HD Radio Time & Level Alignment

If the analog and digital signals of an HD Radio station aren't time-aligned, when analog blends to digital a listener will hear anything from a strange phasey sound to an echo to a momentary drop-out. Misaligned levels can cause an abrupt change in loudness. Correct time and level alignment can make a signal much more listenable near the limits of digital coverage where a receiver may switch back and forth between modes.

A time-alignment error of just 1 millisecond causes these spectral notches to appear during the analog/digital blend. This artifact is distinctly audible on wideband program material.

A 0.1 ms error places the notches well above the midrange. But the response is down 3 dB at 2.5 kHz and the blend still may not be entirely transparent. The iBiquity delay error spec is 0.068 ms maximum.

Mike Bergman and John Kean discuss time-alignment issues in this NAB paper.

I decided to measure the time and level alignment of every FM HD Radio station I could receive in San Marcos, northern San Diego County. On my computer I recorded sound samples from a Sangean HDT-1X tuner in split-audio mode, with the analog signal in one channel and the digital in the other. Then I used a cross-correlation program to measure delay error, level error, peak unlimiting, and correlation. Delay error is positive when the digital signal lags the analog. Level error is the ratio of digital to analog RMS signal levels. Peak unlimiting is the ratio of digital to analog peak/RMS values, a measure of the relaxation of analog peak limiting. Both level error and peak unlimiting may vary somewhat with program content. Correlation ranges from 1 for identical waveforms to 0 for entirely dissimilar signals. The smaller the magnitude, the more the transmit processing differs for analog and digital. A negative value means that the waveforms match best when one is inverted. During blend such signals will suffer a drop-out when time-aligned and a momentary low-frequency loss when not.

This is the frequency response at mid-blend for a delay error of 0.1 ms and correlation = −1.

Peak/RMS for the digital signal on top is 6.1 dB greater than for the corresponding analog signal below. Peak unlimiting is not a measure of blend quality, but higher values identify digital signals that relax audio processing to exploit the wider available dynamic range.

For the following measurements I used a ten-second sound sample (20 seconds or more for very large delay errors). Highlighted stations meet the iBiquity delay error spec, have no waveform inversion, and have a level error magnitude of 1 dB or less. Transition between analog and digital should be transparent for these signals.


