FM MPX out and FM modulation measurement

[Simon G4ELI]

Well I don't see the point in stereo - this is the same signal as shown here, it's before demodulating. Mono 192kHz will be fine I believe.

Had it confirmed that mono is fine.

Assuming I get this going, which programs want this 192kHz via VAC?

RDS Spy - https://rdsspy.com

OK. Will get on with this now - not complex work, easy on the Brain.

[Simon G4ELI]

Well I don't see the point in stereo - this is the same signal as shown here, it's before demodulating. Mono 192kHz will be fine I believe.

Had it confirmed that mono is fine.

Assuming I get this going, which programs want this 192kHz via VAC?

RDS Spy - https://rdsspy.com

OK. Will get on with this now - not complex work, easy on the Brain.

Works, testing with RDS Spy.

[Simon G4ELI]

Have sent a kit to G0OFE (above) for testing, if OK I'll give the test team a kit later today.

[DaveFM]

I inadvertently wiped out both of my posts in error... and could not recover either of them, so here it is best as I can recall the wording. If a moderator could recover I'm ok with that.

Thank you Simon for the great software!

BFM Graph
- Graph tic marks at finer say 1kHz increments near 19k and 67k regions, MPX encoders now put out content to 16.5 kHz.
- The current frequency resolution/response time you have of the MPX display is excellent! Peak hold with a variable decay up to infinite would be wonderful, as often you want to capture spurious signals and maximums.

Audio Info
- L, R , L+R. L-R audio levels in dB, these days these will exceed 0dB.
- Audio loudness level in LKFS.
- MPX power measured in dBr according to Stokkemask/ITU-R SM.1268. This involves a weighted audio filtering and quite a few details to get just right. This is the law in parts of the EU and elsewhere, but useful to all as a relative loudness/density of modulation a station has.

MPX Info
- 19 kHz pilot level
- 57 kHz RDS level
- 67/92 kHz SCA level

The above 4 items are measured differently by region, a selection to display in either % of total modulation or kHz deviation would be ideal.

RF/Reception info
- Multipath in %, important to minimize this as the % modulation/deviation measurements can go very far off in the presence of relatively small amounts of multipath.
- SNR in dB
- Ultrasonic Noise in % is a great indicator of signal reception quality, I've seen mention of monitoring the signal out 150 kHz into the MP, so outside the displayed graph but you are likely oversampling the MPX and reducing to 100 kHz for display.

[DaveFM]

Duplicate post.

[Simon G4ELI]

BFM Graph

• Graph increments depend on available space, it's dynamic.

• At preset there is no peak hold, what you see is an algorithm I threw together.

Audio Info

• L, R etc. levels would be relative, quite easy.

• Stokkemask/ITU-R SM.1268 is interesting, I had not encountered it before. Don't have time now but...

MPX Info

• I could probably display the 19 kHz pilot tone offset, RDS should be 3 x pilot tone. SCA more difficult as I don't have any suitable data.

RF/Reception info

• I don't know how to measure multipath.

• You'll have to give me the exact method to be used for SNR.

• I could probably measure ultrasonic noise.

[G0OFE]

Dave,

Looks like you edited both your posts.. you may want to edit one of them to bring the content back

[DaveFM]

BFM Graph

• Graph increments depend on available space, it's dynamic.

• At preset there is no peak hold, what you see is an algorithm I threw together.

The Peak Hold from a right-click in the Spectrum display could work, but would be great to have variable decay or even just infinite peak hold.

Audio Info

• L, R etc. levels would be relative, quite easy.

• Stokkemask/ITU-R SM.1268 is interesting, I had not encountered it before. Don't have time now but...

I'd imagine this is not an simple undertaking, others have struggled to do it accurately.

MPX Info

• I could probably display the 19 kHz pilot tone offset, RDS should be 3 x pilot tone. SCA more difficult as I don't have any suitable data.

I have both local 67 kHz SCA signals as well as the ability to generate both 67/92 SCA for testing. Once I have enough posts I hope to PM for access to the test team. I'd be happy to test and provide feedback on this any any other FM features including the MPX out on 3.0.26.

RF/Reception info

• I don't know how to measure multipath.

Multipath can be referred to also as DU Ratio or Desired/Undesired, I found this explanation pretty good and if you read the website he references after translating from Japanese it explains with some spectrum analyzer examples.
https://github.com/jj1bdx/rtl_power-fm-multipath

This paper talks about DU in relating to car stereo reception, pretty advanced for the 80's but very common in radio ships now from the likes of SI and NXP to name few.

Some higher end FM tuners had H V Multipath outputs that you would hook up a scope in X-Y mode to observe multipath. The H is the output of the detector, V is a DC voltage relative to signal strength from the IF stage. With no multipath you would get a horizontal line with no wiggles, the width of the line was the amount of modulation and the height on the display the signal level. Real world there is usually a slight curve, but what you don't want is nonlinear peaks and dips.

• You'll have to give me the exact method to be used for SNR.

The SNR meter in Signal Meter of Spectrum display is good, but you cannot display both SNR and dBm/dBuV at once which would be good to have.

• I could probably measure ultrasonic noise.

This would be great as it is a great indicator of reception quality.