Deviation advice

Everything technical about radio can be discussed here, whether it's transmitting or receiving. Guides, charts, diagrams, etc. are all welcome.
Post Reply
piratefm
who u callin ne guy bruv
who u callin ne guy bruv
Posts: 22
Joined: Sun Dec 28, 2014 8:41 pm

Deviation advice

Post by piratefm » Sun Jan 07, 2018 6:13 pm

Hi!!

I don’t know what this means, but on my TX I have a Dial with different options to select and see what the meter reads.

I have one section on Dial that says DEV 100khz and another that says DEV 10khz. At the moment the “100khz setting is reading about “20” on meter and then when selecting 10khz it’s moving between 60 to 100.

What does this mean?? I really haven’t a clue so any help would be appreciated!

Picture attached of transmitter selected to “Dev 10khz” and showing what meter is reading.

Obviously when I stop music, these needles fall to “0”..
You do not have the required permissions to view the files attached to this post.

User avatar
Maximus
tower block dreamin
tower block dreamin
Posts: 346
Joined: Wed Jan 21, 2015 1:29 pm
Location: Where you don't need licenses

Re: Deviation advice

Post by Maximus » Sun Jan 07, 2018 7:51 pm

Deviation is how loud the modulation is. It should be around 75khz in the UK.


Sent from my iPhone using Tapatalk

Albert H
proppa neck!
proppa neck!
Posts: 1026
Joined: Tue Apr 05, 2016 1:23 am

Re: Deviation advice

Post by Albert H » Mon Jan 08, 2018 2:41 am

There's a problem with a simple meter like that - it'll only work with constant tones. It's used for setting up the deviation (75kHz peak in most of the world), and can be used for adjusting your stereo pilot tone (should be 7.5kHz deviation on its own). Unfortunately, the meter won't be fast enough (mechanically) to follow the rapid changes that occur with real audio modulation.

However, it will be good enough to set up the deviation with constant tones of known levels. The meter switch also has positions for left and right audio (again only good enough for steady-state tones). What you will be able to do (assuming the calibration of the meter is OK) is set up your basic levels before applying audio.

If you want to monitor instantaneous levels of audio, you need a PPM (NOT a VU meter - they're too slow and under-read for that reason). The PPM was developed by the BBC back in the 1940s, and is simply calibrated from 0 - 7 with white numbers on a black background. The electronics driving the meter movement compensates for the slowness of the "ballistic" of the meter movement and will therefore read correctly, with a very fast attack and slowed decay to make level watching easy. "0" level - the industry standard 0.775V into 600Ω is indicated by a reading of "4" on the PPM, and each graduation is 4dB, so peak level is at +8dB - PPM "6". Anything over this level is overmodulated.

In radio studios it's common to have as many as three PPMs (sometimes four), usually with pairs of needles - red and green for "right" and "left", white and yellow for "sum" and "difference" (important when broadcasting in stereo) and subsidiary meters for pre-fade level checking, sometimes separated for the "music" and "speech" sub-mixes on the desk (since they'll be processed differently by the compressors and limiters).

Those of you who thought that a radio studio consisted of a disco mixer and a couple of decks and a microphone are wildly under-estimating the equipment that's used in professional broadcasting! :ugeek:
220px-Sifam.type34A.British.ppm.jpg
220px-Spliced.oldppm.jpg
250px-Paqt_meter.ab.png
250px-Paqt_meter.m6.png
You do not have the required permissions to view the files attached to this post.
"Why is my rig humming?"
"Because it doesn't know the words!"
;)

piratefm
who u callin ne guy bruv
who u callin ne guy bruv
Posts: 22
Joined: Sun Dec 28, 2014 8:41 pm

Re: Deviation advice

Post by piratefm » Mon Jan 08, 2018 10:25 am

Albert H wrote:
Mon Jan 08, 2018 2:41 am
There's a problem with a simple meter like that - it'll only work with constant tones. It's used for setting up the deviation (75kHz peak in most of the world), and can be used for adjusting your stereo pilot tone (should be 7.5kHz deviation on its own). Unfortunately, the meter won't be fast enough (mechanically) to follow the rapid changes that occur with real audio modulation.

