Monitoring PPM for Online Streams


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Here's one way to monitor your online PPM signal.

If you engineer at any radio station in one of Arbitron's 48 PPM markets you are already familiar with Arbitron's PPM encoding and encoding monitoring systems. For those unfamiliar, PPM is a clever system that uses acoustical masking techniques to hide codes within a station's program audio for the purpose of continually identifying that broadcast station. While inaudible to humans, these codes are easily heard by the pager-sized Portable People Meters worn by Arbitron panelists for the purpose of generating ratings data. To a station in a PPM market, un-encoded audio is tantamount to being off the air. No codes, no ratings; a worst-case scenario that has already happened at least once and cost that station one whole month of ratings. Yikes!

Arbitron provides an encoding monitor for every encoded audio service; HD Radio, Web streams, AM, FM, etc. These monitors are your only source of confidence that your stations' audio is truly encoded. For this they must be connected to a reliable source of audio - a source representative of what a PPM panelist would hear; what their Portable People Meter will receive. This is a fairly simple and straightforward scheme for the over-the-air broadcast: connect a well-fed tuner to the input of the encoding monitor and connect the encoding monitor to a suitable alerting system. Repeat once for each station you need to monitor.

The scenario changes when you consider needing to monitor encoded Internet audio streams. For us, that adds six more encoded audio paths and leads to a puzzling predicament. For me, it wasn't enough to simply monitor what was hitting our streaming encoder. I wanted to make certain the PPM codes were intact after their long journey through processing, codecs, and distribution channels. Again, representative of what an actual Portable People Meter receives. This meant having to continuously feed audio from something that could tune in the streams.

Previous solution

My first solution was not a bad solution by any measure. I didn't want to run six computers full time so I bought several Roku Soundbridge M1001 network music players, had Middle Atlantic provide rack mount shelves with custom cut faceplates, mounted them and fed them to the encoding monitors. I thought this a well-designed solution and it worked reasonably well for several years. Unfortunately, Roku has discontinued the M1001 and I have found no satisfactory replacement. While the Roku is a reasonable unit for anyone wishing to use such an appliance it was none too happy being pressed into continuous service. Frequent attacks of silence and user interface freeze-ups had us restarting one or more of them almost daily. Not what you want feeding an alerting system. Remember Aesop's fable about the boy who cried wolf?

Six Radio? Sure! Pro applications running at once.

Six Radio? Sure! Pro applications running at once.


Recently our stations made several upgrades to the streams. Most significant was a switch to AAC encoding which the Roku M10001 couldn't decode. The time had come to replace them.

Clearly the need was for a compact, inexpensive and above all reliable source of audio to feed our streams to the PPM encoding monitors. My intention was to feed every encoding monitor from just one computer. The benefit would be low cost, lower power consumption, low space requirement, and single point management. For this to be viable several challenges had to first be met. Obviously a computer that could accommodate six simultaneous audio outputs would have to be found. I would need to find software that was not averse to "tuning" several streams and outputting them each through independent audio ports.

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