FM IBOC Processing/compression

[strangelove]

I know that the audio on a given HD channel is digitally compressed but how is that compounded by the existing processing at the station's studio and TX site? There's a station here that sounds great in analog but when the HD kicks in it sounds as though the music has been fed into a into an industrial press. Is there any sort of additional tuning that's done at the IBOC stage or is it just a passive input or is it just being affected by the way the studio/tx is set up?

 

[SUPERCASTER]

The 2 kinds of compression:

Audio- Reduction of audio dynamics including "limiting".

Data- Data bitrate compression to make audio files smaller and reduce the bandwidth necessary to send digital audio. Example= compressing audio "wave" files to much smaller "MP3" files and streams. This is "lossy" data bitrate compression, and does reduce audio fidelity.

HD radio uses lossy data bitrate compression.

 

[Play Freebird]

There are several possible explanations for the "industrial press" effect you're hearing on HD-1:

1) The station has chosen a very low HD-1 bitrate to make room for multicasting. Remember, there's only 96 kbps available to carry audio in the standard hybrid system, so quality of the main channel must be compromised to allow for HD-2 (and even more so for HD-3). New iBiquity software allows partitioning of 32-32-32 which is bad news for those who prefer quality over quantity.

2) The station's digital STL may include some form of lossy compression which is "fighting" with the HD codec. Meanwhile, the analog may be traveling through a linear PCM link (like CD recording, a more proper use of digital technology) or -gasp- an "old fashioned" composite FM STL, which doesn't generate these artifacts. Several months ago, our friend David Eduardo questioned whether any Top 100 Market station could still be running an analog STL (such as the Moseley 606, standard of the industry back in the 80s and 90s when FM stations were raking in big profits), but a survey of the FCC database shows these systems are still very common, and in most cases they sound fine.

3) The station may be using lossy MP3 compression to store its music on hard drive. You don't notice this when listening to the analog signal, but the addition of the HD codec causes the "dualing algorithm" problem.

4) The station may be driving its digital equipment with improper levels. If driven too hard, clipping will result in nasty digital artifacts, but if driven too low, the effective resolution decreases and the sound becomes "grainy".

5) Or perhaps the audio processing ahead of the digital codec is set up poorly. There are MANY things that can go wrong in these systems, they are not as simple as the old console-into-Audimax-into-Volumax-into-transmitter arrangement that we relied on back in the 60s.

About four years ago, I installed an FM HD transmitter for one of the local public stations and I recall the digital sounded very good at that time because the entire 96 k was dedicated to HD-1. We fed the digital exciter directly from the linear STL (Harris CD Link), while the analog exciter was fed by an Optimod 8200. The only processing ahead of the STL was an Aphex Compellor -- and with no additional dynamic crunching at the transmitter site, the digital channel retained much of the "open" quality of the music, and my overall impression was favorable. When my receiver blended from digital to analog, I could detect somewhat of a disparity in levels, but it wasn't really objectionable.

A year later, the station decided to add an HD-2 at 32k (which reduced the HD-1 bitrate to 64k) and I immediately noticed a decline in quality when my receiver switched from analog to HD-1.

We replaced the 8200 a few weeks ago with an Omnia 6 (set conservatively for this market) and the digital output of the Omnia now feeds the HD-1. It sounds better than most of the local commercial stations, but at this point I still prefer the FM analog.

 

[strangelove]

Thanks for the info On the very station in question today there was an ad for HD radio, I believe it was the 'emergency broadcast' spot, and, well, there was a ground buzz running throughout the entire spot. Not very good PR for the system that causes so much grief on AM.

 

[KB1OKL]

Thanks for the info On the very station in question today there was an ad for HD radio, I believe it was the 'emergency broadcast' spot, and, well, there was a ground buzz running throughout the entire spot. Not very good PR for the system that causes so much grief on AM.

I think I heard that ad today, it sounds either like Beavis or Butthead, something about needing 8 oz of HD everyday? Really moronic, these are as bad as the old "Radio with a boob job". Sounds like the same ad agency.

