Using Am/mw Bands For Digital Radio?

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Using Am/mw Bands For Digital Radio?

2002-10-05 06:33:16

Not sure if anybody can answer this question, but it occured to me that if the AM band (520-1720Khz) were to be cleared of analog stations, the resulting band could be used for higher-quality digital streams, much like XM and Sirius are doing in the 4Ghz range. My question is: how do you convert analog, RF bandwidth (i.e. 10Khz/channel for AM stations) to bandwidth in bits? In other words, how many bps can be broadcasted over a small analog bandwidth like 5 or 10Khz? Obviously, since we are not talking analog anymore, musical content with frequencies up to 20Khz could be possible, which is not possible with AM (after the modulation components/carrier wave, only 5Khz is left for audio, resulting in the famous AM sound). I'm not sure how to convert analog bandwidth to data bandwidth. Anybody care to take a stab at this??

Using Am/mw Bands For Digital Radio?

Reply #1 – 2002-10-05 07:07:34

That's a very interesting question. There are a lot of variables here. Using digital the stations could be placed closer together without crosstalk. There would also be no requirement to stay with amplitude modulation. Or maybe a phase modulated sub-carrier. Error control (resending bad blocks) would not be an option. A good portion of the bandwidth might be needed for error correction codes. Hmmm. Looks like I really don't have answers just more questions. Hope someone knowledgable about digital radio step in here. Inquiring minds want to know.

Using Am/mw Bands For Digital Radio?

Reply #2 – 2002-10-05 07:38:42

Er, i think that for conversions from the digital to the analog realm, in that sense, you have to study the use of: Modulator/Demodulator (aka. modem )

How Modems Work

For example, the dialup modems can only use the bandwidth allocated for voice, which is tipically from something higher than 0 (do you remember the highpass value for phones?) to 3400Hz (the lowpass value).

If im not mistaken typical AM stations use something like 9~10Khz, more than twice that.

Of course we would be talking about unidirectional broadcasts, without no way of correcting errors (like a typical UDP broadcast over the net), so it better be very prone to them.

Using Am/mw Bands For Digital Radio?

Reply #3 – 2002-10-05 07:58:48

Hmmmm...in that case, let's say that we stuck with amplitude modulation for our carrier, with the same bandwidth and spcacing (10Khz in N America, 9Khz elsewhere). That leaves about 5000hz for data, which is not much more than a phoneline's 3400. Assuming 56kbps max. for a 3200Hz bandwidth, we would get ~82kbps with 5000Hz by using the same ratio. Any data correction would have to be very robust (as people have been pointing out, we're talking about a unidirectional feed), so if we use 1/4 of the bandwidth for error correction, we are left with about 60kbps. Looks like CT-accPlus (AAC+SBR) would come in handy here

This is all just guesswork....I'm sure somebody has more exact figures; it's just a matter of finding them. My prediction is that terrestrial radio will go this route eventually, including FM, whose stations have a much larger 200Khz bandwidth. Perhaps this could be used with SW radio also?

Using Am/mw Bands For Digital Radio?

Reply #4 – 2002-10-05 08:00:41

Quote

Not sure if anybody can answer this question, but it occured to me that if the AM band (520-1720Khz) were to be cleared of analog stations, the resulting band could be used for higher-quality digital streams, much like XM and Sirius are doing in the 4Ghz range. My question is: how do you convert analog, RF bandwidth (i.e. 10Khz/channel for AM stations) to bandwidth in bits?

Good question, but already been solved... Take a look at Digital Radio Mondiale (DRM, don't mix this abbreviation up with you know what):

http://www.drm.org

They do exactly this (using all analog radio frequencies below FM for digital broadcasting) with the use of AACplus (AAC combined with Spectral Band Replication) which gives them a near-FM sound quality with bitrates way below 64 kbps and a lot of available channels. This project is still in beta phase, so you can't tune into these broadcastings yet, as far as I know. But nevertheless there are lots of examples and explanations (fine documentation files about SBR in PDF format and so on) to be found on this website.

Using Am/mw Bands For Digital Radio?

Reply #5 – 2002-10-05 08:06:10

That's really cool..thanks for the info!!

And yes, I did feel a momentary shiver down my spine when I saw the URL was "drm.org"

{Edit: By the way, those samples are amazing for 15-20kbps. Perhaps what is most amazing is that once you move away from the analog broadcast, channel crosstalk, modulation noise, and interference go away. Comparing the digital version with the analog is like night and day, especially for the samples braodcast on the so-called "graveyard" frequencies from 1400-1500Khz where crowding is common}.