However, it will be good enough to set up the deviation with constant tones of known levels. The meter switch also has positions for left and right audio (again only good enough for steady-state tones). What you will be able to do (assuming the calibration of the meter is OK) is set up your basic levels before applying audio.

If you want to monitor instantaneous levels of audio, you need a PPM (NOT a VU meter - they're too slow and under-read for that reason). The PPM was developed by the BBC back in the 1940s, and is simply calibrated from 0 - 7 with white numbers on a black background. The electronics driving the meter movement compensates for the slowness of the "ballistic" of the meter movement and will therefore read correctly, with a very fast attack and slowed decay to make level watching easy. "0" level - the industry standard 0.775V into 600Ω is indicated by a reading of "4" on the PPM, and each graduation is 4dB, so peak level is at +8dB - PPM "6". Anything over this level is overmodulated.

In radio studios it's common to have as many as three PPMs (sometimes four), usually with pairs of needles - red and green for "right" and "left", white and yellow for "sum" and "difference" (important when broadcasting in stereo) and subsidiary meters for pre-fade level checking, sometimes separated for the "music" and "speech" sub-mixes on the desk (since they'll be processed differently by the compressors and limiters).

Those of you who thought that a radio studio consisted of a disco mixer and a couple of decks and a microphone are wildly under-estimating the equipment that's used in professional broadcasting! :ugeek:

220px-Sifam.type34A.British.ppm.jpg


220px-Spliced.oldppm.jpg
250px-Paqt_meter.ab.png
250px-Paqt_meter.m6.png
Thanks AH!

I have a professional set up, with an Omnia and an Alice Monitoring with PPM, so I can switch between the output of the omnia and the off air fm receiver.

The omnia does also have a test tone to set up things like this.

I’m just confused as to how I should be reading that meter on the tx......would I need to set it to the “Dev 10khz” or “Dev 100khz” setting and then run the test tone?....

Then I guess I would then adjust the output from the omnia so that the reading on the meter of the TX reads around 75?

User avatar
thewisepranker
tower block dreamin
tower block dreamin
Posts: 405
Joined: Fri Aug 22, 2014 12:53 pm

Re: Deviation advice

Post by thewisepranker » Mon Jan 08, 2018 11:28 am

I think the 10 kHz setting might be for setting the level of the stereo pilot tone and only displays 10 kHz of deviation at full scale deflection. Think of it as being zoomed in ten times when compared to the 100 kHz reading. There may also be a bandpass filter on the 10 kHz setting to stop full audio being included in that particular deviation measurement, so that the pointer doesn't smash into the housing as it tries to display 750%.

100 kHz should be the full MPX and the one you are mainly interested in.

I don't know if this is correct though as I don't have one to mess about with.

piratefm
who u callin ne guy bruv
who u callin ne guy bruv
Posts: 22
Joined: Sun Dec 28, 2014 8:41 pm

Re: Deviation advice

Post by piratefm » Mon Jan 08, 2018 12:36 pm

thewisepranker wrote:
Mon Jan 08, 2018 11:28 am
I think the 10 kHz setting might be for setting the level of the stereo pilot tone and only displays 10 kHz of deviation at full scale deflection. Think of it as being zoomed in ten times when compared to the 100 kHz reading. There may also be a bandpass filter on the 10 kHz setting to stop full audio being included in that particular deviation measurement, so that the pointer doesn't smash into the housing as it tries to display 750%.

100 kHz should be the full MPX and the one you are mainly interested in.

I don't know if this is correct though as I don't have one to mess about with.
Thanks for your help! I will check it out. That’s given me more of an understanding.

radium98
tower block dreamin
tower block dreamin
Posts: 268
Joined: Fri Aug 26, 2016 7:01 pm

Re: Deviation advice

Post by radium98 » Mon Jan 08, 2018 8:19 pm

I agree with the wisepranker

Post Reply