8.33 KHz comm spacing in P3d5

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jabloomf1230
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8.33 KHz comm spacing in P3d5

Post jabloomf1230 »

Do any of the P3d5 compatible A2A aircraft support this? Someone must have asked this before about P3d5, but the forum search function doesn't yield any threads. A Google search on 8.33 for the whole a2a website only returns much older posts asking about whether 8.33 support is forthcoming. Thanks.

jabloomf1230
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Re: 8.33 KHz comm spacing in P3d5

Post jabloomf1230 »

jabloomf1230 wrote:
18 Jul 2020, 17:57
Do any of the P3d5 compatible A2A aircraft support this? Someone must have asked this before about P3d5, but the forum search function doesn't yield any threads. A Google search on 8.33 for the whole a2a website only returns much older posts asking about whether 8.33 support is forthcoming. Thanks.
Apparently, no one here has any idea about 8.33 KHz comm spacing, so I researched it further and here's what I discovered:
  • 1. The default P3d5 C-130 comm radio supports it, as does both the default ATC and VOXATC. (LM lists this in the change log for P3d5)
    2. The Garmin GTN trainer supports it and the Jeppesen navigation db includes the 8.33 frequencies. (Just like the real world GTN unit)
    3. The RXP GTN trainer does not support 8.33. RXP is working on it. (Confirmed by RXP support)
    4. No one seems to know if the Flight 1 GTN Complete gauge supports it.
    5. Lockheed Martin was not all that thorough when it updated P3d5 airports and often did not include 8.33 KHz frequencies.
    6. If LM did not use the correct 8.33 frequency for an airport, Navigraph used what LM used, even in the most recent AIRAC. (Confirmed by Navigraph support)

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AKar
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Re: 8.33 KHz comm spacing in P3d5

Post AKar »

jabloomf1230 wrote:
29 Jul 2020, 20:11
3. The RXP GTN trainer does not support 8.33. RXP is working on it. (Confirmed by RXP support)
I'm pretty sure I've successfully tuned such frequencies in RXP GTN. One needs to change the appropriate setting in radio options somewhere in the units' configuration menus.

TreeTops
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Re: 8.33 KHz comm spacing in P3d5

Post TreeTops »

I have changed the setting in the GTN 650 and tuned 123.33 and 123.66 successfully.
I havent used these frequency type before.
Cheers
Trev

jabloomf1230
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Re: 8.33 KHz comm spacing in P3d5

Post jabloomf1230 »

@AKar, @Treetops,

Thanks for the replies. Without changing the settings on the GTN Trainer, one can't tune to the 8.33 KHz comm frequencies. I already did that.

But Treetops has actually hit on what the problem is. The way that 8.33 KHz comm frequencies are implemented worldwide is that the actual frequency and the displayed frequency are different, because some of the new frequencies are irrational numbers (like 1/3 which repeats indefinitely as a decimal). When the 8.33 KHz frequencies were being added, some genius thought that it would be better for aircraft radios to display at even intervals. You can read about it here, but the last digit that is displayed is always a 0 or a 5:

https://833radio.com/news/show/7

In 8.33 KHz mode, The RXP GTN gauge lets you tune to a frequency like 125.405, but the gauge changes the display back to 125.400. It also doesn't set the sim's comm frequency variable correctly. Try flying the stock C-130 in P3d5 and you'll see what the correct behavior of the comm radio should be. The Garmin GTN trainer in offline mode also works correctly.

One of the reasons that I asked about this on the A2A forum (other than I have 6 A2A aircraft) is that the A2A aircraft have legacy comm radios in addition to possibly having the GTN and I was wondering if accidently changing the frequency with the legacy radio would cause some issue, even if the GTN gauge worked correctly.

