Accuracy of 19 kHz stereo pilot tone

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reverend
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Accuracy of 19 kHz stereo pilot tone

Post by reverend » Mon Feb 03, 2025 2:05 pm

Does anyone know whether there is a specification for the required accuracy of the 19 kHz stereo pilot tone frequency?

I was trying to figure out whether it is possible to use the same crystal to provide a reference for a PLL and for a stereo-coder. Using an 8 MHz crystal for example, it's possible to get 38095 / 19047 kHz tones (divide by 210 then by 2) which is around 0.25% (or 2500 ppm) off frequency.

I guess I could try using this and see whether most receivers will work OK, however I wondered if anyone had seen a standard which specifies what is actually/theoretically needed.
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Re: Accuracy of 19 kHz stereo pilot tone

Post by 1608cc » Mon Feb 03, 2025 3:40 pm

According to "Technical Standards and Requirements for FM Broadcasting Transmitters" from 2005:
A.2.2.2.1 The frequency of the pilot signal shall be 19,000 ±2 Hz.
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Re: Accuracy of 19 kHz stereo pilot tone

Post by Albert H » Tue Feb 04, 2025 4:05 am

The spec. is +/- 2Hz, but many commercial coders don't manage this. One Bext coder I have here at the moment is 17 Hz high. Practically, it doesn't seem to matter much, but if the spec says 2Hz.....

I've used a 10 MHz reference (with GPS steering from a "Navman" satellite receiver) with a couple of PLLs to give 19 kHz and 57 kHz at very high accuracy. It's also easy enough to divide the 10 MHz to a sensible reference frequency for a transmitter PLL, so yes - it's possible and is done in some commercial gear.

My other favourite frequency accurate off-air source (198kHz longwave) is being closed down, so I'm going to have to rebuild some of my gear to use GPS receiver modules. :(
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Re: Accuracy of 19 kHz stereo pilot tone

Post by jvok » Tue Feb 04, 2025 8:17 am

Albert H wrote: Tue Feb 04, 2025 4:05 am My other favourite frequency accurate off-air source (198kHz longwave) is being closed down, so I'm going to have to rebuild some of my gear to use GPS receiver modules. :(
Or make a low power 198khz rig and GPS lock that :smoke

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Re: Accuracy of 19 kHz stereo pilot tone

Post by reverend » Tue Feb 04, 2025 1:12 pm

Thank you @1608cc and @Albert H - 2 Hz is more of a challenge without adding the complexity of an additional PLL. Time for some creative thinking on division ratios to try and get a suitable reference frequency and the necessary 19 & 38 kHz tones.
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Re: Accuracy of 19 kHz stereo pilot tone

Post by reverend » Tue Feb 04, 2025 2:01 pm

It turns out it's quite easy... With a 4864 kHz crystal, you can divide by 4864 (4096 + 512 + 256) (just 3 bits on a counter) to get a 1 kHz reference. With a divide-by-100 pre-scalar you can then operate the PLL for the FM synthesiser. Equally, you can divide by 128 to get 38 kHz and 256 to get 19 kHz. Luckily I have a bag of 4864 kHz crystals so can give it a try. Divide by 100 is more complicated but something like an MC12080 (div-by-10) followed by any-old-CMOS divider would do the job. I still have a couple of SP8629's kicking around but not enough to be useful.
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Re: Accuracy of 19 kHz stereo pilot tone

Post by 1608cc » Tue Feb 04, 2025 3:28 pm

Consider also the 8,664MHz, 4,332MHz and 456kHz resonators, which are quite easy to get online. One guy from polish forum used 11,0592MHz with 8bit DAC that gives - as he said - 1,13ppm off from ideal.

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Re: Accuracy of 19 kHz stereo pilot tone

Post by Albert H » Wed Feb 05, 2025 10:37 pm

Years ago, I made a run of stereo coders (mixer type with MC1496 for the DSSC modulator) using 455kHz IF filters for the reference. I found that if they were lightly loaded, it was quite easy to get a nice, stable 456kHz out of them.

A couple of CMOS divider ICs later, I had 19 and 38 kHz. I divided first by 6 (with a 4017) to give 76 kHz, but with an assymmetrical squarewave - almost a pulsewave. This fed both halves of a 4013 to give 50 : 50 M/S 38 kHz and 19 kHz. I used simple LC filters to make the squarewaves into pretty good sinewaves, and found that I had to tweak the inductor cores to get the 19 and 38 kHz in phase.

The downside of this type of stereo coder was that the 1496 couldn't handle "line-level" signals, so I had to attenuate the difference signal to prevent distortion in the analogue mixer, and then amplify it back up to match the sum signal. This, of course, gave a noise penalty - the "Side" (modulated difference) component was about 16dB noisier than the "Mono" path, leading to an overal degradation in the noise performance. I used the quietest op-amps I could find (NE5532 or LM833), but still wasn't entirely happy with the result.

Soon afterwards, I moved to oversampled, switching coders, and got much better noise performance. I still used the 455 kHz resonators or filters for the reference, though.
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Re: Accuracy of 19 kHz stereo pilot tone

Post by radionortheast » Thu Feb 06, 2025 11:21 am

i’ve missed this talk of resonators, sounds abit like something mechanical :D I guess the 455 ones are what you find in old am/fm radios, you could make your own version of a clock opera. :lol:

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