Perhaps some of the folks with broadcast industry experience can comment on this.
I note on good receivers that FM HD audio has some annoying high-frequency (~10-20 KHz) residues or artifacts. It almost sounds like a reverb at those frequencies and is more noticeable with speech than with music.
If I turn off HD mode on the receiver, I get a lot less of this (but it's still there to some degree) but regardless I still end up turning the treble all the way down because these artifacts drive me nuts. I have heard this on multiple receivers. I do not hear the same artifacts on non-HD stations. I can say that it may depend upon the transmitter being listened to but not 100 % sure on that.
I don't know the exact source of this. It might be due to phase shifts in that frequency range but I am not sure. (I wonder what a group delay plot of the audio spectrum on HD modulated audio looks like.) HD audio is supposed to be flat and not use the high-frequency pre-emphasis used in analog FM so, in theory the group delay before the encoder would be kinda flat (...but after restoration to analog at the receiver it might look
very different). I'm guessing that it is some artifact of the audio sampling and the bit-rate stream, perhaps aliasing issues. I did some searching but was unable to come up with a discussion of something similar. I did find this document on HD audio processing and alludes to artifacts but doesn't get into specifics:
https://www.telosalliance.com/images/Omnia%20Products/White%20Papers/Audio_Processing&HD-Radio.pdfEmploying a clipper as a peak limiter in a HD Radio system will work, but there are sonic penalties to be paid. Any clipping process yields harmonics of the fundamental source signal, and even with distortion masking some second order harmonic content will remain. This adds to the audio spectrum and aggravates the encoder, which in turn spawns additional sonic artifacts. This is particularly noticeable in the high-frequency range – where most codec artifacts exist – and is very noticeable with certain program material; therefore, another form of peak limiter is needed.
(Bottom of page 2, right column)
Does anyone recognize these artifacts and know the origin?
Show Posts