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AKar
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Re: 8.33 KHz comm spacing in P3d5

Post AKar »

jabloomf1230 wrote:
30 Jul 2020, 11:13
But Treetops has actually hit on what the problem is. The way that 8.33 KHz comm frequencies are implemented worldwide is that the actual frequency and the displayed frequency are different, because some of the new frequencies are irrational numbers (like 1/3 which repeats indefinitely as a decimal). When the 8.33 KHz frequencies were being added, some genius thought that it would be better for aircraft radios to display at even intervals. You can read about it here, but the last digit that is displayed is always a 0 or a 5:
This is actually fairly common in telecommunications world. The nominal band carrier frequency can be different from the actual. (In fact, some systems transmit "nothing" on the nominal frequency, but let's leave that for now! :mrgreen:)

Simple COM radios were not sufficiently tightly filtered to allow for very tight frequency spacing. Back in the 30ies, where much of our aviation tech dates from, this could not have been reasonably expected either. Later on, the loose filtering was done on purpose, as the wide channel spacing allowed for picking up the right one with less tuning precision if such a loose filtering was used. Nowadays it makes no difficulties to actually have the radios the size of the ones used in airplanes both tuned at the correct frequencies, and to have them filtered narrowly around it. Hence, they came up with the idea of increasing the amount of frequencies usable for COMs by introducing this narrower channel spacing.

Of course, it would be stupid to have too many decimals, and also, it makes sense to limit the 'count' according to what is really needed. The channel spacing is still wide enough to allow for the last digit be either 0 or 5. Importantly, this reduces errors in getting the frequency right when you are given one in a garbled speech.

-Esa

jabloomf1230
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Re: 8.33 KHz comm spacing in P3d5

Post jabloomf1230 »

AKar wrote:
30 Jul 2020, 12:13
...The channel spacing is still wide enough to allow for the last digit be either 0 or 5.
-Esa
The problem with what was adopted is that not all frequencies that end in a 0 or 5 are valid . Notice in the link that I posted above, there is no frequency at 118.020. This poses a challenge for flightsim gauge designers, as they normally use a simple case statement in C combined with modular arithmetic to set or read each digit. Now they have to additionally check if certain frequencies are valid, based on the 5th right hand digit. It's not a coding nightmare, but it needs to be factored into the gauge logic. A digital gauge like the Garmin GTN is easier to handle than something with knobs that one twists.

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AKar
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Re: 8.33 KHz comm spacing in P3d5

Post AKar »

They simply skip some that won't come up. It should be trivial, probably via rounding to the count, but I haven't checked if this is always valid with every channel. Certainly, if I did run into a problem, that would be something I checked the first, for this very often in general is not the case.

In any case, these channels are defined in pubic. So, any simulation of the 8.33 spacing done wrong is just plain bad job. :) There ought to be nothing difficult about. If there is a platform issue, I've not yet run into it on v5, using RXP gauges and some other stuff as well.

I'll check how the 8.33 spacing is defined tomorrow, as it is interesting. In fact, I've never resorted into the spec.

-Esa

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AKar
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Re: 8.33 KHz comm spacing in P3d5

Post AKar »

Well, I quickly browsed through some of the stuff I had at hand (I haven't yet recovered my full document collection after my mobo decided to start rejecting certain RAID drives). There appears to be nothing too funky about 8.33 kHz channel spacing definition.

Perhaps the most confusing thing is that, indeed, let us say, 125.400 and 125.405 tune exactly the same carrier frequency, which would be 125.4 MHz. What selecting 125.405 tells the box is to use 8.33 kHz channel spacing instead of 25 kHz, which would be used if 125.400 was tuned.

In other words, where the same frequency is shared by the 25 kHz and 8.33 kHz spacing, the use of 8.33 instead of 25 is indicated by that added .005 to the dialed nominal frequency.

I don't know how FSX/P3D handles these exactly, but in general, the frequencies in the sim are exact, for radio and bandwidth stuff is not simulated. Hence, perhaps, they simply model 125.4 and 125.405 as the same frequency / channel - which in reality they precisely are / are not at the same time. :mrgreen:

-Esa

jabloomf1230
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Re: 8.33 KHz comm spacing in P3d5

Post jabloomf1230 »

FSX and earlier versions of P3d have no ability to use 8.33 KHz comm spacing, so it's only an issue in P3d5. Since the freestanding Garmin GTN Trainer standalone displays 125.405 correctly, the reason that the RXP GTN gauge displays it as 125.400 might be due to the behavior that you explained. I passed this information along to RXP and I'll see what they say.

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AKar
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Re: 8.33 KHz comm spacing in P3d5

Post AKar »

Nice, let us know, if you will, what's the answer. :)

Could be some P3D thing, if they indeed merged the .000 and .005 (etc).

Has anyone attempted how some frequency on 8.33 kHz spacing comes through to a set tuned to the same frequency spaced 25 kHz, and vice versa, with real aviation COM radios? It could be interesting.

-Esa

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