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.
According to Beezley, 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.” He also discovered that some stations are transmitting the analog and digital signals in opposite phase polarity. His results found that the time alignment can vary with the analog leading the digital and vice versa. His test setup included a computer sound card fed from a Sangean HDT-1X in split-channel mode with the analog signal on one channel and the digital on the other. He visually compared waveforms and used a cursor to measure errors greater than 100ms. For smaller errors he used a waveform normalizing and differencing program. He made the measurements between Oct. 14 and 19.
This should serve as a reminder to stations to verify the time alignment of the HD Radio signals. Even if they were set once, it is wise to check them again to ensure that the alignment has not drifted.