Audible blending artifacts, caused by timing and level misalignment between the analog and digital versions of the main channel audio signal, are the top complaint from auto manufacturers and consumers regarding the HD Radio listening experience. The majority of the problems can be traced back to individual broadcast stations. The Radio Technology Forum session at this year’s NAB Broadcast Engineering Conference (April 16-21, 2016, Las Vegas, NV) includes a paper, excerpted here, discussing some of the causes of drift and how those can be resolved. The paper is entitled HD Radio™ Diversity Delay Field Observations: The Need For Automatic Alignment, and is authored by Alan W. Jurison, iHeartMedia, Cincinnati, OH.
INTRODUCTION – the hybrid digital broadcasting solution deployed in the US, FM and AM IBOC, more commonly known as HD Radio from DTS, Inc, requires simultaneous transmission of main channel audio broadcast radio signals in both the analog and digital domain. A key component is that receivers can immediately acquire the analog signal and then transition (or “blend”) into the digital signal after it has acquired and buffered. The transition process is called blending. In order to have this blend be seamless and transparent to listeners, broadcasters employ delays on analog AM and FM signals to time align it to the digital. This delay is typically called “diversity delay” because the HD Radio transmission system uses a variety of techniques and buffering to add redundancy or diversity to the system for signal robustness.
This robustness and signal processing creates significant delays compared to the near-instantaneous analog transmissions, typically anywhere from 6–10 seconds depending on hardware, software, equipment settings, and data links in use. A properly configured AM or FM station broadcasting in HD will have a device in the analog air chain providing this delay.
MEASURING THE PROBLEM – while there have been a few commercially-available signal monitoring products on the market that would measure diversity delay, the measurements being taken were single point in time values. The premise behind these first products was an automated extension of the initial manual alignment process. Precision tools to monitor diversity delay in real-time started arriving on the marketplace in the last few years. These devices were made by well-known manufacturers in the broadcast monitor and receiver business. With these products, we had a value of what the diversity delay was at the exact moment we were observing the reading, as opposed to a single measurement at a point in time. But even equipped with continuous measurement products, the industry still did not have a very good handle on what was really going on.
“If you’ve been saying things like ‘…the station is off –0.0001 seconds… close enough,’ then you’ve been doing it wrong.”
TIME ALIGNMENT SPECS – the official specification from DTS is that the analog and digital signals time alignment should be 0 samples, plus or minus 3 samples. So the permitted range is –3 to +3 samples, or a range of 6 samples with the center of 0 (0 ± 3). One sample refers to 1 out of 44,100 samples per second in the 44.1 kHz digital audio bit stream. That is an extreme amount of precision required to get within specification. One sample is 22.7 μsec. 3 samples means within 68 μsec. Many broadcast monitoring products show the measurement in seconds and samples. If you’ve been measuring your diversity delay in seconds from any of these products and been saying things like “…the station is off –0.0001 seconds… close enough,” you have been doing it wrong. The best way to avoid these issues is to focus on the number of samples and stop looking at the time. The industry needs to be focused on samples, not seconds. And when you look at it that way, that level of precision is only available consistently if we automate the process.
EXAMPLES OF THE PROBLEM – research with these new measurement devices on time alignment is really in its infancy. We have only looked at a small percentage of implementations currently. As time goes on, more measurement devices are obtained and deployed, the industry will learn a significant amount of information. Two figures presented here illustrate how various configurations of the same equipment yield dramatic differences in diversity delay stability.
The first figure shows wide variations in the diversity delay shift over time. In this particular set up, everything was installed in what was considered at the time as accepted industry practice. It indeed was a near perfect configuration, the Exporter and Exciter were on the latest versions of software. The Exporter to Exciter link was across an Intraplex LAN bridging card on an isolated VLAN network. Wireshark captures showed the only traffic competing for bandwidth was the Exporter to Exciter data. Careful analysis of this particular system actually indicated this configuration allotted more bandwidth than what was typical. The data in this first figure is from 6am until 10am, or 4 hours; numbers along the y-axis are audio samples.
The diversity delay drift on this station was extreme. Impossible to maintain in spec by manual means.
This installation was lacking a GPS disciplined 10 MHz reference source at the transmitter site into the Exciter to assist with clocking. The station subsequently obtained a loaner unit which had a GPS antenna input and a 10 MHz BNC output, which was brought to the exciter. The exciter was reconfigured to use the external 10 MHz for clocking sync. You can see the dramatic results of adding the 10 MHz reference in the second figure.
HOW TO ADDRESS THE PROBLEM – For years, broadcast engineers have been working with DTS and transmitter hardware manufacturers to resolve the alignment problems discussed in this paper. Progress has been made, and some of the many common issues that cause drift have been addressed. As I have transitioned from a traditional broadcast engineer to an HD Radio implementation specialist for iHeartMedia, I have discovered there are just too many variables beyond the control of all the individual parties involved. The above items are just some most common issues that can cause drift. There needs to be an industrywide push for updated, precision and continuous diversity delay measurement systems and automatic corrective systems. iHeartMedia and other broadcasters have been asking for products and solutions in this area, and we have seen the industry respond in the last few years. At the April 2015 NAB Show in Las Vegas, we saw a large collection of solutions being introduced to the marketplace. Moving forward, many new products and integration efforts continue to be introduced.
This paper will be presented on Sunday, April 17, 2016 at 1:30PM PDT in room S227 of the Las Vegas Convention Center, and will be included in its entirety in the 2016 NAB BEC Proceedings, available at the NAB Show and afterwards, on-line at the NAB Store.
For additional conference information visit the NAB Show web page at.