Prism dScope Aids Search for Noise

User Report: Test instrument helps in pursuit of high-quality digital audio
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SAN ANTONIO I began working in radio in Oklahoma City in 1970 after more than eight years in the U.S. Air Force, which is where I got my basic electronics training. I have been with Cox Radio since 1977 and I am C.E. of seven stations including two AM directional stations.

After installing a wireless Ethernet bridge between our transmitter and studio we were hit with pops and glitches on-air once every few days — or worse, every few minutes. Debating how to make a test that would track down the cause became a hot topic in our engineering department.

Then we heard about the Wednesday Webinar series hosted by Prism Sound, which included a session covering this very thing: Test strategies for reliable, high-quality AES digital audio in HD Radio.

During the webinar we learned about proven test techniques and measurement instruments designed for this kind of job.

The dScope Series III digital/analog audio test instrument, designed and manufactured by Prism Sound, was the tool we needed to help find what was causing these elusive pops and glitches. Although the instrument can measure both analog and digital performance, we were interested in its comprehensive digital analysis capabilities. We used the instrument's digital measurement tools both on-air and off to help us get to the bottom of what was plaguing our broadcast.

Pop & click hunt

After we received the instrument we were impressed with how lightweight and compact it was. We immediately put it to work probing our signal chain.

Although it has the usual essentials such as signal generation and analysis of channel status, frame rate and the rest, we soon discovered that the dScope Series III had some powerful and unique capabilities.

This included a "Watchdog" function providing us with a comprehensive log of failures over an indefinite amount of time. This log consists of state changes such as biphase violations, eye narrowing and the like, as well as numerical results measuring our digital audio transmission path. The log also included any limits that we defined that were breached with a stamp of the date and the time of occurrence.

Now we had a way of putting those pops and glitches together with any digital audio technical hiccups occurring in our broadcast chain.

Another feature unique to the dScope Series III was the ability to determine whether the jitter in the chain was clock jitter stemming from a specific piece of equipment or intersymbol interference induced by our cable runs. This proved invaluable in determining and isolating the contribution each piece of equipment and its associated cable runs contributed to any jitter in our chain, another probable cause of those frustrating pops and glitches.

Out at the transmitter site we used Prism Sound's DSA-1 portable analyzer. This is a handheld, battery-operated instrument that makes getting around tight rack spaces easy. Like the dScope Series III, the DSA-1 analyzes and measures digital performance and has built-in automated tests for troubleshooting the integrity of digital audio transmission lines and equipment.

The Prism Sound dScope Series III and DSA-1 were invaluable in helping us narrow down the culprit causing our pops and glitches. After all this work did we solve the problem? No, but we sure proved what it wasn't.

We now believe the issue is with the way clock/timing packets are handled across the link. We are working with the two manufacturers to help solve this problem.

Paul K. Reynolds is a chief engineer with Cox Radio.

For information, contact Prism Sound at (973) 983-9577 or visit
www.prismsound.com.

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