A recent commentary article in Radio World addresses the issue of AM bandwidth. I believe there are hidden agendas.
In the North America, AM stations are assigned to frequencies from 530 KHz to 1700 KHz in 10 KHz increments (other parts of the world use 9 KHz increments). When a radio signal is amplitude modulated, sidebands are generated at the carrier frequency+modulating frequency and the carrier frequency-modulating frequency. For example, if a station at 1000 KHz modulates the transmitter with a 1 KHz tone, it will create sideband frequencies at 999 KHz and 1001 KHz.
Since stations are spaced 10 KHz apart, any modulation frequency in excess of 5 KHz results in sideband products spilling over into the adjacent channels spectrum. Years ago, the audio modulating AM transmitters was rolled off after 5 KHz to prevent interference with adjacent channels.
FM, with it’s capability of reproducing audio frequencies up to 15 KHz, began to compete with AM. The fidelity of AM became more important. Stations were allowed to modulate up to 7.5 KHz improving the fidelity. This did cause some interference to adjacent channels but normal wideband receivers were not selective enough to separate adjacent channels. Limiting modulation to 7.5 KHz still meant the adjacent channels spectrum was clean out to 2.5 KHz from it’s carrier allowing narrow bandwidth receivers to receive an adjacent channel without interference.
FM continued to gain market share. AM stations were allowed to go up to 10 KHz which means they’d splatter over an entire sideband of an adjacent channel resulting in objectionable interference even for a listener with a narrow bandwidth receiver. AM stations even boosted high frequencies to compensate for high frequency roll-off in receivers.
Recently, the NRSC performed limited listening test using only three receivers and thirty subjects. Adjacent channel interference used simulated noise instead of real program material. Simulated noise made it possible to bias the tests towards a listener preference for constrained bandwidth by exaggerating the resulting interference. These tests determined that there was no advantage to a 10 KHz bandwidth. I guess that’s why broadcasters pushed to go to 10 KHz from 7.5 KHz.
HD Radio, iBiquity’s digital radio system for AM, utilizes spectrum from either 5 or 7.5 KHz to 15 KHz from the carrier, depending upon mode. To use this system you must constrain the audio bandwidth of analog audio. Ibiquity’s system only achieves a data rate of 36 Kb/s with 7.5 KHz – 15 Khz and 56 Kb/s with 5 KHz – 15 KHz. This is very poor use of precious AM broadcast spectrum, the total bandwidth utilized by an hybrid iBiquity digital AM signal is 30 KHz.
The HDC codec is a multi-streaming version of HEACC, which is AAC+ with SBR, the same Codec used for XM satellite. I’ve read in the XM blogs, 48Kb/s provides high quality encoding, at 40KB/s the definition is lost, at 32Kb/s quality is seriously impaired.
SBR, spectral band replication, reduces encoding bandwidth by discarding high frequency audio detail. At the decoding end it replicates the lower frequency spectrum to replace the lost high frequencies. Most high frequency program material is harmonically related to lower frequencies. The problem is that the exact nature of the harmonic content varies greatly in music from instrument to instrument. Flute producing almost a pure sine wave with almost no harmonic content. A brass horn or stringed instrument is rich in harmonics. The decay envelope for instruments is not the same for harmonics as for the fundamental.
Ogg Vorbis encodes good quality stereo at 45 Kb/s, the whole spectrum, no need for SBR. It is an open format and iBiquity can not extract licensing fees.
DRM, Digital Radio Mondale, is able to achieve a 30 Kb/s in a 9 Khz spectral space using using QAM64 modulation of the CODFM carriers. 15 KHz of spectrum would yield 50 Kb/s which would suffice for full spectrum Ogg Vorbis stereo encoding. The DRM format is used for LW, MW, and SW digitial broadcasts in the rest of the world.
I am all for digital radio, just not 30 KHz wide inefficient hybrid proprietary digital radio with garbage audio . We can have high quality audio in half the bandwidth with a non-proprietary encoding scheme. The F.C.C. was mandated with the task of regulating the public airwaves in the publics interest. The AM version of HD radio is not in the public interest. A DRM digital stream and Ogg Vorbis encoding would utilize bandwidth more efficiently and provide higher quality audio. Receivers and transmitting hardware would be less expensive since no royalties would need be paid to iBiquity.