HD Radio Time Alignment
HD Radio stations are supposed to time-align their analog and digital signals. If they don't, when analog blends to digital a listener will hear anything from an echo to a strange phasey sound to a momentary drop-out. Correct time alignment makes listening much easier near the limits of digital coverage where a receiver frequently switches back and forth between modes.
A time-alignment error of just 1 ms causes the spectral notches shown above to appear during the analog/digital blend. This artifact is distinctly audible. A reasonable time-alignment error limit is 0.1 ms, which will place any delay-induced spectral notches well above the midrange. Mike Bergman and John Kean discuss time-alignment measurement issues in an NAB paper here.
I decided to measure the time alignment of every FM HD Radio station I could receive here in San Marcos, northern San Diego County. I had hoped to list errors in microseconds, but after tuning around a bit I realized that I had to relax my expectations. I rounded larger errors to the nearest millisecond and smaller values to 0.1 ms. Positive means that the digital signal lags the analog. I drove a computer sound card with a Sangean HDT-1X in split-audio mode, with the analog signal in one channel and the digital in the other. I used a cross-correlation program to measure the delay error and level match. Level match is the ratio of digital to analog RMS signal levels. The value varies with program content when the analog and digital dynamic ranges differ, as they do for nearly all stations.
Some of the errors below may be due to broadcast engineers who don't take signal alignment seriously. Others may be due to data transport clock slips that in some cases can unrecoverably undo a careful alignment.
Delay Level
Freq Call Error Match Date Organization Market
MHz ms dB
88.1 KKJZ -0.1 -1 3-04-08 Cal State Long Beach Los Angeles
88.7 KQSC 40 10-15-07 University of Southern California Santa Barbara
89.3 KPCC -13 3-04-08 Pasadena City College Los Angeles
89.5 KPBS -4* 0 3-04-08 San Diego State University San Diego
89.9 KCRW 4 -2 3-04-08 Santa Monica City College Los Angeles
91.1 KDSC 153 3-04-08 University of Southern California Ventura
91.5 KUSC -145 1 3-04-08 University of Southern California Los Angeles
92.3 KHHT 0.1 10-14-07 Clear Channel Los Angeles
93.1 KCBS 11 0 3-04-08 CBS Los Angeles
93.3 KHTS -29* 5 3-04-08 Clear Channel San Diego
93.9 KMVN -0.2* -1 3-04-08 Emmis Los Angeles
94.1 KMYI -14 3 3-04-08 Clear Channel San Diego
95.5 KLOS 0.0 -3 3-04-08 Citadel Broadcasting Los Angeles
95.7 KUSS 4 -3 3-04-08 Clear Channel San Diego
96.5 KYXY -31* -1 3-04-08 CBS San Diego
96.7 KWIZ >300 0 11-04-07 Liberman Broadcasting Orange County
97.1 KLSX 0.0 -3 3-04-08 CBS Los Angeles
97.3 KSON 0.2 2 3-04-08 Lincoln Financial Media San Diego
97.5 KLYY >300 -6 3-04-08 Entravision San Bernardino
98.7 KYSR 0.0 2 3-04-08 Clear Channel Los Angeles
99.1 KGGI 90 2 3-04-08 Clear Channel San Bernardino
99.7 XHTY >300 -4 3-04-08 Uniradio Tijuana/San Diego
100.3 KRBV 0.2 -1 3-04-08 Radio One Los Angeles
101.1 KRTH 3 1 3-04-08 CBS Los Angeles
101.5 KGB 29* 1 3-04-08 Clear Channel San Diego
101.9 KSCA -0.5 -1 3-04-08 Univision Los Angeles
102.7 KIIS 0.1 1 11-04-07 Clear Channel Los Angeles
102.9 KLQV >300 -5 3-04-08 Univision San Diego
103.7 KSCF 78 -1 3-04-08 CBS San Diego
103.9 KRCD -0.1 3 11-03-07 Univision Los Angeles
104.3 KBIG -0.1 1 3-04-08 Clear Channel Los Angeles
105.1 KKGO 0.1* 3 3-04-08 Mt. Wilson FM Broadcasters Los Angeles
105.3 KIOZ -0.8* -2 3-04-08 Clear Channel San Diego
105.9 KPWR 0.2 0 3-04-08 Emmis Los Angeles
106.7 KROQ 0.0 1 3-04-08 CBS Los Angeles
107.1 KSSD >300 -8 3-04-08 Entravision San Diego
107.5 KLVE 0.0 1 3-04-08 Univision Los Angeles
107.9 KWVE -0.8 -3 3-04-08 Calvary Chapel of Costa Mesa Orange County
* Analog and digital signals are out of phase. This will cause an audio drop-out during blend
when the channels are time-aligned.
More is here.
Updated March 18, 2008
