Note: All these files has been "compressed" with help of flac.
This doesn't mean that the information as been lost. The benefit of using flac is that the file can be
decompressed _exactly_ as it was before you compressed the file!
Flac stands for: Free Lossless Audio Codec.
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You would only like to transmit using a bandwith of either 18Khz or 20Khz.
- 18Khz of bandwith:
QAM Codeing rate kbit/s 16 0,62 34 64 0,60 49 64 0,78 ?
- 20Khz of bandwith:
QAM Codeing rate kbit/s 16 0,62 38 64 0,60 55 64 0,78 72
Note: Robustness mode A is assumed. The prefered bandwith is 20Khz and to use QAM-64 with an codeing rate of 0,6.
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Well, it should be possible and quite easy too. All you need to start transmitting in DRM is this:
- First an soundcard which samples your audio (48Khz 24Bits Stereo).
- Second, encode that audio with vorbis (OGG) (We don't like patents, now do we?)
- Third, Now that we have our audio in a compress format, we must encode it using the same
method as we would if we were to transmitt on AM, that is, encode the bitstream as QAM-16 or QAM-64.
(And possible some other "headers" that should exists, like station name etc.) - Forth, Output the QAM stream to the transmitter, on the MPX input, from a soundcard
That's it! Your're now transmitting in digital mode on FM!
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Let's first look into the FM signal itself. We have today:
- 0Hz - 15Khz Mono signal (L+R)
- 19Khz Possible pilot tone, if your're transmitting in stereo
- 23Khz - 38Khz Left channel information (L-R)
- 38Khz - 53Khz Right channel information (R-L)
- 57Khz - 63Khz Possible RDS Data
What we can do is that we skip the stereo part and get an "empty" space between
23Khz and 53Khz, which we can use to transmit our digital data on.
If we use this "compability mode", all existsing radio can still hear your station,
that is, in mono, and even have the RDS information that you transmit today.
In the other mode, digital mode, we would transmit our digital signal as the only information.
The RDS information should be moved into the header of the bitstream.
Here we get can get 60Khz of bandwith for our digital signal.
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Now we can talk about the bitrate, that is, how much (data) information we can transmit at once.
- We shall first start with the compability mode (30Khz of bandwith):
QAM Codeing rate kbit/s 16 0,62 57 64 0,60 82 64 0,78 108
- And this is the digital mode (60Khz of bandwith):
QAM Codeing rate kbit/s 16 0,62 114 64 0,60 165 64 0,78 216
- I think it should be possible to have an digital mode with 80Khz of bandwith:
QAM Codeing rate kbit/s 16 0,62 152 64 0,60 220 64 0,78 288
Note: Robustness mode A is assumed on all examples and the bitrate is also here as an example!
This depends on which method you would like to use and also, which of QAM-16 or QAM-64 you would like to use.
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Well, that depends on how much bitrate you have and also the quality of the sound that is encoded
you would like to use. I should probably only have one radiostation.
If you would like the hear how it would sound, you could download the oggencoder from vorbis and encode
your sound files with the following command:
oggenc -M "bitrate from the table above" yoursoundfile.wav
Now listen and judge for your self. You will probably be amazing how "good" it does sound.
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If you look at the bandwith that DAB use, 1.5 MHz, you can have 8 digital channels
within this bandwith. Each channel has a bandwith of 100Khz and the space between each
channel is 100Khz.
However, once again, this doesn't count on the number of audio channels (or radiostation) that you can have! It is depending on the bitrate that you're going to use!
And another thing is that ogg sounds (much) better than mpeg1 layer II does (which is what DAB use today).