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