Thursday, November 15, 2007

Measuring and logging HD Radio interchannel delay

The analog FM program audio and the main digital audio are supposed to be synchronized very closely to produce a clean blend between analog and digital when necessary at the receiver. With some work, you can synchronize these down to within a few samples. The problem now, depending on your installation, is keeping it synchronized. A lot of stations are finding that their digital signal drifts in time against the analog. The drift problem is being worked on, but for now, my concern is "How do I keep aware of how well I am synchronised?"

A ham operator and FM enthusiast, Brian Beezley (K6STI), wrote a command line program to cross-correlate the delay error between the two channels of a wave file and output the results to a text file. It seems logical to me that one could automate the recording of audio from a receiver in split mode to create many samples of audio for analysis. Then, another automated script could analyze the sample audio files and output the results in a statistical format. Since it doesn't have to be done in real time, all these data points (timestamped delay error values) could be saved and graphed, showing a nice graphical view of the drift. If you use a package like OpenNMS to collect and log the data, you could even get notification when it drifted outside certain limits. That is one way to keep on top of the drift and know how much it changes.


Anonymous said...

The program outputs results to the screen, not a text file.


John McMellen said...

Thanks for the correction Brian. I believe that in Windows it is possible to run a command line program in a way that can save the screen output to memory or a text file. Didn't mean to mislead anybody.