San Marcos, CA - Oct 15, 2007 - Brian Beezley, an engineer in southern California, has a passion for RF. He has reviewed several HD Radio receivers and has posted his research on his website. On Oct. 14 and 15, he took advantage of his location to investigate a common setting for HD Radio: the analog/digital delay.
The hybrid-mode signals should be time-aligned to make the blend from analog to digital a smooth one. When the time alignment is not set properly, the blending can be an unusual phase-shifting effect during the transition to a complete dropout. The severity of the effect depends on the mismatch of the signals.
Beezley posted his results online, and the details are not complimentary to many of the stations he received. His post is quoted below.
I decided to measure the time alignment of every FM HD Radio station I could receive here in San Marcos, northern San Diego County.
A reasonable time-alignment error limit is a few samples (less than 100 microseconds). This will place any delay-induced spectral notches well above the midrange. I had hoped to list errors in microseconds, but after tuning around a bit I realized I had to relax my expectations. Below are the errors I measured rounded to the nearest millisecond. Positive means that the digital signal lags the analog. I drove a computer sound card with a Sangean HDT-1X in split-channel mode, with the analog signal on one channel and the digital on the other. I visually compared waveforms and used a cursor to measure large errors. To measure errors below 100ms, I used a waveform normalizing and differencing program.
Note that a timing error of just 1ms causes pronounced spectral notches to occur at 500Hz, 1.5kHz, 2.5kHz, etc., during the analog/digital blend. This artifact is distinctly audible. Some of the errors below may be due to broadcast engineers who don't take signal alignment seriously. Others may be due to clock slips that in some cases can irrevocably undo a careful alignment.
* Analog and digital signals are out of phase resulting in audio drop-out during blend.
The results may be indicative of a wide-spread problem. There are several known cases of stations not properly aligning their signals. Beezley's report is posted at ham-radio.com/k6sti/roster.htm.