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NEWQPSK modem for the Usermode Soundcard Modem driver
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=====================================================
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This package implements the NEWQPSK protocol (also known as Q15X25) as
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designed and defined by Pawel Jalocha, SP9VRC, <pawel.jalocha@cern.org>.
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Pawel's original modem is written in Motorola 56k assembler for the
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DSP56002EVM evaluation module or the Alef Null DSP Card 4. This code
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is a port of the original code to C.
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Most of the features of the original modem are implemented. The most
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important feature still lacking is the support for tuning the transceiver.
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The modem has been used for real life QSOs with performance comparable to
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Soundmodem configuration
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========================
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There is an IMPORTANT detail in soundmodem core configuration concerning
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NEWQPSK. In the "Channel Access" section one should set "TxDelay" to a
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non-zero value. The actual value does not matter, the modem will ignore
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it. (If your transmitter is slow and you need to compensate that then you
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can increase "Tune length" as explained later.) Equally important is that
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one sets "TxTail" to zero. Failing to set these will cause the NEWQPSK
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modem to behave erratically.
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The actual NEWQPSK modulator and demodulator are subject to five
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configuration parameters that can be set with the `soundmodemconfig' tool:
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This sets the modem raw bit rate. The available range is 1000 to
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5000. Rates 2500bps and 3000bps correspond to sample rates of
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8000sps and 9600sps in the original code respectively.
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Sets the interleave depth. Allowed range is 0...16. Zero
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disables interleaving.
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Sets the used error correcting code. Possible values are:
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Note that BCH (15,7) is NOT compatible with the original
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The FEC overhead can be calculated from the figures in
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parenthesis. For example Walsh (15,5) lowers the user data
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rate to 2500*5/15 = 833.3 bps.
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On modulator side this sets the length of the tune preamble
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in symbols. Allowed range is 0...64.
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On demodulator side this sets the minimum of received
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tune preamble symbols needed to make a tuning decision.
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Allowed range is 0...32. Zero means the demodulator will jump
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directly to waiting sync state.
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On modulator side this sets the length of the sync preamble
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in symbols. Allowed range is 16...64.
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On demodulator side this sets the minimum of received
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sync preamble symbols needed to make a symbol sync decision.
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Allowed range is 8...32.
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More detailed configuration of the modem parameters can be done at compile
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time with the modemconfig.h file in this directory. The parameters there
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are not to be changed unless you know what you are doing... :)
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NEWQPSK modem was designed primarily with HF in mind. You are supposed to
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connect it to a normal HF SSB rig. However NEWQPSK might also find some
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use in VHF/UHF FM packet. As FM has no frequency ambiquity the tune
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preamble can be disabled (tune length = 0). Also it might make sense to
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shorten the sync preamble.
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NEWQPSK is an AX.25 packet modem and can be used for anything regular
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packet can, including TCP/IP. Here is a short list of NEWQPSK highlights:
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- 15 carriers spaced by 125 Hz (150 Hz) modulated with DQPSK
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(differential 4-level phase shift keying) at 83.33 baud (100 baud)
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- Total raw data rate 2500 bps (3000 bps)
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- Two phase preamble for frequency shift correction and fast symbol
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sync. The modem can correct frequency shift up to about 100 Hz.
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- Forward Error Correction: three simple schemes with different levels
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- Time/frequency diversity (interleave) to de-localize burst errors
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(The figures in parenthesis above are for sample rate of 9600 sps.)
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Note that the envelope of the transmitted signal is NOT constant. The
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modulation has a crest factor of about XX dB which means that with a 100W
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transmitter and a power meter that reads average power you should adjust
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the output reading to about YY Watts. Exceeding this will cause clipping
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that will not only cause interference to other band users but also make
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copy at the other end worse.
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Also keep in mind that this is a WIDE mode (about 2000...2400Hz wide). You
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should be very careful not to transmit over other band users and you
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should probably keep away from the narrow band digital segments.
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The sole purpose for me to write this modem implementation was to learn
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about DSP in general and Pawel's novel modem in particular. I'm fairly new
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to DSP and this code is mostly a result of countless hours of staring at
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the original Motorola 56K assembler source and banging my head against the
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wall... I can't claim I understand all aspects and every detail in the
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modem. There are probably misunderstandings and errors in the code and if
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you spot one, please tell me. I want to learn more. Also if you have ideas
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for further development please share them with me. I have already some
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Tomi Manninen, OH2BNS <tomi.manninen@hut.fi>
added
removed
newqpsk/README.newqpsk
