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. 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. 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 relax audio processing to exploit the wider available dynamic range. Some stations may trade level equality during blend for this audio quality enhancement.

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    -9     1.1  0.6  0.96  12-14-15  Cal State Long Beach          Los Angeles
 88.3  KSDS   269     0.7  0.5  0.96  12-14-15  San Diego City College        San Diego
 88.7  KDRW     0.05  0.2  0.9  0.98  06-21-15  Santa Monica College          Santa Barbara
 89.1  KUOR  7163     3.1 -4.2  0.68  10-23-13  University of Redlands        Inland Empire
 89.3  KPCC    -1    -1.3  5.1 -0.35  07-15-15  Pasadena City College         Los Angeles
 89.5  KPBS     0.48  0.1  1.6  0.99  12-11-15  San Diego State University    San Diego
 89.9  KCRW    -0.05 -1.9  0.1  0.99  12-11-15  Santa Monica College          Los Angeles
 91.1  KDSC    -1    -0.3  0.0  0.99  06-22-15  Univ of Southern California   Ventura County
 91.5  KUSC    -0.48 -0.5  0.1  1.00  12-14-15  Univ of Southern California   Los Angeles
 91.9  KVCR    -4    -0.3  3.2  0.92  06-13-14  San Bernardino CC District    Inland Empire
 92.1  KSOQ   -21    -3.2  4.9  0.95  12-14-15  Entercom                      San Diego County
 92.3  KRRL     7    -0.7  2.5 -0.59  05-30-15  iHeartMedia                   Los Angeles
 92.9  KXFG    -0.66  0.3  0.1  0.98  12-11-15  CBS                           Riverside County
 93.1  KCBS     0.00 -1.5  3.4  0.95  08-18-14  CBS                           Los Angeles
 93.3  KHTS    -0.02  0.3  3.0  0.93  12-14-15  iHeartMedia                   San Diego
 93.9  KXOS     0.02 -2.0  3.9  0.91  12-14-15  Emmis                         Los Angeles
 94.1  KMYI   128    -0.3  3.8  0.52  12-14-15  iHeartMedia                   San Diego
 94.7  KTWV     1    -0.6  2.7  0.97  08-13-13  CBS                           Los Angeles
 94.9  KBZT    -3     2.3  4.2  0.96  12-14-15  Entercom                      San Diego
 95.1  KFRG     0.52  0.3  0.2  0.99  12-14-15  CBS                           Inland Empire
 95.5  KLOS    -0.27  0.3  0.6  1.00  12-14-15  Citadel                       Los Angeles
 95.7  KSSX  -115    -2.8  3.9  0.94  12-14-15  iHeartMedia                   San Diego
 96.3  KXOL   186    -1.8  2.9  0.96  08-31-09  Spanish Broadcasting System   Los Angeles
 96.5  KYXY    45     0.5  1.4  0.52  12-14-15  CBS                           San Diego
 96.7  KWIZ    30    -4.4  2.0  0.92  08-09-12  Liberman                      Orange County
 97.1  KAMP    -0.02 -0.4  3.5  0.96  12-11-15  CBS                           Los Angeles
 97.3  KSON    -5     0.8  3.5  0.96  12-14-15  Entercom                      San Diego
 97.5  KLYY    -0.02 -1.9  5.8  0.94  05-26-16  Entravision                   Inland Empire
 97.5  KYGA     0.18 -1.6  1.4  0.98  09-29-15  Educational Media Foundation  Santa Barbara
 98.1  KXSN    -3     1.4  4.2  0.97  05-03-16  Entercom                      San Diego
 98.7  KYSR    34    -1.4  4.1  0.95  12-11-15  iHeartMedia                   Los Angeles
 99.1  KGGI   172    -0.1  0.9  0.97  05-26-16  iHeartMedia                   Inland Empire
 99.7  XHTY   -21     0.6  3.6  0.81  12-14-15  Uniradio                      Tijuana
100.3  KSWD     0.02  0.2  1.4  0.94  12-14-15  Entercom                      Los Angeles
101.1  KRTH    -0.02 -1.1  3.5  0.94  12-14-15  CBS                           Los Angeles
101.5  KGB      0.05  0.9  2.6  0.87  12-14-15  iHeartMedia                   San Diego
101.9  KSCA    -0.02  1.3  2.5  0.96  12-11-15  Univision                     Los Angeles
102.5  XHUAN15002    -3.1  0.0  0.42  12-14-15  Inst Mexicano de la Radio     Tijuana
102.7  KIIS     0.68  1.8  0.2  0.95  12-14-15  iHeartMedia                   Los Angeles
102.9  KLQV  9986     1.8  1.0 -0.94  12-14-15  Univision                     San Diego
103.5  KOST    -0.07 -3.1  3.3  0.92  12-20-10  iHeartMedia                   Los Angeles
103.7  KEGY -1346     1.8  2.5  0.90  09-29-15  CBS                           San Diego
103.9  KRCD     2     1.3  1.6  0.95  01-02-09  Univision                     Los Angeles
104.3  KBIG    -1     0.4  1.0  0.96  12-14-15  iHeartMedia                   Los Angeles
105.1  KKGO    -8     0.7 -0.3  0.90  05-30-15  Mt Wilson FM Broadcasters     Los Angeles
105.3  KIOZ     0.61 -0.8  2.6  0.92  12-14-15  iHeartMedia                   San Diego
105.9  KPWR     0.02  1.6  1.1  0.91  12-11-15  Emmis                         Los Angeles
106.3  KRRF   -55     0.6  0.1  0.98  07-25-12  Cumulus                       Ventura
106.5  KLNV  9217    -1.2  4.2  0.96  12-14-15  Univision                     San Diego
106.7  KROQ     0.00 -0.3  1.0  0.99  12-14-15  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     0.00  0.0  4.2  0.99  12-14-15  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               55         7         1.00      1.0     889.55     -0.2      2.1      0.89
iHeartMedia       11         2        34.00      1.8      20.57     -0.5      2.5      0.87
CBS                9         2         0.52      0.5    -144.46     -0.1      2.0      0.91
Entercom           5         1         3.00      1.4      -6.40      0.3      3.6      0.96
Univision          5         1         2.00      1.3    3841.00      0.6      2.7      0.96
Entravision        3         0         5.00      1.0      23.99     -0.7      3.6      0.96

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.


May 26, 201688108 MHz