 

[Mike Walker]

I'm a longtime supporter of HD Radio, but if the bitrate for the HD1 channel is set too low, it can indeed sound quite a bit worse than the same programming in analog. Of course if the station does it right, it can also sound dramatically better. Sadly, the FCC didn't set minimum bitrate standards for HD1. They should have! Personally, I'd NEVER send out an HD1 signal at less than 64kbps, because there's a strong possibility that average listeners WILL prefer analog if you go too low. Besides, the point is supposed to be to offer enhanced, not degraded service!

 

[dumber than a box of hair]

There are several possible explanations for the "industrial press" effect you're hearing on HD-1:

1) The station has chosen a very low HD-1 bitrate to make room for multicasting. Remember, there's only 96 kbps available to carry audio in the standard hybrid system, so quality of the main channel must be compromised to allow for HD-2 (and even more so for HD-3). New iBiquity software allows partitioning of 32-32-32 which is bad news for those who prefer quality over quantity.

2) The station's digital STL may include some form of lossy compression which is "fighting" with the HD codec. Meanwhile, the analog may be traveling through a linear PCM link (like CD recording, a more proper use of digital technology) or -gasp- an "old fashioned" composite FM STL, which doesn't generate these artifacts. Several months ago, our friend David Eduardo questioned whether any Top 100 Market station could still be running an analog STL (such as the Moseley 606, standard of the industry back in the 80s and 90s when FM stations were raking in big profits), but a survey of the FCC database shows these systems are still very common, and in most cases they sound fine.

3) The station may be using lossy MP3 compression to store its music on hard drive. You don't notice this when listening to the analog signal, but the addition of the HD codec causes the "dualing algorithm" problem.

4) The station may be driving its digital equipment with improper levels. If driven too hard, clipping will result in nasty digital artifacts, but if driven too low, the effective resolution decreases and the sound becomes "grainy".

5) Or perhaps the audio processing ahead of the digital codec is set up poorly. There are MANY things that can go wrong in these systems, they are not as simple as the old console-into-Audimax-into-Volumax-into-transmitter arrangement that we relied on back in the 60s.

6) Someone whose cheese has slipped off his cracker is using analog processing, pushed beyond its limits, on the digital signal. Given the number of stations I hear whose HD operations are effectively an afterthought, it wouldn't surprise me a bit.

 

[Play Freebird]

Sadly, the FCC didn't set minimum bitrate standards for HD1. They should have! Personally, I'd NEVER send out an HD1 signal at less than 64kbps, because there's a strong possibility that average listeners WILL prefer analog if you go too low. Besides, the point is supposed to be to offer enhanced, not degraded service!

When digital radio was first discussed back in the '80s, that's what most of us hoped for -- a significant enhancement in service. It's sad to see how it actually turned out in the US; not only is IBOC's audio quality disappointing, but the hybrid system degrades analog reception. And it does nothing to help AM broadcasters offer the same night service as they provide in the daytime; the only fix for that problem is analog FM or the Internet.

I'm off to Germany later today for this weekend's Friedrichshafen electronics show/hamfest -- the "Dayton" of Europe (except it has a beautiful view of the Alps and the building isn't falling down):

http://www.hamradio-friedrichshafen.de/html/en/

I hope to listen to some digital radio while I'm over there at the show and get an idea of how the Eureka-147 system is working out. Will check in with all of you in a couple of weeks.

 

[Mike Walker]

Doesn't Eureka 147, being an older standard, use MPEG-2 compression, which is far less efficient than HDC, and sounds far worse at low bitrates???

 

[ajc_trw]

Doesn't Eureka 147, being an older standard, use MPEG-2 compression, which is far less efficient than HDC, and sounds far worse at low bitrates???

It uses MPEG* audio but they use layer 2 (mp2) rather than layer 3 (mp3) because it's less complex thus easier to encode and decode and is more robust in a broadcast environment. All of their stations started at 192kbit but quickly reduced to 128 to make room for more stations on each mux. Most agree that 192kbit is the minimum for good quality.

*IIRC MPEG-1 audio covers sample rates of 32, 44.1 and 48kHz. MPEG-2 covers 16, 22.05 and 24kHz for low bitrate use.