HD Radio Time & Level Alignment

If the analog and digital signals of an HD Radio station are offset in time, when analog blends to digital a listener will hear anything from a strange phasey sound to an echo to a momentary drop-out. A level difference 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 most program material.

A 0.1 ms error places the notches well above the midrange, but the response is still down 3 dB at 2.5 kHz, which may be noticeable. The maximum delay error spec is 0.068 ms.

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. 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. I wrote a cross-correlation program to measure delay error, level error, peak unlimiting, and correlation. I define delay error as 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 unrelated signals. The smaller the magnitude, the more analog and digital transmit processing differ. 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 maximum 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 unnoticeable for these signals.


                Delay Level Peak Corre-
 Freq  Call     Error Error  Unl lation   Date         Organization                    Region
  MHz             ms    dB   dB
 88.1  KKJZ       0.00  0.6  1.5  0.96  10-01-23  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      -2     0.6  3.7  0.98  10-01-23  Saddleback College                 Orange County
 88.7  KDRW       0.00 -4.3  4.7  0.96  09-23-23  Santa Monica College               Santa Barbara
 89.1  KUOR       0.00 -1.2  2.0  0.96  06-12-23  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       0.00  1.5  0.0  0.98  10-01-23  San Diego State University         San Diego
 89.9  KCRW       0.02 -1.4  2.1  0.98  10-01-23  Santa Monica College               Los Angeles
 91.1  KDSC       0.00  0.5 -0.1  0.99  08-22-22  University of Southern California  Ventura County
 91.5  KUSC       0.05  0.6  0.2  0.99  10-01-23  University 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 -1.1  3.6  0.94  10-01-23  Educational Media Foundation       San Diego County
 92.3  KRRL       0.00  0.7  2.0  0.95  10-12-23  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      -0.07  0.9  3.0  0.97  10-09-23  Audacy                             Los Angeles
 93.3  KHTS       0.45  0.1  3.5  0.93  10-01-23  iHeartMedia                        San Diego
 93.5  KDAY      -0.29  0.4  1.9  0.93  09-20-23  Meruelo Media                      Los Angeles
 93.5  KDEY      23     1.2  1.6 -0.91  02-11-19  Meruelo Media                      Inland Empire
 93.9  KLLI       1     1.0  2.5  0.95  02-14-19  Meruelo Media                      Los Angeles
 94.1  KMYI      -2    -1.6  3.1  0.96  10-01-23  iHeartMedia                        San Diego
 94.7  KTWV       0.00  0.1  3.3  0.96  07-15-16  Audacy                             Los Angeles
 94.9  KBZT      -0.14  0.4  3.8  0.95  10-01-23  Audacy                             San Diego
 95.1  KFRG       0.02 -0.1  0.6  0.98  10-01-23  Audacy                             Inland Empire
 95.5  KLOS     333     1.5  1.7  0.96  09-11-23  Meruelo Media                      Los Angeles
 95.7  KSSX       0.05 -2.8  7.1  0.63  10-01-23  iHeartMedia                        San Diego
 96.3  KXOL     186    -1.8  2.9  0.96  08-31-09  Spanish Broadcasting System        Los Angeles
 96.5  KYXY      -0.02  1.0  3.4  0.95  10-01-23  Audacy                             San Diego
 96.7  KWIZ      30    -4.4  2.0  0.92  08-09-12  Universal Church                   Orange County
 97.1  KNX        0.02 -0.9  3.9  0.96  10-01-23  Audacy                             Los Angeles
 97.3  KWFN       0.05 -0.3  3.3  0.97  10-01-23  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
 97.9  KLAX       0.02 -0.8  3.3  0.99  10-02-23  Spanish Broadcasting System        Los Angeles
 98.1  KXSN      -0.18  0.2  2.7  0.96  10-01-23  Audacy                             San Diego
 98.7  KYSR      -0.02  0.7  3.0  0.93  10-01-23  iHeartMedia                        Los Angeles
 99.1  KGGI       0.02  1.6  2.2  0.97  10-01-23  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     -37    -2.4  4.2 -0.58  10-10-23  Uniradio                           Tijuana
100.3  KKLQ     -12     0.6  1.8 -0.55  10-01-23  Educational Media Foundation       Los Angeles
101.1  KRTH       0.05  0.2  1.9  0.97  10-01-23  Audacy                             Los Angeles
101.5  KGB        0.02 -0.8  4.5  0.90  10-01-23  iHeartMedia                        San Diego
101.9  KSCA       0.02 -0.2  3.9  0.95  10-01-23  Univision                          Los Angeles
102.1  KLVJ       0.00  0.7  0.6  0.96  05-08-23  Educational Media Foundation       San Diego
102.3  KJLH       0.00  0.1  2.9  0.97  09-20-23  Taxi Productions                   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  0.4  0.8  0.99  10-01-23  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    -0.8  4.3  0.98  10-01-23  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 -0.1  5.0  0.92  10-11-23  iHeartMedia                        Los Angeles
105.1  KKGO      -0.02  0.3  0.7  0.90  10-09-23  Mt Wilson FM Broadcasters          Los Angeles
105.3  KIOZ      -0.14  0.4  2.1 -0.95  10-01-23  iHeartMedia                        San Diego
105.9  KPWR       0.00  0.0  2.7  0.95  09-20-23  Meruelo Media                      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       0.00 -0.8  2.8  0.96  09-23-23  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 -1.7  2.9  0.97  10-10-23  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               64         27         0.05      0.7     296.22     -0.2      2.4      0.91
Audacy            12          9         0.04      0.4       0.15      0.0      2.8      0.97
iHeartMedia       11          6         0.02      0.7      -0.15     -0.2      3.4      0.91
Univision          5          1         0.09      0.7     120.18      0.1      1.6      0.96
Meruelo            5          1         1.00      1.0      71.34      0.8      2.1      0.94
EMF                5          2         0.05      0.6      -4.99      0.1      1.8      0.78

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.


October 21, 202388108 MHz