                Delay Level Peak Corre-
 Freq  Call     Error Error  Unl lation   Date           Licensee                      Region
  MHz             ms    dB   dB
 88.1  KKJZ       0.02  0.5 -1.1  0.96  08-20-22  Cal State Long Beach              Los Angeles
 88.3  KSDS    -690     1.7  3.7  0.88  10-27-16  San Diego City College            San Diego
 88.5  KSBR       0.09  0.7  3.1  0.98  08-20-22  Saddleback College                Orange County
 88.7  KDRW      53    -3.7  5.1  0.97  08-21-22  Santa Monica College              Santa Barbara
 89.1  KUOR       0.05 -0.6 -0.2 -0.95  02-22-22  University of Redlands            Inland Empire
 89.3  KPCC      -5     0.3  1.6 -0.47  08-21-22  Pasadena City College             Los Angeles
 89.5  KPBS      -0.02  0.5  0.1  0.99  05-18-22  San Diego State University        San Diego
 89.9  KCRW       0.05 -1.0  2.4  0.97  08-20-22  Santa Monica College              Los Angeles
 91.1  KDSC       0.00  0.5 -0.1  0.99  08-22-22  Univ of Southern California       Ventura County
 91.5  KUSC      -0.07  0.5  0.0  1.00  08-08-22  Univ of Southern California       Los Angeles
 91.9  KVCR       0.00 -0.3 -0.1  0.97  12-14-21  San Bernardino Community Colleges Inland Empire
 92.1  KARJ      -0.02 -0.7  2.7  0.98  08-18-22  Educational Media Foundation      San Diego County
 92.3  KRRL      -0.05 -0.6  3.1  0.98  07-29-18  iHeartMedia                       Los Angeles
 92.7  KYLA     -13     0.5  1.8 -0.49  08-19-22  Educational Media Foundation      Orange County
 92.9  KXFG       0.02  0.5  0.1  0.98  07-27-18  Audacy                            Riverside County
 93.1  KCBS      -0.05 -0.3  4.1  0.92  07-27-18  Audacy                            Los Angeles
 93.3  KHTS      -2    -1.1  3.9  0.94  08-18-22  iHeartMedia                       San Diego
 93.5  KDAY       0.02  0.4  1.8  0.96  08-18-22  Merueldo Radio Holdings           Los Angeles
 93.5  KDEY      23     1.2  1.6 -0.91  02-11-19  Merueldo Radio Holdings           Inland Empire
 93.9  KXOS       1     1.0  2.5  0.95  02-14-19  Emmis                             Los Angeles
 94.1  KMYI     -13    -1.4  4.5  0.92  08-18-22  iHeartMedia                       San Diego
 94.7  KTWV       0.00  0.1  3.3  0.96  07-15-16  Audacy                            Los Angeles
 94.9  KBZT      -0.77 -1.2  2.7  0.95  08-18-22  Audacy                            San Diego
 95.1  KFRG       0.00  0.3  0.4  0.98  08-21-22  Audacy                            Inland Empire
 95.5  KLOS       0.48  1.3  2.3  0.97  08-18-22  Citadel                           Los Angeles
 95.7  KSSX       0.02 -3.9  7.4  0.37  08-18-22  iHeartMedia                       San Diego
 96.3  KXOL     186    -1.8  2.9  0.96  08-31-09  Spanish Broadcasting System       Los Angeles
 96.5  KYXY       0.79  1.1  2.6  0.96  08-18-22  Audacy                            San Diego
 96.7  KWIZ      30    -4.4  2.0  0.92  08-09-12  Liberman                          Orange County
 97.1  KNX        0.07  0.2  0.8  0.94  02-22-22  Audacy                            Los Angeles
 97.3  KWFN       0.05  0.2  2.8  0.98  02-12-19  Audacy                            San Diego
 97.5  KLYY      -0.02 -1.0  5.4  0.93  04-04-18  Entravision                       Inland Empire
 97.5  KLSB       0.05 -0.3  1.0  0.94  08-16-22  Educational Media Foundation      Santa Barbara
 98.1  KXSN       0.66 -0.3  3.7  0.83  08-18-22  Audacy                            San Diego
 98.7  KYSR       0.00  0.4  2.3  0.95  08-18-22  iHeartMedia                       Los Angeles
 99.1  KGGI      -0.20  1.9  2.1  0.98  08-20-22  iHeartMedia                       Inland Empire
 99.1  KBUU-LP -278     0.7  2.9  0.96  08-16-22  Zuma Beach FM Broadcasters        Malibu
 99.7  XHTY     -38    -1.6  4.1  0.72  08-18-22  Uniradio                          Tijuana
100.3  KKLQ       0.27  0.6  1.7  0.95  08-22-22  Educational Media Foundation      Los Angeles
101.1  KRTH      -2    -0.9  0.5  0.93  08-20-22  Audacy                            Los Angeles
101.5  KGB       -2    -1.2  4.6  0.91  08-18-22  iHeartMedia                       San Diego
101.9  KSCA      -0.02  0.8  1.7  0.96  08-19-22  Univision                         Los Angeles
102.3  KJLH       0.00 -0.3  2.9  0.98  08-19-22  Taxi License Corp                 Los Angeles
102.5  XHUAN  14478    -2.9  3.2 -0.65  02-12-19  Instituto Mexicano de la Radio    Tijuana
102.7  KIIS       0.00  0.1  2.3  0.96  10-05-22  iHeartMedia                       Los Angeles
102.9  KLQV     683     2.0  1.9 -0.95  02-12-19  Univision                         San Diego
103.5  KOST      -0.02 -0.8  3.7  0.91  10-05-22  iHeartMedia                       Los Angeles
103.7  KSON      -2     0.3  0.4  0.99  08-18-22  Audacy                            San Diego
103.9  KRCD      -0.09 -0.2 -2.9  0.97  08-19-22  Univision                         Los Angeles
104.3  KBIG       0.05 -0.7  3.8  0.97  10-05-22  iHeartMedia                       Los Angeles
105.1  KKGO       0.05  0.2  1.5 -0.91  10-05-22  Mt Wilson FM Broadcasters         Los Angeles
105.3  KIOZ       0.07  0.7  1.2  0.96  02-12-19  iHeartMedia                       San Diego
105.9  KPWR       0.05 -0.7  3.2  0.97  08-19-22  Emmis                             Los Angeles
106.3  KALI    4217    -2.6  1.2  0.98  08-19-22  Multicultural Radio               Orange County
106.3  KRRF     -55     0.6  0.1  0.98  07-25-12  Cumulus                           Ventura
106.5  KLNV     -82     0.7  2.4  0.96  02-12-19  Univision                         San Diego
106.7  KROQ       0.07 -1.2  2.1  0.97  08-19-22  Audacy                            Los Angeles
107.1  KSSD      -5    -1.0  3.7  0.98  04-07-09  Entravision                       San Diego County
107.1  KSSE      77     0.7  1.2  0.95  08-11-12  Entravision                       Los Angeles
107.3  XHFG     110    -2.4  4.1 -0.66  08-18-22  Uniradio                          Tijuana
107.5  KLVE       0.02 -1.1  2.6  0.97  10-05-22  Univision                         Los Angeles
107.9  KWVE      -1     1.1  1.4  0.96  12-11-15  Calvary Chapel of Costa Mesa      Orange County


Organization    Number     Number     Median    Median     Mean      Mean     Mean      Mean
                  of        with       Delay     Level     Delay     Level    Peak     Corre-
               Stations  Transparent   Error     Error     Error     Error   Unlimit   lation
                            Blend     Dev ms    Dev dB      ms        dB       dB     Magnitude
All               62         25         0.09      0.7     301.16     -0.3      2.2      0.92
Audacy            12          6         0.07      0.3      -0.26     -0.1      2.0      0.95
iHeartMedia       11          6         0.05      0.8      -1.56     -0.6      3.5      0.90
Univision          5          1         0.09      0.8     120.18      0.4      1.1      0.96
Edu Media          4          2         0.16      0.6      -3.17      0.0      1.8      0.84
Entravision        3          1         5.00      1.0      23.99     -0.4      3.4      0.95

Median delay error deviation is the median delay error regardless of sign. Median level error deviation similarly ignores sign. Mean values include the sign and may reveal systemic bias.


November 8, 202288108 MHz