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/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 0.00 1.7 -2.0 0.96 02-22-22 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 KCSN 0.09 0.5 2.8 0.86 07-11-21 Cal State Northridge Los Angeles 88.5 KSBR 0.05 0.5 3.2 0.97 07-29-18 Saddleback College Orange County 88.7 KDRW -2304 -1.5 2.1 0.98 11-18-20 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 223 0.5 1.0 0.95 03-08-22 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 9 -3.5 7.0 0.97 01-26-22 Santa Monica College Los Angeles 91.1 KDSC 0.02 0.5 0.0 0.99 11-18-20 Univ of Southern California Ventura County 91.5 KUSC -0.05 0.6 0.2 1.00 07-29-18 Univ of Southern California Los Angeles 91.9 KVCR 0.00 -0.3 -0.1 0.97 12-14-21 San Bernardino Comm Coll Dist Inland Empire 92.1 KARJ 0 -4.5 1.2 0.99 02-12-19 Educational Media Foundation San Diego County 92.3 KRRL -0.05 -0.6 3.1 0.98 07-29-18 iHeartMedia Los Angeles 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 0.09 -0.7 3.7 0.91 02-12-19 iHeartMedia San Diego 93.5 KDAY -0.05 1.2 1.6 0.96 06-11-19 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 0.00 -1.5 3.1 0.94 02-12-19 iHeartMedia San Diego 94.7 KTWV 0.00 0.1 3.3 0.96 07-15-16 Audacy Los Angeles 94.9 KBZT 26 3.9 3.1 0.96 02-12-19 Audacy San Diego 95.1 KFRG 0.00 0.2 1.1 0.97 04-04-18 Audacy Inland Empire 95.5 KLOS 0.05 0.8 2.4 0.95 07-29-18 Citadel Los Angeles 95.7 KSSX -2 -3.3 5.0 0.65 02-12-19 iHeartMedia San Diego 96.3 KXOL 186 -1.8 2.9 0.96 08-31-09 Spanish Broadcasting System Los Angeles 96.5 KYXY -11 2.2 2.7 0.95 02-12-19 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 952 0.4 0.9 0.98 11-18-20 Educational Media Foundation Santa Barbara 98.1 KXSN 0.52 0.0 3.0 0.95 02-12-19 Audacy San Diego 98.7 KYSR 0.02 -1.3 3.9 0.95 07-29-18 iHeartMedia Los Angeles 99.1 KGGI 0.07 0.5 1.3 0.98 07-29-18 iHeartMedia Inland Empire 99.7 XHTY -41 -1.0 3.1 0.54 05-18-22 Uniradio Tijuana 100.3 KKLQ -0.25 0.5 3.3 -0.36 07-29-18 Educational Media Foundation Los Angeles 101.1 KRTH 0.02 -0.9 3.0 0.94 04-03-18 Audacy Los Angeles 101.5 KGB 3 -1.1 5.4 0.95 02-12-19 iHeartMedia San Diego 101.9 KSCA -52 0.7 3.1 0.97 03-17-18 Univision Los Angeles 102.3 KJLH 0.00 -0.5 3.2 0.93 08-03-21 Taxi License Corp Los Angeles 102.5 XHUAN14478 -2.9 3.2 -0.65 02-12-19 Inst Mexicano de la Radio Tijuana 102.7 KIIS 0.02 0.2 3.2 0.94 04-03-18 iHeartMedia Los Angeles 102.9 KLQV 683 2.0 1.9 -0.95 02-12-19 Univision San Diego 103.5 KOST -0.07 -3.1 3.3 0.92 12-20-10 iHeartMedia Los Angeles 103.7 KSON 4 0.4 0.6 0.94 02-12-19 Audacy San Diego 103.9 KRCD 209 0.5 1.9 0.97 04-23-18 Univision Los Angeles 104.3 KBIG -0.02 0.9 2.0 0.98 07-29-18 iHeartMedia Los Angeles 105.1 KKGO -7 0.3 0.0 0.85 07-12-16 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 0.02 0.1 1.0 0.92 03-17-18 Emmis Los Angeles 106.3 KALI 1211 -0.6 1.8 0.96 06-11-19 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 0.02 -0.9 3.0 0.96 07-29-18 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 6 -4.9 3.5 -0.28 07-14-16 Uniradio Tijuana 107.5 KLVE 697 -0.8 3.1 0.97 03-17-18 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 61 22 0.09 0.7 255.22 -0.2 2.3 0.91 Audacy 12 8 0.05 0.4 1.64 0.5 2.3 0.95 iHeartMedia 11 5 0.07 0.9 0.10 -0.8 3.2 0.92 Univision 5 0 209.00 0.7 291.00 0.6 2.5 0.96 Entravision 3 1 5.00 1.0 23.99 -0.4 3.4 0.95 Ed Med Fdn 3 0 0.25 0.5 317.25 -1.2 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.