Friday, December 24, 2010

Ibiquity proposes "AM RDS"

Ibiquity (the HD Radio/IBOC folks) and the NAB have released an "AM Digital Data Service System Study Report". This report proposes a data system for AM radio similar to the RDS system available on FM.

Read the report here.

THIS SYSTEM SHOULD **NOT** CREATE ADJACENT CHANNEL INTERFERENCE.

(hopefully the above sentence will quell any wild rumors of ADDS being the death of AM DX(grin)!)

The ADDS proposes to provide:

- Station Message Service.
- Station Name (call letters) up to 8 characters.
- Station Message up to 158 bytes. "Check us out at http://www.wzmf.com", something like that.

Presumably the SMS is for information about the station itself. It can change (there are provisions for notifying the receiver the Station Message has changed) but probably not very often.

- Program Service Data.
- Title
- Artist
- Album
- Genre
- Content
- Commercial

The first four are probably familiar to anyone who's worked with CD-ripping software or MP3 files. Ibiquity proposes to broadcast the ID3 tags for a musical selection. Tags are valid up to 1,024 bytes. The proposal calls for the commercial tag to contain:

- Price
- Date until which the price is valid
- URL (web address to locate the product online)
- Type of delivery (some products, like music downloads, can be delivered online -- others may require visiting a bricks-and-mortar store)
- Name of seller
- Description of product
- *picture* of product (in 1,024 bytes???)
- Logo

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This system does NOT propose to extend the station's signal beyond its current bandwidth. If the station is already using IBOC, it will continue to occupy the bandwidth it already does. If the station is NOT using IBOC, adding ADDS will NOT widen the station's occupied bandwidth.

Three sets of data carriers are proposed. Only the first set is obligatory; the other two sets are optional but will greatly increase the speed at which data can be transmitted. (at the expense of potential self-interference -- it is possible adding ADDS will cause the station to interfere *with itself*.)

The obligatory set of carriers is at 181.7Hz either side of the main carrier/station's quoted frequency. Since few AM receivers have much response below 300Hz this should be fairly well rejected. These carriers will be 26dB below main carrier.

The optional carrier sets are at twice and three times this separation -- 363.4 and 545.1Hz. These will be 40 and 45dB respectively below main carrier. This gets into territory not rejected by the receiver's filtering -- but in most cases 40dB attenuation should be enough to make them inaudible below ambient noise levels.

With just the obligatory carrier set, a data rate of 91.5 baud should be possible. If the optional carrier sets are added, up to 1,098 baud should be possible.

The system is designed to be compatible with IBOC.

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At this point this is simply a study. It's not (yet) part of the standard, let alone ready for any station to broadcast it, let alone for any receivers to be available.

IMHO this is good news for the AM DXer. It is essentially "RDS for AM". Many American FM DXers (myself included) have found FM RDS indispensable for identifying stations running continuous classical music/NPR talk shows/etc... European FM DXers don't believe there are still American FM DXers who *don't* use RDS! (it seems just about every FM DXer in Europe does) It does run the risk of generating additional self-interference, but that's not going to make things any worse for us.

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