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Author Topic: HD FM Audio Drives Me Nuts  (Read 1327 times)

Offline KaySeeks

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HD FM Audio Drives Me Nuts
« on: December 10, 2019, 2043 UTC »
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.pdf

Quote
Employing 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?
Just somebody with a radio, a computer and a pair of headphones...

Offline redhat

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Re: HD FM Audio Drives Me Nuts
« Reply #1 on: December 10, 2019, 2115 UTC »
The HD and analog paths are processed very differently.  Any lossy codec as a rule does not like clipped audio, as it contains more harmonic energy that it must process, with varying success.  Instead, look-ahead limiting is used, but as a consequence, produces some intermodulation distortion.  The analog path is processed conventionally.  The codec for HD also makes use of spectral band replication, which can cause all sorts of wierd artifacts on its own.  It all depends on what data rate the station has set their importer/exporter up for, and this is largely determined by how many HD sub channels they have.

There is a long standing theory that American DAB (HD) has a self-noising property.  Some more detail can be found here http://ham-radio.com/k6sti/hdrsn.htm

Also within the last few years, the Arbitron PPM system has been toyed with by several manufacturers in an attempt to boost the host stations' ratings.  They do this by making the PPM signal louder, and in many cases unmasking the tones.  It seems to be most noticeable on dry voice, and makes the announcer sound as if they are in a broom closet, or talking through a paper towel tube.  The device responsible for this is made by Telos, called the voltair.  https://www.telosalliance.com/25-Seven/Voltair

It's a race to the bottom.  The voltairs running on large corporate stations here have made the stations unlistenable for me.  My favorite station here switched on HD about 18 months ago, and I immediately noticed their audio quality drop.  Everything got sort of fuzzy sounding.

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« Last Edit: December 10, 2019, 2121 UTC by redhat »
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Offline refmo

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Re: HD FM Audio Drives Me Nuts
« Reply #2 on: December 10, 2019, 2359 UTC »
I am glad to see its not just me.  I find it appalling that any broadcast engineer would accept such degradation in audio.
Have they not heard good audio??
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Offline KaySeeks

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Re: HD FM Audio Drives Me Nuts
« Reply #3 on: December 11, 2019, 0108 UTC »
There is a long standing theory that American DAB (HD) has a self-noising property.  Some more detail can be found here http://ham-radio.com/k6sti/hdrsn.htm

Quote
An HD Radio station's digital sidebands may interfere with reception of its own analog signal in two ways. First, FM detection of the sidebands may generate intermodulation products that overlap the analog signal. Interference also may occur when the stereo decoder demodulates the digital sidebands along with the stereo subchannel signal. Because the digital subcarriers are numerous and their data randomized, analog detection yields noise. This form of HD Radio self-noise is the subject of this article.

OK, but what I am hearing happens when listening to the digital signal, not the analog signal.

Also within the last few years, the Arbitron PPM system has been toyed with by several manufacturers in an attempt to boost the host stations' ratings.  They do this by making the PPM signal louder, and in many cases unmasking the tones.  It seems to be most noticeable on dry voice, and makes the announcer sound as if they are in a broom closet, or talking through a paper towel tube. The device responsible for this is made by Telos, called the voltair.  https://www.telosalliance.com/25-Seven/Voltair

OK, yes, what I hear is more noticeable with voice because here is less spectral content to clutter the air. Not sure about the closet, but maybe there is a little bit of paper towel tube there, yes, but only at high frequency. At lower frequencies there is no problem.

I'm still thinking it's more about artifacts of the restoration from digital.
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Offline JimIO

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Re: HD FM Audio Drives Me Nuts
« Reply #4 on: December 11, 2019, 0129 UTC »

Offline redhat

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Re: HD FM Audio Drives Me Nuts
« Reply #5 on: December 11, 2019, 0219 UTC »
If what your hearing exists only in the digital portion, then it is most likely a codec issue.  It could also be caused by sample rate conversion happening somewhere in the airchain.

PPM encoding happens mostly in the midrange frequencies, from about 1.5-4 KHz.  The exact mechanism is a trade secret.

Quote
am glad to see its not just me.  I find it appalling that any broadcast engineer would accept such degradation in audio.
Have they not heard good audio??

Usually that decision is made farther up the chain either in the management or programming end.  Engineering has little to do with it, outside of implementation.  Further, the majority of HD installations these days are being done to support feeding translators.

+-RH
« Last Edit: December 11, 2019, 0222 UTC by redhat »
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Offline BoomboxDX

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Re: HD FM Audio Drives Me Nuts
« Reply #6 on: December 11, 2019, 0507 UTC »
The HD and analog paths are processed very differently.  Any lossy codec as a rule does not like clipped audio, as it contains more harmonic energy that it must process, with varying success.  Instead, look-ahead limiting is used, but as a consequence, produces some intermodulation distortion.  The analog path is processed conventionally.  The codec for HD also makes use of spectral band replication, which can cause all sorts of wierd artifacts on its own.  It all depends on what data rate the station has set their importer/exporter up for, and this is largely determined by how many HD sub channels they have.

There is a long standing theory that American DAB (HD) has a self-noising property.  Some more detail can be found here http://ham-radio.com/k6sti/hdrsn.htm

Also within the last few years, the Arbitron PPM system has been toyed with by several manufacturers in an attempt to boost the host stations' ratings.  They do this by making the PPM signal louder, and in many cases unmasking the tones.  It seems to be most noticeable on dry voice, and makes the announcer sound as if they are in a broom closet, or talking through a paper towel tube.  The device responsible for this is made by Telos, called the voltair.  https://www.telosalliance.com/25-Seven/Voltair

It's a race to the bottom.  The voltairs running on large corporate stations here have made the stations unlistenable for me.  My favorite station here switched on HD about 18 months ago, and I immediately noticed their audio quality drop.  Everything got sort of fuzzy sounding.

+-RH

But do stations use Voltairs on their HD channels?
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Offline redhat

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Re: HD FM Audio Drives Me Nuts
« Reply #7 on: December 11, 2019, 0924 UTC »
Yes.  The main HD channel is encoded with the same PPM data and passes through the same enhancement (voltair) chain in most cases.

+-RH
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