Is pre-emphasis used on HD FM?

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kcbooboo
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Is pre-emphasis used on HD FM?

Post by kcbooboo » Mon Dec 22, 2014 7:30 pm

I know that 75usec pre-emphasis is standard on FM analog audio in the US. Most mono transmitters and exciters have this. Composite audio processors usually provide it as an option, and it's probably defaulted to ON.

What about digital HD? I realize that the digital audio doesn't have to be pre/de-emphasized, but is it done anyway?

Bob M.
Last edited by kcbooboo on Tue Dec 23, 2014 7:20 am, edited 1 time in total.

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BroadcastDoc
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Re: Is pre-emphasis used on HD FM?

Post by BroadcastDoc » Mon Dec 22, 2014 8:25 pm

Nope - just plain old flat audio!
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kcbooboo
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Re: Is pre-emphasis used on HD FM?

Post by kcbooboo » Tue Dec 23, 2014 7:22 am

Thanks. That makes sense but who knows what the IBOC folks threw in just to make it proprietary.

So an HD FM receiver would have de-emphasis in the analog demodulator but NOT on the HD channels.

Bob M.

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Re: Is pre-emphasis used on HD FM?

Post by BroadcastDoc » Tue Dec 23, 2014 10:35 am

Correct - there's no analog component to speak of. HD sends a constant stream of data that could be audio, picture, data, whatever. The radio then decodes the data stream.

So the station multiplexes the data: MPS (Main Program service - HD1) along with PSD (Program Service Data - title/artist and transmission information) and, if you're sending it, SPS (Supplemental Program Services - HD Sub channels, data services such as traffic info and album art) and sends it off as a single data stream. Then the receiver decodes the stream and separates the services back out again for the user.

Pre-emphasis was devised to reduce static in analog audio by pre-emphasizing the audio on transmission and de-emphasizing it (and the static) on the receive end. It was simply to make the audio more pleasing to human ears after analog transmission. With HD, humans aren't listening to any sort of analog reproduction - they're listening to data decoded from a stream. Therefore there's no benefit from pre-emphasis.

I'm sure you knew a lot of this already - just including it for those who might not!
Christopher "Doc" Tarr CSRE, DRB, AMD, CBNE
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