FDMDV

1
FDMDV

• A New Digital Voice Mode for HF Marc C. Tarplee,
  Ph.D. N4UFP
• SC Section Technical Coordinator

2
What is FDMDV?

• FDMDV is a new digital voice mode developed by
  Peter Martinez, G3PLX, and HB9TLX
• Advantages of FDMDV
• Narrower bandwidth than SSB (1.1 KHz)
• Should provide good copy at SNR of 3 dB

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How does FDMDV work?

• Voice data is sent over 15 carriers
• 75 Hz carrier separation gives 1125 Hz BW
• 50 baud QPSK is used on 14 carriers for an
  overall data rate of 1400 bit/sec
• Center carrier is used for synchronization
• There is no Forward Error Correction (FEC)
• Analog voice signals are not directly converted
  to a digital signal, but are encoded by a vocoder

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Vocoders

• A vocoder is a device that
• Creates a mathematical approximation to a human
  voice and outputs the key parameters of that
  approximation as data
• Creates voice sounds based on its internal
  mathematical model and numerical data that it
  receives
• Vocoding is not simple A/D conversion
• The output is not digitized signal amplitude
• The sampling rate does not have to be twice the
  highest audio frequency present

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Vocoder Model

• Broadband buzzing sound source (vocal cords)
• Tube with multiple resonances (formants)
• Additional basis sounds
• Plosives (puffs of air b, d, p sounds)
• Fricatives

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Vocoder Operation

• Vocoder analyzes the incoming voice signal to
  determine
• Number of formants
• Quality of formants
• Amplitude of basis sounds
• Numerical parameters describing the items above
  are the data output
• Data output rate is much less than the Nyquist
  sampling rate required by traditional A/D
  conversion

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Vocoder Limitations

• The choice of basis sounds in somewhat
  language-dependent.
• FDMDV vocoder is optimized for Germanic languages
  (English, German, Dutch, etc.)
• The received quality of non-Germanic languages is
  poorer
• The vocoder must fit incoming audio to its
  internal human voice model
• Non-voice sounds such as music are not faithfully
  reproduced

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FDMDV Signal
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FDMDV Station Requirements

• Amateur transceiver that has low frequency drift
• Any modern solid state rig is OK (IC-706, IC-746,
  TS-940, TS2000, FT-1000, Omni VI, etc.)
• Vacuum tube rigs can be problematic, unless ample
  warm-up time is allowed (15 30 minutes)
• PC and Soundcards
• Operating System - Windows 98, ME, 2000, XP
• Sound Card(s)
• 1 Sound card with USB mic/headset or 2 Sound
  cards
• Sound cards should be able to sample at 12Khz
  rate
• An interface that connects the PC and the
  transceiver
• Software
• FDMDV
• MELP1400.dll

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Audio Interface Circuits

• 1. Acoustic Coupling (Tayloe Interface)
• Rigs speaker is placed near the PCs microphone
• Rigs microphone is placed next to the PCs
  speaker
• VOX is used for T/R switching
• Advantages
• Absolutely no extra equipment required.
• Good electrical isolation between PC and rig
• Disadvantages
• Transmit levels can be tricky to adjust
• Ambient noise degrades signal

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Where to hear FDMDV

• FDMDV activity is concentrated around 14.236 MHz
• There are regular nets that begin at 2000Z on
  Saturday and Sunday afternoons on 14.236 MHz

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Tuning in a PSK-31 signal

• Turn on the equipment
• Run the PSK software
• Select the waterfall display mode
• Use the mouse to place the cursor on a signal
• Adjust the squelch until text appears in receive
  window

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Where to Find More Information

• www.arrl.org (members only, search for PSK31)
• www.ssiserver.com\info\psk31 (copy of
  thispresentation as an Adobe PDF document)
• www.psk31.com (official home page)
• www.aintel.bi.ehu.es/psk31.html (official home
  page)
• www.arrl.org/tis/info/HTML/psk31 (Steve Fords
  original QST article)
• Email reflector send email to psk31-request_at_ainte
  l.bi.ehu.es. Put subscribe in the subject line.

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Where to Find Software and Information

• Hamscope may be downloaded from the following
  website
• http//www.qsl.net/hamscope/
• W1SQLPSK may be downloaded from the following
  website
• http//www.faria.net/w1sql/
• Digipan may be downloaded from the following
  website
• http//mywebpages.comcast.net/hteller/digipan/
• PSK31 Official Homepagean excellent source for
  background information
• http//aintel.bi.ehu.es/psk31.html
• Good reference material and links
• http//www.arrl.org/tis/info/psk31.html