FDMDV

A New Digital Voice Mode for HF Marc C. Tarplee, Ph.D. N4UFP

SC Section Technical Coordinator

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

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

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

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

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

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

FDMDV Signal

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

Audio Interface Circuits

1. Acoustic Coupling (Tayloe Interface)

– Rig’s speaker is placed near the PC’s microphone

– Rig’s microphone is placed next to the PC’s 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

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

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

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 Ford’s original QST article)

•Email reflector: send email to psk31-request@aintel.bi.ehu.es. Put “subscribe” in the subject line.

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 Homepage”an 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