Digital Radio Mondaile

Digital Radio Mondaile is a new technology for broadcasting on medium wave (AM broadcast band) and short wave frequencies that looks interesting. It uses a form of modulation called Coded Orthogonal Frequency Division Multiplex which is claimed to be resilient to common forms of interference.

Click on the link above to read the details. The description of this modulation scheme sounds to me very much like the modulation scheme used by the old Telebit Worldblazer and Trailblazer modems except at RF rather than audio frequencies.

I wonder how this will work in practice with distant stations at night where you usually have several stations coming in at once and interfering with each other.

I’m not excited about the encoding scheme used for the audio, AAC compression followed by Spectral Band Replication.

I know a lot of people have good things to say about AAC, but I’ve compared to AAC to OGG and MP3, and of the three usually OGG is better even at substantially lower bit rates. There are rare instances where AAC does a better job, particularly things like picked acoustic guitar. For some reason OGG seems to suppress the fundamental relative to the harmonics leaving the instrument sounding hollow. But this only happens if there aren’t other simultaneous sounds. The beginning of the Beatles Here Comes the Sun is a good example of something OGG chokes on. On the whole though OGG does better than the other two even at much lower data rates.

Then the other part of that is pretty ugly too. Spectral Band Replication, that’s a buzz word for not transmitting the high frequencies and then trying to guess where they should have existed based upon the lower frequencies present. The idea is that most higher frequencies are harmonically related.

I have an audio processing program that is useful for trying to make really bad or mangled recordings more tolerable, things like scratch removal, hiss removal, and subtractive noise removal. It also has this spectral band replication capability; and if you take a really old recording where everything over 7 Khz or so is rolled off, it does make it sound “better”, but it doesn’t necessarily sound like it is supposed to.

In real music, while it is true that most high frequencies are harmonics of lower frequencies in the source, the relative intensity of the harmonics is not fixed, nor is the envelope of the harmonics necessarily the same (and more often than not it is not) as the fundamentals.

Think of a gong. You first hear the lower frequency fundamental tones prominently. Harmonics are present but as time progresses the fundamentals fall off at a higher rate than the harmonics and they seem to peak a second or two after the gong is struck. This is the sort of thing that spectral band replication is not good with.

I would have rather seen them go with OGG, and if they had they might have been able to transmit the high frequencies and still stay within the target bit rate.

Still, if it resurrects AM and makes it usable for the transmission of music again, then that’s a positive thing.

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