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Analog On The Air, Digital With No Audio.

stewie

stewie

Heard something odd on a local HD station I was hoping one of you engineers could help me figure out.

Tuned a station and the analog sounded fine. When my radio finished buffering the digital, the HD lock came on and the audio went away. For grins, I jumped to the HD-2 and it was working just fine.

Can someone walk me through the "typical" (if there is such a thing) audio flow? I called the station and in a few minutes the HD was off entirely.

Curious how something like that happens.
 
Typically for the main channel:
Primary and backup STLs (T1 and digital microwave) to two STL switchers (Titus 3-DRX) as AES digital audio
STL switcher outputs to primary and backup audio processors
Processor "analog" outputs as composite audio to two processor switchers (BDI CDS-302)
Processor switchers to main and backup transmitter exciters - analog composite input
Primary processor "digital" out (MPS) to HD Exporter as AES digital audio
HD Exporter data as Ethernet to exciter

It's common for only one processor to get upgraded to have the separate HD processing chain and output during an HD conversion. The HD output on the processor has a lookahead limiter instead of a clipper and allows a few other tweaks to the audio that are specific to the HD MPS feed. The upgrade also typically includes the diversity delay for the analog signal to match the inherent ~8 seconds delay to the MPS HD program. If there aren't two HD processors and the main one fails... no audio to the HD MPS channel. The right thing to do in such a situation is to turn off the HD entirely so what you're hearing doesn't happen. Or better yet, upgrade both the main and backup processors and put an AES switcher between them to feed the Exporter so you have a backup MPS source.

SPS (HD2/3/etc) programming typically feeds the Importer which may have an internal processor card, or a standard processor will be used. This feeds the Exporter via ethernet.

There are a few different ways to do all of this but I'd call this layout a typical installation. It's possible to have the HD Importer, Exporter and processor at the studio and feed the HD data as Ethernet to the transmitter and the analog audio as composite or AES over a microwave STL or T1, but then you're introducing various forms of network/WAN issues to the HD link. Nautel introduced Reliable HD Transport to help with that. I like having everything at the transmitter and controlling it via WAN. It's also possible to do the diversity delay in the Exporter but requires separate processors for analog and HD MPS, or with an external microprecision delay. It can also be done in the analog input section of a Nautel transmitter without adding the diversity delay function to the processor.
 
If you have two transmitters combined, and the exgine in the HD loses its mind, you can develop this condition. The cure is a silence sensor on the HD which kills the HD carriers if it runs silent for an appreciable amount of time. Since virtually all HD radios now switch to HD when there are carriers present and cannot easily be forced back to analog in the presence of HD carriers, some sort of monitoring is essential.
 
While not entirely on topic.. In SLC, there's Clear Channel and Bonneville who provide most of the HD radio in the market. I don't have much understanding on how the delay syncing happens. Most of the CC stations lose their sync quite often. On my radio, I hear a song or some programming and then it switches to HD-1 and it skips ahead. Sometimes more than a second or two. I've contacted the station and they pass it along and usually it's back in sync. Then by the next day, it's off again. The Bonneville stations are NEVER out of sync and the transitions between FM and HD-1 are seamless, sync-wise. What could be causing the trouble for CC vs. B'ville? I assume they are using somewhat similar equipment. But is there some monitoring or sync control systems that CC is opting not to use? Please explain. Thanks!
 
They sync issue is one we all deal with, we check ours once a month and re-aline as necessary.
Allot of stations are more short staffed and its one thing that can fall by the wayside until a listener notices and calls the station.
 
At my recent installation, we have a Harris CD Link STL. I'm sending analog audio into the STL transmitter and getting AES out of the STL that goes to our Omnia 9. The Omnia 9 offers up to three cores, so it processes our analog, HD1, and HD2 all in the same box. From there the analog composite goes to the Flexstar exciter. HD1 and HD2 goes via AES up to the importer computer, then it's all UDP packetized audio from the importer to the exporter and then the Flexstar exciter. I set up delay in the Omnia 9, bypassing any of it within the HDI-200. It has been VERY VERY VERY stable. The time sync never moves. I haven't had to reboot at all, ever. I did have an exciter issue (bad capacitor), but I still didn't have to really mess with the HD stuff. It just sits there and runs. One thing that might help this is that we are using a double-conversion UPS to run everything in the rack. I think that helps stop any power oddities that might upset things. I am also in agreement that putting everything at the tower is the best way to go. It stops all finger-pointing and issues that confuse a complicated technology (HD radio) even more than it has to be. Many of the choices I made in assembling and designing our setup come from listening to others moan about their HD transmission issues. I took mental notes and tried to avoid their early technology mistakes because I don't want to have to drive 100 miles round-trip to fix the damn thing LOL!
 
Is it a BE Importer/Exporter combo that's doing it? Been there, rebooted that.
 
bradgoehl said:
They sync issue is one we all deal with, we check ours once a month and re-aline as necessary.
Allot of stations are more short staffed and its one thing that can fall by the wayside until a listener notices and calls the station.
Click to expand...

Which stations are yours?
 
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