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 Santa Barbara
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.
88–108 MHz