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The AM Digital Data Service
Mapping option 1:
Mapping option 2:
So naturally the question arises about the system performance with respect to an AM station's coverage. There is a trade-off in the system design between the strength of the ODFM sub-carriers (and thus the system’s robustness) and the potential for interference to analog program signal itself. With respect to the carrier levels shown in Table 1 (above) mathematically it can be shown that the BER for Pair 1 is such that data can still be passed on them while the analog audio signal-to-noise ratio is far less than what is typically considered acceptable (26dB). The QPSK pairs (2 and 3) are still useable at an audio SNR of 1dB. In other words, the listener will have given up on the station (due to noisy conditions) before the receiver is unable to decode the digital signals.
With respect to the same OFDM carrier levels shown in Table 1, how much interference (for lack of a better phrase) or noise will be noticed in the audio output of the receiver, by the typical listener? This is of obvious importance to an AM broadcaster interested in the capabilities of ADDS. According to the text of the report, "The proposed modulation for AM Digital Data Service will require digital sub-carriers transmitted under the analog modulation. These sub-carriers have the potential to generate noise on certain receivers tuned to the analog broadcast. Because the digital sub-carriers are transmitted in quadrature (complementary sub-carrier pairs) to the DSB analog audio signal, their effect on coherent AM detectors is theoretically null. A coherent AM detector will outperform the envelope detector."
Since an envelope detector is considered to be the worst case, the effects of the OFDM sub-carriers in the output of such a detector were analyzed. Again through mathematical analysis it is shown that (in an envelope detector) that the level of analog audio corresponding to 181.7Hz is -52dBc (which I presume means below the output of a single tone, amplitude-modulated at 100 percent). It is further assumed that, due to audio processing, that the program audio average is -13dBc. This then leaves an audio SNR between the 181.7Hz tone and the rest of the audio program of 39dB. Factoring in the difference between the human ear’s sensitivity to 181.7Hz to and 1kHz (15dB) you are left with an apparent SNR of 54dB. When Pairs 2 and 3 are also transmitted, more "noise" shows up in the output of the envelope detector; and since the frequencies of Pairs 2 and 3 are higher, their presence in the audio output of an envelope detector is more noticeable to the ear. In fact, the ear is about 6dB more sensitive at Pair 2 than it is at Pair 1, and about 9.5dB more sensitive at Pair 3 than at Pair 1. So in consideration of those factors, the transmitted levels of sideband Pairs 2 and 3 are lower than that of Pair 1. See Table 2 for the calculated transmission levels of the pair combinations that will cause an apparent SNR in the audio output of the envelope detector of 54dB. Notice you can also increase the levels of Pairs 2 and 3 for slight degradations in the overall SNR.
Without ever having heard an ADDS system on the air, I can't say unequivocally that I would be able to hear this noise, though I believe I could. However, the real question is whether or not the average listener could – and even if they could, would it be objectionable to them?
All the technology described herein is "heavily borrowed" from AM IBOC (according to the report). As such, it seems reasonable that transmission equipment, and perhaps more importantly, compatible receivers, could be readily manufactured and gotten in to the hands of consumers. One of the first things I notice with an RDS-capable receiver or an IBOC receiver (AM or FM) is the scrolling message, which, though simple, seems very necessary now. Without even a call-letter display, the vast majority of AM receivers seem very old-fashioned indeed.
Irwin is transmission systems supervisor for Clear Channel NYC and chief engineer of WKTU, New York. Contact him at firstname.lastname@example.org.
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