Maintaining Proper HD Radio Time Alignment

A proactive approach is required to stay within spec
Author:
Publish date:
Social count:
0

Thus 2017 has arrived, and the New Year is always a good time to reflect both personally and professionally on what we can do better. One area on which the entire industry should focus more is the housekeeping of our HD Radio installations.

HD time alignment and blending issues continue to be a top consumer/listener complaint to automakers. There is a growing HD Radio receiver base out there, particularly in the automotive segment. Many of your listeners are listening with an HD Radio. It’s time we focus on providing them a better listening experience, and one of the most critical items the industry needs help on is time alignment.

Remember, most listeners will experience more than one or two blends. Particularly on the edge of coverage, listeners are constantly in and out of HD reception. This means the time alignment of a station is very important.

The last two years of research by many in the industry has taught us a lot of best practices when it comes to time alignment. I strongly suggest you review each station on which you have HD Radio and make sure you follow the best time alignment practices. Some go against concepts we learned over a decade ago. These topics were introduced at large at the 2016 NAB Broadcast Engineering Conference (see related article at http://tinyurl.com/rw-hddelay). Look for more on this topic soon; there will be information forthcoming from a variety of sources regarding best practices in more detail.

In the meantime, here are the biggest items found:

• The Exporter should be co-located with the Exgine exciter at the transmitter site, on an isolated network. Connect the 10 MHz cabling between the two devices (1 PPS as well if you have GatesAir/Harris equipment). Make sure the Exciter is configured properly to use the external 10 MHz or 1 PPS reference. An Exporter across an STL simply will not achieve the accuracy required to remain in specification alone; automatic correction is required.

• Use of a single, integrated FM+HD audio processor is very important. If separate processors must be used, use exact same model, firmware and presets. Separate, dissimilar processing between the FM and HD channels causes blending artifacts on stations that are otherwise perfectly time-aligned.

• Make sure the Exporter and the Exciter are on a UPS. Reboots of either device will cause time alignment jumps. Some Exporters and Exciters were pre-installed in transmitters and might not be on a UPS. Reconfigure the wiring, or relocate these devices outside of the transmitter to supply these devices with UPS power.

• Make sure you are on the latest versions of software for your Exporter, Exciter and Exgine interface, and also for your Importer.

AUTOMATED SOLUTIONS

If you haven’t already done so, start researching and devising a plan for automated time alignment. There are many products on the market that support automated time alignment. You might already own one and need only to update the firmware and pair it with a second product. There are also single-box, dedicated solutions that reduce the complexity of the install and are easy to maintain.

Across the board, manufacturers have stepped up to the plate to provide multiple approaches in solving this problem for the industry. Research what the best fit is for your stations. Automated time alignment is something every station should implement.

Know the specifications for HD time alignment. The official specification is that the analog and digital signals time alignment should be zero samples, plus or minus three samples. So the permitted range is –3 to +3 samples, or a range of 6 samples with the center of zero (0 ± 3). One sample refers to 1 out of 44,100 samples per second in the 44.1 kHz digital audio bit stream.

In my research, I have found most stations are out of tolerance. Yes, even in the largest markets, some of the biggest stations and brands you can think of have problems. It’s time for us as an industry to take this problem a lot more seriously then we have.

I presented some market-based research at the 2016 IEEE Broadcast Technology Symposium.

A TELLING STUDY

With the help of my colleagues, we started taking market-wide diversity delay readings. Essentially, the project involved locating new diversity delay monitoring equipment in a variety of locations across the country. These are located mainly in large and medium-sized markets. The idea was to learn as much as we could about diversity delay issues by looking at every station in a market. The results out there are pretty stunning.

The figures below were generated by the Inovonics Justin 808 Time Alignment Processor. For the purpose of this research, instead of the processor going in the air chain as designed, it was only hooked up to an FM antenna in each market. Every hour, we set it to seek up to the next available HD station, and the device would plot the results.

In a nutshell, we could look at the delay ecosystem of each station and plot it an hour at a time. So instead of looking at just one station for hours in the plots, we can look at the entire market. This allows us to monitor 24 stations per day.

The goal behind these graphs is not to embarrass any specific station, owner or market, so I have chosen to keep the market names and offending station labels off the graphs. This information is available and recorded by the device, but the goal here is to get everyone motivated to look at their own stations in a new light.

