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 transparent 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 -0.25 0.2 1.0 0.58 09-22-24 Pasadena City College Los Angeles
89.5 KPBS -0.36 1.4 -0.1 0.98 07-19-24 San Diego State University San Diego
89.9 KCRW 0.00 -4.3 7.2 0.96 09-22-24 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 -0.02 1.6 0.5 0.98 06-29-24 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 0.11 0.6 1.7 0.97 07-19-24 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 26 0.05 0.7 295.82 -0.2 2.5 0.92
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.
88–108 MHz