The author is president/CEO of Inovonics Broadcast. This article is from the free Radio World ebook “HD Radio Best Practices 2025.”
Nothing drives me crazier than skipping or stuttering audio when blending from FM to HD, or HD to FM. The blend is designed to be seamless. Even if not perfect, it should never be jolting!
Unfortunately, there are a lot of earlier-generation of exporters and transport systems still in use today. If you have one and you think everything is locked down tight, first, congratulations, second, don’t power cycle anything.
Even in a system that has been perfectly set up, with GPS locked, 10 MHz locked or 1 PPS locked, things can still drift. In our lab we have an exporter in which the PLL clock is always moving, and nothing short of replacing the board will fix it even though it is set up with GPS, 10 MHz and 1 PPS.
As outlined in the NRSC-G203-A guidelines published in 2021 (a great resource), alignment of the FM and HD Radio audio is extremely important.
What is really useful is to look at the alignment over time. If you take a quick sample and the number comes back indicating that your HD audio is 10 samples ahead of your FM audio, you may make the adjustment in your FM air chain and think everything is set. But if you take another sample and see that fixing the 10 samples of delay didn’t really “fix” it, you are stuck trying to figure out what is going on.
One way would be to continue to take samples at regular intervals, write them down, then take more samples, and then eventually plot something on a graph. The real solution is much easier if you have a piece of gear to do that for you automatically.
You can see from Fig. 1 that if you were simply taking readings every 5 minutes, your delay samples would be all over the place. Without the means to look at your delay over time, it is extremely hard to make sense of what is going on.
The other “issue” with FM and HD blend is audio levels. If these are not matched, the experience can be quite annoying for listeners. Fortunately, this one is not as hard to monitor or fix, even if you don’t have the proper equipment since you can use your ears or audio level meters.
See Fig. 2. The yellow graph shows that the Peak (dB) levels are –4.0 dB below the FM level. FM is always the level to compare against because you are limited in your modulation to ±75 kHz deviation and can’t really turn that level up or down. The blue line shows the relative Loudness (dB) is –10.0 dB lower than the FM level.
The most important thing you can do is to make sure your gear is GPS, 10 MHz and 1 PPS locked together. Using one “Master Clock” helps to keep things in sync. If you lose power or need to reboot things, always recheck your alignment and levels. Next, monitor your system on a daily basis. If you have the gear to do that automatically, fantastic. If you don’t, you need to get some.
Bottom line, you monitor and course-correct what is important to you.
