HD Radio Time & Level 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. Misaligned levels will cause a sudden 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. Experiment with a blend simulator to hear what various delay errors sound like.

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 so the values are more suggestive than definitive. 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 unlimiting: the signals have the same RMS level, but the peak level of the digital signal on top is 5.7 dB greater than that of the analog signal on the bottom. Peak unlimiting is not a measure of blend quality, but higher values identify digital signals that exploit the wider available dynamic range by relaxing the audio processing.

For the following measurements I used a ten-second sound sample (20 seconds 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 stations.


              Delay Level Peak Corre-
 Freq  Call   Error Error  Unl lation   Date          Organization              Region
  MHz           ms    dB   dB
 88.1  KKJZ   -15    -6.6  4.6 -0.86  08-13-13  Cal State Long Beach         Los Angeles
 88.3  KSDS   -62     0.7  1.0  0.94  08-13-13  San Diego City College       San Diego
 88.7  KQSC  -213    -1.8 -0.5  0.97  07-25-12  Univ of Southern California  Santa Barbara
 89.1  KUOR  7163     3.1 -4.2  0.68  10-23-13  University of Redlands       Inland Empire
 89.3  KPCC    -0.05 -0.7  2.3  0.93  08-13-13  Pasadena City College        Los Angeles
 89.5  KPBS    38    -0.1 -0.4  0.99  08-13-13  San Diego State University   San Diego
 89.9  KCRW     7    -0.5 -0.2  0.99  08-13-13  Santa Monica College         Los Angeles
 91.1  KDSC    91    -4.5 -0.4  0.98  11-19-12  Univ of Southern California  Ventura County
 91.5  KUSC    -0.59 -1.4  0.6  0.61  08-13-13  Univ of Southern California  Los Angeles
 91.9  KVCR     0.91  0.4  3.0  0.86  06-20-12  San Bernardino CC District   Inland Empire
 92.1  KSOQ   379    -2.1  2.9  0.95  08-13-13  Lincoln Financial Media      San Diego County
 92.3  KHHT    -0.18 -0.3  3.3  0.88  08-13-13  Clear Channel                Los Angeles
 93.1  KCBS     0.09 -0.7  3.1  0.96  08-13-13  CBS                          Los Angeles
 93.3  KHTS    -5     0.5  3.6  0.90  08-13-13  Clear Channel                San Diego
 93.9  KXOS     0.00  0.8  1.5  0.95  08-13-13  Emmis                        Los Angeles
 94.1  KMYI    -0.02 -1.0  4.0  0.82  08-13-13  Clear Channel                San Diego
 94.7  KTWV     1    -0.6  2.7  0.97  08-13-13  CBS                          Los Angeles
 94.9  KBZT    -0.50  1.4  3.9  0.96  08-13-13  Lincoln Financial Media      San Diego
 95.1  KFRG     0.36  0.2  1.4  0.98  08-19-13  CBS                          Inland Empire
 95.5  KLOS    -0.14 -1.2  4.6  0.95  08-13-13  Citadel                      Los Angeles
 95.7  KSSX     0.05 -2.1  5.4  0.92  08-13-13  Clear Channel                San Diego
 96.3  KXOL   186    -1.8  2.9  0.96  08-31-09  Spanish Broadcasting System  Los Angeles
 96.5  KYXY   -47     2.2  2.4 -0.54  08-13-13  CBS                          San Diego
 96.7  KWIZ    30    -4.4  2.0  0.92  08-09-12  Liberman                     Orange County
 97.1  KAMP    -0.02  0.1  4.3  0.96  08-13-13  CBS                          Los Angeles
 97.3  KSON    -2     0.8  3.4  0.91  08-24-13  Lincoln Financial Media      San Diego
 97.5  KLYY    47    -3.6  6.4  0.89  08-13-13  Entravision                  Inland Empire
 98.1  KIFM    -0.61  0.5  3.9  0.97  08-13-13  Lincoln Financial Media      San Diego
 98.7  KYSR    -3     2.3  1.6  0.91  08-13-13  Clear Channel                Los Angeles
 99.1  KGGI   188    -0.6  1.0  0.97  08-19-13  Clear Channel                Inland Empire
 99.7  XHTY 10002    -7.0  4.6  0.70  05-07-12  Uniradio                     Tijuana
100.3  KSWD     4     1.0  3.3  0.94  08-13-13  Bonneville                   Los Angeles
101.1  KRTH     0.05 -1.9  3.4  0.98  08-13-13  CBS                          Los Angeles
101.5  KGB     -0.23  0.2  3.5 -0.92  08-13-13  Clear Channel                San Diego
101.9  KSCA     0.18  0.4  3.5  0.94  03-06-13  Univision                    Los Angeles
102.5  XHUAN14001     2.4  0.6 -0.41  11-16-13  Inst Mexicano de la Radio    Tijuana
102.7  KIIS     0.07  1.2 -0.7  0.97  08-13-13  Clear Channel                Los Angeles
102.9  KLQV  1087     1.9  1.5  0.96  08-24-13  Univision                    San Diego
103.5  KOST    -0.07 -3.1  3.3  0.92  12-20-10  Clear Channel                Los Angeles
103.7  KEGY  -234     0.0  4.1  0.91  08-13-13  CBS                          San Diego
103.9  KRCD     2     1.3  1.6  0.95  01-02-09  Univision                    Los Angeles
104.3  KBIG    -3    -1.0  2.8  0.91  08-13-13  Clear Channel                Los Angeles
105.1  KKGO     0.16 -0.7  0.6  0.90  08-13-13  Mt Wilson FM Broadcasters    Los Angeles
105.3  KIOZ   -19    -1.2  1.7  0.96  08-13-13  Clear Channel                San Diego
105.9  KPWR     0.07  2.3  0.7  0.95  08-13-13  Emmis                        Los Angeles
106.3  KRRF   -55     0.6  0.1  0.98  07-25-12  Cumulus                      Ventura
106.5  KLNV    51     0.4  2.9  0.97  08-13-13  Univision                    San Diego
106.7  KROQ     0.00 -1.8  2.5  0.97  08-13-13  CBS                          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  8204     0.5  1.5  0.21  08-11-12  Uniradio                     Tijuana
107.5  KLVE     0.73 -0.2  2.5  0.98  08-13-13  Univision                    Los Angeles
107.9  KWVE    -0.91 -0.5  2.0  0.92  03-21-13  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               53         4         3.00      1.0     771.59     -0.5      2.3      0.89
Clear Channel     11         1         3.00      1.2      14.33     -0.5      2.7      0.92
CBS                8         1         0.23      0.6     -34.94     -0.3      3.0      0.91
Univision          5         0         2.00      0.4     228.18      0.8      2.4      0.96
Lincoln            4         0         1.31      1.1      93.97      0.2      3.5      0.95
Entravision        3         0        47.00      1.0      39.67     -1.3      3.8      0.94
USC                3         0        91.00      1.8     -40.86     -2.6     -0.1      0.85

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 systematic bias.


March 6, 201488108 MHz