The HD Radio system has an inherent audio delay due to the processing time required in the receiver. Because of this delay, when a station operates in hybrid mode, the analog audio must be delayed to match the digital audio signal so the blend function from analog to digital and back is smooth and transparent. In October, Brian Beezley, an engineer in southern California, applied his passion for RF to evaluate the current state of HD Radio time alignment for the more than 30 stations that he is able to receive at his home. The stations cover the Los Angeles and San Diego markets.
When the time delay is not set properly, the resulting effect can be an annoyance to the listener. When the difference is small, a comb filter effect is applied to the audio. In extreme cases — like that when there is no time delay at all — the digital transition will repeat the previous 8 seconds of audio.
Beezley's results did not show well. A table showing the measured variations is posted on his website at ham-radio.com/k6sti/roster.htm.