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Thursday, April 25, 11:24:44am [ Login ] [ Main index ] [ Post a new message ] [ Search | Check update time | Archives: 1, 2, 3, 4, 5, [6] ]

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Date Posted: Sunday, December 18, 02:04:06am
Author: Gus Katsoris
Subject: Re: HD Radio Future Comes to New York City
In reply to: Steve Green 's message, "Re: HD Radio Future Comes to New York City" on Saturday, December 17, 02:29:17am

The FM station can actually send more than three signals. This is fairly complex topic.

First, the analog signal which has been in use for decades can be partitioned into multiple signals. The most basic is the stereo signal that we're all used to.

For Stereo audio, the station transmits two signals. The first is a composite signal made up of the left channel audio added to the right channel audio. This is the normal mono audio which is compatible with non-stereo radios. A stereo radio however, needs one additional component to create the stereo audio from this "Left plus Right" blend.

The station broadcasts a second "piggybacked" signal which contains the information used to make the mono audio into stereo. It is sent on something called a subcarrier (you can't hear it, but it's always in the background). The subcarrier contains is the same music in a different format...it is sent with the left channel audio subtracted from the right channel audio.

Inside the radio, a bit of manipulation takes place to make stereo audio from these two signals (it's actually algebraic addition and subtraction, just like in 9th grade math class). The signals are added and subtracted and the result is separate left and right audio channels. In simple math terms, add (L+R) to (L-R) you get L+R+L-R=2L and then subtracting (L+R) from (L-R) you get L+R-L-(-R)=2R.

To properly decode the stereo information, receivers use a signal called a pilot signal (which is broadcast by the station). The pilot helps the radio to stay in synchronization with the station (not needed for mono radios).

Other subcarriers can carry all sorts of information such as extra music tracks, stock quotes, information on the CD/Arist being played etc. In the 70's there were several construction articles in electronic magazines which showed how to build a decoder for these subcarriers (called "SCA").

Now we have HD radio. Here in the US, it's a product of a company called Ibiquity. They did a terrific job of designing the digital radio system being introduced in the US. They found a way insert digital programming into unused portions of the FM station's channel.

In the trade, HD radio is called "IBOC" which stands for "In-Band/On-Channel".

What makes the Ibiqity system so nice is that it's compatible with existing analog radios and the old channel structure. Analog-only radios can coexist nicely with the digital broadcast.

The IBOC HD audio is supposed to be superior (I haven't personally heard it yet) and is said to be close to CD quality! (as a side note---I always have to remind myself that digital can sound really bad too, such as most digital cellphones or slow data-rate webcasts).

HD stations can subdivide the digital programs into "multicasts"...they can broadcast two or more programs simultaneously. You may have a choice of, a ballgame or news and music from one radio station. This is in addition to the analog broadcast.

There are several modes of HD radio (IBOC), Hybrid IBOC, Extended Hybrid IBOC, and All Digital IBOC. I think that stations are currently using the first mode (hybrid).

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