevrolet should relabel the "D" to "Go Really Fast" and Jeep should relabel to "Go Really Slow Up A Telephone Pole"

My Jeep does indeed have a "D" on the "gear shifter," but it also has another lever connected to a military-grade transfer case to select from three variations of 4WD.  (All analogies break down at some point, but I love the legs this one has!)   Grin

73
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N4OI
Member

Posts: 214




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« Reply #59 on: September 21, 2013, 05:39:26 AM »

Latency with SDR is a non-issue except for CW.

For some of us, this statement effectively reads, "Latency with SDR is a non-issue except for [ham radio]."

73


Low ground clearance of the Corvette is a non-issue except for off-roading.

And you will not find off-roading anywhere in the Corvette's published features or specs -- perhaps SDR vendors should re-label their CW buttons to "Amp Tune" ...   Grin

73

True. What you will find is a gearshifter that has the same designations as a Jeep gearshifter. Totally different vehicles. Same "D" on the shifter. (or 1, 2, 3, 4, etc.)

Perhaps Chevrolet should relabel the "D" to "Go Really Fast" and Jeep should relabel to "Go Really Slow Up A Telephone Pole"

My Jeep does indeed have a "D" on the "gear shifter," but it also has another lever connected to a military-grade transfer case to select from three variations of 4WD.  (All analogies break down at some point, but I love the legs this one has!)   Grin

73

Oh, and I think one of the 4WD options is labeled "QSK"  Grin !!
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