Manchester encoding isn't really necessary. However, if we use NRZI (without
bit stuffing) we would not lose any bandwidth and would not have a problem if
the GMSK signal gets inverted. The demodulator would work on shifts or lack of
shifts rather than what symbol it sees.
Kristoff says there is a randomizer in the system, so the addition of NRZI
might not significantly effect the AC performance.
Thanks
Bruce
"Stuart Longland (VK4MSL)" <[email protected]> wrote:
>[Gah, keep forgetting to switch the From: address]
>Hi,
>On 13/04/13 08:44, Kristoff Bonne wrote:
>> On 12-04-13 20:04, Bruce Perens wrote:
>>>> >> The low-pass filter should be passing everything below about
>22.05 or
>>>> >> 24 kHz, right? Even an imperfect filter should be fine.
>>> > Oh. Kristoff, is the modem not using Manchester encoding?
>> No, I might be wrong but I guess this would double the required
>baudrate.
>>
>> The 4800 bps modem is simply 4800 baud at 1 bit/symbol. For
>Manchester
>> encoding, you would needs 2 bits per symbol.
>
>This might be an absurd suggestion, I don't know... but would the
>1600bps Codec2 scheme work?
>
>This has the side-effect of being directly compatible with HF FreeDV;
>the bit-stream merely needs re-modulation and framing as the actual
>audio data is the same, making HF-to-VHF/UHF gateways trivial.
>
>Manchester encoding would mean an effective rate of 2400; which would
>still allow for 800 bits of headroom for things like synchronisation
>and
>ID headers, etc.
>--
>Stuart Longland (aka Redhatter, VK4MSL)
>
>I haven't lost my mind...
> ...it's backed up on a tape somewhere.
>
>--
>Stuart Longland (aka Redhatter, VK4MSL)
>
>I haven't lost my mind...
> ...it's backed up on a tape somewhere.
>
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