Using Am/mw Bands For Digital Radio?

Reply #6 – 2002-10-05 08:19:33

Quote

That's really cool..thanks for the info!!

And yes, I did feel a momentary shiver down my spine when I saw the URL was "drm.org"

The test examples for a listening experience can be found here:

http://www.drm.org/system/globsample.htm

I just downloaded No. 2 which was done with AAC+, I'm very curious how it will sound... The samples seem to have a reasonable size (about 450 KB) and are all transcoded to MP3, so everyone should be able to listen to them.

Using Am/mw Bands For Digital Radio?

Reply #7 – 2002-10-05 09:15:18

Uhm... i think that the Open Source community could bring us something like "OpenDRM" (well or "freeDRM") to us, using the Vorbis technology...
Any volunteer ?

Using Am/mw Bands For Digital Radio?

Reply #8 – 2002-10-05 10:04:18

OpenDRM? The university of Darmstadt is working on a GPL DRM decoder. They use FAAD2, which is ready for DRM with just a few modification.

edit: http://www.tu-darmstadt.de/fb/et/uet/fguet...vf/DRM/DRM.html

Menno

Using Am/mw Bands For Digital Radio?

Reply #9 – 2002-10-05 11:13:03

Quote

By the way, those samples are amazing for 15-20kbps. Perhaps what is most amazing is that once you move away from the analog broadcast, channel crosstalk, modulation noise, and interference go away. Comparing the digital version with the analog is like night and day, especially for the samples braodcast on the so-called "graveyard" frequencies from 1400-1500Khz where crowding is common.

Yes, they are... B) I've listened to No. 2-8 now (25 and 22 kbps) and am really flabbergasted. But appraising the bitrate you have to take into account that these are mono recordings, so a possible stereo broadcast would double the bitrate/bandwidth in need and we're talking about 48 kbps then.

And the test files are far from "transparent" of course (ABX'ed them 32/32 ), because sharp treble attacks in the voices of both speakers and in the music are showing those typical flanging artifacts. By the way, I already knew this sound from a weekly FM radio broadcast when a sports commentator is reporting from the football stadium in Hanover, but wasn't sure if it is AACplus (probably at an even lower bitrate) what I heard. But all in all this is really astounding and promising, too...

Using Am/mw Bands For Digital Radio?

Reply #10 – 2002-10-05 11:19:04

Tnx !

Using Am/mw Bands For Digital Radio?

Reply #11 – 2002-10-05 11:28:12

Quote

OpenDRM? The university of Darmstadt is working on a GPL DRM decoder. They use FAAD2, which is ready for DRM with just a few modification.

edit: http://www.tu-darmstadt.de/fb/et/uet/fguet...vf/DRM/DRM.html

Yeah... B) By the way, DRM starts an open field test right now where everyone interested can participate after registering in the pop-up window that comes up right after the initial homepage. Here's an excerpt from the PDF file that can be downloaded in that window, too:

DRM Software Radio Project

The DRM Software Radio, designed for private use, is a downscaled version of an existing, professional Fraunhofer receiver. Its features include: audio MPEG-4 AAC +SBR decoding, multimedia reception, selection of service and the possibility to log the reception quality (which can later be sent back to DRM). Its audio decoding library has been provided by Coding Technologies.

Radio enthusiasts may register their interest in the Software Radio Project at the DRM Booth during IBC 2002, or at the DRM Web site at www.drm.org. When the project launches in December, the web site will be set up to support and distribute the software for a 2-year period, and to analyse received DRM transmissions. It will also contain a discussion forum, worked examples of receiver modifications, an analysis section, and general information about the project.

Participants must meet the following technical criteria:
Operating system: Windows 2000, Windows XP, or Windows 98. An AT-compatible PC with 500 MHz Intel Pentium processor (or equivalent), 64 MB RAM, 50 MB free disk space,16-bit SoundBlaster (or compatible) soundcard that supports full duplex at 48 kHz sampling rate for input and output; the input must be without AGC (Automatic Gain Control); for example: Creative SoundBlaster Live! or "USB One" USB audio interface. Notebook soundcards are not always compatible, and a compatibility list will be published.

A front-end receiver is also required, an AOR7030 or another receiver which can be modified. The modification is the addition of an extra circuit board to produce an extra Intermediate Frequency (IF) of 12kHz. Several companies already sell these conversion boards. Participants will need a receiver with a 12kHz IF, because this frequency is within the range of PC soundcards. Many radio receivers have an IF of 455kHz, so the extra circuit board mixes this down to the soundcard range. The PC soundcard is used to analyse the DRM signal.

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