HD Radio Self-Noise Levels
Nearly all stereo decoders use a 38-kHz square wave for demodulating the L-R subchannel, which lies between 23 and 53 kHz. An undesired consequence is that the waveform's fifth harmonic demodulates power near 190 kHz. HD Radio digital sidebands, which occupy spectrum from 129 to 198 kHz after FM detection, can cause an annoying audio background noise when demodulated by the fifth harmonic. Extended-hybrid HD Radio signals, whose detected spectrum may extend as low as 102 kHz, can cause additional noise when demodulated by the third harmonic at 114 kHz.
Calculations below yield the analog signal-to-noise ratio (S/N) due to this HD Radio self-noise for service mode MP1, which provides a 96-kbps program channel that may be subdivided for multicasting, as well as extended-hybrid modes MP2 through MP7 that offer additional channels at lower bit rates.
This is the RF spectrum of KUSC in Los Angeles on an HP 141T/8553B/8552B spectrum analyzer. The horizontal scale is 100 kHz/div and vertical is 10 dB/div. At the center of the screen is the analog carrier at 91.5 MHz. 50 to 100 kHz away are RDS and SCA subcarriers. 130 to 200 kHz out are the digital sidebands. The lower analog sideband of a weak station at 91.7 mixes with KUSC's upper digital sideband. To the right is a station at 91.9 with SCA.
This is the detected spectrum in a Yamaha T-1020 tuner, which uses two 250-kHz ceramic IF filters and a quadrature detector. The frequency span is 0 to 200 kHz and the analysis-filter bandwidth is 3 kHz. The leftmost pip is DC. The next pip is the 19-kHz pilot signal. Then comes a low-level RDS subcarrier at 57 kHz, followed by two SCA subcarriers. Finally, at 130 kHz is an abrupt rise in the background noise level. This is the digital signal, which extends to about 200 kHz.
I adjusted the trace so that the pilot was 20 dB down. Later I learned that the pilot was set at 8.5% deviation, the SCAs at 9.2% each, and RDS at 1.3%. Total subcarrier deviation was 19.7%. If peak carrier deviation was increased beyond 100% by 0.5% for each 1% of subcarrier deviation as permitted by FCC rules, it would be 109.85%. This leaves 81.65% for the program channel, putting it 19.7 dB above the pilot, or 0.3 dB below the top of the screen. The reference for FM tuner S/N is a maximum-deviation 1-kHz sine wave. The 75-microsecond audio deemphasis network attenuates this signal 0.9 dB. So the RF level corresponding to 0 dBr, the audio S/N reference level, is 1.2 dB below the top of the screen.
Rather than waiting for a perfectly quiet program segment to measure S/N, I calculate it by relating the measured spectrum to the digital sideband levels using the HD Radio signal specification. Each digital sideband is -23 dBc (dB below the analog carrier), so both sidebands incoherently sum to -20 dBc. Each sideband is 69 kHz wide. Therefore spectrum analysis with 3-kHz analysis-filter bandwidth should show the noise level 10*LOG(69/3) = 13.6 dB lower. Including 1.05 dB for undermeasurement in the HP 8552B due to average instead of RMS detection, the noise level is -20 - 13.6 - 1 = -34.6 dBc.
The detected noise near 190 kHz is about 38 dB below full scale, or -36.8 dBr. This noise level corresponds to -34.6 dBc. Therefore to determine the audible noise level, I set dBr = dBc - 2.2. This relation is valid for this tuner and signal near 190 kHz.
A stereo decoder with a squarewave oscillator waveform will demodulate noise near 190 kHz to audio with the fifth harmonic of 38 kHz. The audio is rolled off by the 75-microsecond deemphasis network. The digital sidebands are composed of OFDM subcarriers at -45.8 dBc spaced 363.373 Hz apart. I used these facts in a simple computer program to calculate the total deemphasized noise contributed by the subcarriers between 175 kHz, which demodulates to 15-kHz audio, and the last one at 198.402 kHz. The calculation yields -33.7 dBc.
A constant-deviation sine wave demodulates to the same predeemphasis level in the L+R and L-R passbands. Therefore no level adjustment is needed for the single-sided L-R response to noise. A square wave's fifth harmonic is 14.0 dB below the fundamental, so the MP1 noise level is -14 - 33.7 dBc - 2.2 = -49.9 dBr after stereo decoding and deemphasis.
How much additional noise does service mode MP2 bring? In this mode the digital sidebands extend down to 122.457 kHz. The third harmonic of 38 kHz at 114 kHz will demodulate the noise between this frequency and 129 kHz, which corresponds to 15-kHz audio, as -47.6 dBc. A square wave's third harmonic is 9.5 dB below the fundamental, so the demodulated and deemphasized noise level is -9.5 - 47.6 dBc - 2.2 = -59.3 dBr. This figure must be adjusted for the difference in IF-filter response between 190 kHz, where the 2.2-dB relation was derived, and the MP2 region centered at 126 kHz.