Image placeholder title

Fig. 1a

In Fig. 1a, this comprises a large Top 25 market of 18 HD Radio stations. The graph spans from midnight until 6 p.m. You will note we revisit the same station that was tuned in at the midnight hour (00:00–00:59) at the end of the graph (18:00–18:59).

This station has a distinct slope to it in both sides, indicating a drifting pattern. In that 17- to 18-hour time period, the station slowly drifted about 2,500 samples. The station has a serious problem, but like many is not equipped with a modern delay device to understand that they have a problem and how they could fix it.

There is also a station in the 17:00 hour that also has a drifting pattern. You’ll note that few stations have the blue line on or near zero samples. With the vertical Y-Axis auto-scaled to over 9,000 samples, you really can’t see the center of this where most of the stations indeed are not within specification.

Image placeholder title

Fig. 1b

Fig. 1b is the same market, same day, but the graph is zoomed in from 2 a.m. until 12:59 p.m., showing us 11 different HD Radio stations in this market. Now, the auto-scaling of the graph on the Y-axis is adjusted to +200 samples and –700 samples. You can see very few stations are still near the zero sample line. There are four stations with drifting as well.

In all, after reviewing all 18 stations in this market, only five were in specification. After consulting with the specific stations, we learned that the ones that were in tolerance are running automatic correction.

Image placeholder title

Fig. 2

Image placeholder title

Fig. 3

Fig. 2 is another large Top 20 market, a few thousand miles away. In the view I have highlighted, you can see one station (circled in green) with dramatically separate, dissimilar processing on FM vs HD. There are also three stations that have drifting issues (circled in red), and then there are two other stations a few hundred samples out of spec. Even those that appear to be close to zero samples in this graph are not in specification. All told, this market has 16 HD radio stations, only of which two are within specification.

Fig. 3 comes from a medium Top 75 market. Here, there are fewer HD stations — only eight in the market. None are in specification. You’ll note two adjacent stations are drifting and jumping quite a bit. The station circled in red is the same station, sampled three different times for an hour each. Similarly, the station circled in yellow is a different station, sampled three different times for an hour each. These stations have severe alignment issues that need to be addressed. But, the other stations in the market are also not in specification, although they are closer.

Image placeholder title

Fig. 4

Fig. 4 is a Top 5 market. Here things seem to be in a little better order than the others we have seen thus far. But, as you can see in the 16:00 hour, we have a station drifting (circled in red), and in the 11:00 and 12:00 hours we have two adjacent stations that are 425 and 1002 samples out, respectively. In this market, there are nineteen HD radio stations, only three of which are within specification.

Image placeholder title

Fig. 5

Image placeholder title

Fig. 6

Fig. 5 is also another Top 5 market, also with 19 HD radio stations. In this figure, we quickly can identify four stations with drifting problems (circled in red) and one with jumping problems (circled in yellow). In total, nine of the 19 are within spec. Overall, that is impressive, but with five of those, their engineer has been following the advice above, along with automated alignment tools to achieve these goals. So, it does pay off to follow the best practices and use the latest measurement and alignment tools to stay within specification.

Fig. 6 is a Top 20 market. Here you can see one station is 20,000 samples (or 0.45 seconds) out of alignment. There is another station circled in yellow and red that is seen drifting and jumping its delay. This market has 12 HD stations, five of which are in spec.

All told, while Fig. 6 is the only graph I’ve presented showing a station with a significant delay out of spec, the truth is that each market above had at least one station that was 20,000 or more samples out; many had two or three. I filtered them out so to present other trends.

Image placeholder title

Table 1

COMPLIANCE TAKES WORK

A summary of our findings is in Table 1.

It’s pretty scary out there. Unless you are already familiar with the latest research on this issue, it is more than likely that you have one or multiple stations under your supervision that are not in specification.

Follow the best practices above. Work on obtaining one of the newer precision devices that can measure diversity delay accurately. Come up with a game plan for your station(s).

HD Radio is here to stay; the penetration rate in new vehicles continues to increase. Millions of new HD Radios are introduced to the marketplace each month, only to be subjected to a broadcast environment that is far from friendly. It’s time to get serious about solving this issue.

A new year is upon us. I strongly encourage everyone to educate themselves more on this topic, examine your own stations and work on ways to fix this issue.

Alan Jurison is a senior operations engineer for iHeartMedia’s Engineering and Systems Integration Group. He also chairs the NRSC RDS Usage Working Group (RUWG). He holds several SBE certifications including CPBE, CBNE, AMD and DRB. His opinions are not necessarily those of iHeartMedia, the NRSC or Radio World.

Related