Hi all,
Just wondering this question : How come nobody uses DTMF as digital network's keying ?
I think rather than QPSK, which can only transmit 2-bits at once, the DTMF can transfer up to 4-bits and it's proven to be very effective over copper wires.
I think if it is used over RF network, we can improve the network's throughput. Does anyone has experience with this ?
-kunil
It's very slow, that's all.
100 millisec per digit (4 bits). i.e. equivalent to 50 baud.
DTMF signaling was used in the early RF trunked networks. It was very slow and unreliable.
Vladimir Vassilevsky
DSP and Mixed Signal Design Consultant
How did you arrive at this number? I would guess that one wave time of the lowest allowable frequency would be enough.
DTMF decoders are very slow (in order to provide noise immunity).
DTMF was designed to allow Ma Bell to build cheap phones that would transmit OK, and have mostly-accurate results. Time didn't matter much, because humans can't push buttons that fast.
It is, however, not the most bandwidth efficient method of data transmission.
For a really over-the-top multi-tone signaling protocol, and one that is in commercial use, check out OFDM (search for it on the web).
For the vast majority of engineering folks, it is lot easier to put 8880 on the board compared to implementation a basic PSK/FSK modem in the software.
You don't say that. It is a rocket science...
Vladimir Vassilevsky
DSP and Mixed Signal Design Consultant
built the first digital decoder for it, which was designed to be integratable, and was described by me in an IEEE meeting. The tone must last 40 mS, and the rep rate must be under 10 per sec. The quiet period must be at least 40 mS. The major problem is false hits, and my system was very hard to fool.
I get 40 baud, but close enough. ;)
IIRC, that's still the spec for currently available decoders like the Clare 8880.
40 baud allows you to transmit about 300 bytes per minute minus whatever overhead you build in for error detection/correction.
DSP and Mixed Signal Design Consultant
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Very good info , thanks .....
Speed is syncing clocks on both ends . Very low cost today .
Run length limited and then huge decoding /decompression databases at both ends can up the speed .
Phone lines can then send 100 K baud or more ..
Cool..I didn't expect anybody has tried it before..I couldn't find any records in google about this.
Thanx
-daniel
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