To determine the response difference, I fed two signal generators through a power combiner to the T-1020. I set the tuner and one generator at 98.5 MHz, modulated with a 9% pilot. I set the other generator at 98.69 MHz and adjusted its RF level until the detected signal at 190 kHz was about 20 dB below the pilot, roughly the level of the digital sidebands. Then I set the spectrum analyzer to 2 dB/div and measured the level change with the second generator at 98.626 MHz. I repeated the test with the second generator below 98.5 MHz and averaged the two readings. The result was 4.5 dB. This is the change in tuner response between 190- and 126-kHz offsets for a low-level, flat, additive RF input spectrum.
The adjusted MP2 result is -59.3 + 4.5 = -54.8 dBr. Summing the -49.9 dBr noise from the fifth harmonic yields -48.7 dBr total. This is 1.2 dB higher than for MP1.
Service mode MP3 extends the digital sidebands down to 115.553 kHz. Noise from this frequency to 129 kHz calculates as -38.8 dBc, which becomes -50.5 dBr when demodulated. IF response at 122 kHz was 4.6 dB higher than at 190 kHz, yielding -45.9 dBr. Summing the -49.9 dBr noise from the fifth harmonic gives -44.4 dBr total. This is 5.5 dB higher than for MP1.
Service modes MP4-7 extend the digital spectrum down to 101.744 kHz. Noise to 129 kHz calculates as -33.6 dBc, and demodulated noise as -40.7 dBr, including 4.6 dB greater IF response at 115 kHz. Total noise from the third and fifth harmonics is -40.2 dBr, 9.7 dB higher than for MP1.
HD Radio Analog S/N
MP1 49.9 dB
MP2 48.7 dB
MP3 44.4 dB
MP4-7 40.2 dB
The calculated figures depend on measurements made for a particular tuner, but they should be accurate to within a few dB for any tuner with two wideband ceramic IF filters, no postdetection filter, and a squarewave stereo decoder. This covers the great majority of home tuners and receivers made in the last 30 years. S/N could be a dB higher for signals with no SCAs. Narrower IF filters help a lot. I've seen a worthwhile reduction in MP1 self-noise when replacing 280-kHz filters with 230s, and a substantial reduction with 180s. I've never heard HD Radio self-noise in a car radio, perhaps due to the narrow IF filters commonly used. Finally, HD Radio self-noise disappears with the addition of a postdetection filter or 38-kHz harmonic canceller.
The S/N figures are for a full-deviation 1-kHz sine wave. RMS levels for uncompressed program material can be up to 25 dB lower. For stations with conservative audio processing, MP1 self-noise may be only 25 dB below the program level and distinctly audible. I've found HD Radio signals to be unlistenable on every tuner I've tried that lacked a special stereo decoder, narrow IF filter, or postdetection filter.
The stereo S/N of some high-quality tuners is in the low 90s. Thus S/N may degrade 40-50 dB when tuned to an HD Radio signal. An S/N loss of this magnitude is mostly hypothetical, though, because few stations today transmit with S/N so high. On the other hand, I can't imagine a station with S/N so low that HD Radio self-noise would be inaudible with a decent RF signal level.
Program to calculate HD Radio noise levels:
DEFINT I-N ' Variables in this range are integers fc = 1 / (2 * 3.141593 * 75E-6) ' 75-us deemphasis corner freq s = 363.373 ' OFDM subcarrier spacing in Hz FOR i = 175000 / s TO 546 ' 15 kHz below 5th harmonic to last subcarrier f = i * s - 190000 ' Freq decoded to audio x = f / fc a = a + 1 / (1 + x * x) ' Sum power through deemphasis filter NEXT i FOR i = 337 TO 129000 / s ' First MP2 subcarrier to 15 kHz above 3rd harmonic f = i * s - 114000 x = f / fc b = b + 1 / (1 + x * x) NEXT i FOR i = 318 TO 129000 / s ' First MP3 subcarrier to 15 kHz above 3rd harmonic f = i * s - 114000 x = f / fc c = c + 1 / (1 + x * x) NEXT i FOR i = 280 TO 129000 / s ' First MP4-7 subcarrier to 15 kHz above 3rd harmonic f = i * s - 114000 x = f / fc d = d + 1 / (1 + x * x) NEXT i PRINT 10 * LOG(a) / LOG(10) - 45.8 ' MP1-7 5th-harmonic noise PRINT 10 * LOG(b) / LOG(10) - 45.8 ' MP2 3rd-harmonic noise PRINT 10 * LOG(c) / LOG(10) - 45.8 ' MP3 3rd-harmonic noise PRINT 10 * LOG(d) / LOG(10) - 45.8 ' MP4-7 3rd-harmonic noise
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Updated April 1, 2008
