1. Codec 2 open source speech codec
low bit rate (2500 bit/s down to 1200 bit/s)
main application is digital speech for HF and VHF radio
fills gap in open source speech codecs beneath bit/s
work in progress
samples & source: rowetel.com/codec2.html

2. Ham Radio and Open Source
Ham radio is an experimental service
Key to the hobby is the need to be able to experiment, understand, and modify
Evolving from analog to digital communications
But current digital speech modes use proprietary codecs
Understanding them is discouraged, experimenting with them is illegal

3. Patents and Codecs
The authors of proprietary/patented codecs borrowed heavily from the public domain
Perhaps 5% of the algorithms they use are original and patented
95% of the algorithms in these codecs are public domain algorithms
For that 5% we are being locked into their codec and being told to pay them royalties forever.
To build an equivalent codec, we simply need alternatives for the 5% that is patented

4. Speech Coding
Take speech samples (e.g. 16 bit samples at 8 kHz sampling rate)
Compress to 1400 to 2400 bit/s
What can we throw away?
Retain intelligible speech
Retain natural speech
Use a model of speech, send model parameters, more efficient than coding waveform

5. Model Parameter example of a model parameter is pitch
for humans in the range 50 to 500 Hz
can be accurately represented with 7 bits
updated every 20 ms
so 7/0.02 = 350 bit/s to represent pitch

6. Sinusoidal Speech Model
Amplitude 1 Phase 1 Frequency 1
Amplitude 2 Phase 2 Frequency 2
In practice speech consists of voiced speech (like vowels) and unvoiced speech (like consonants)
For voiced speech the frequencies are harmonics of the pitch frequency
However if the number of harmonics L is high, noise can be accurately generated using harmonics with random phases
So instead of sending the speech waveform, we send the pitch frequency, and the harmonic amplitudes and phases
These parameters are time varying, so we update them every 10-20ms
Amplitude L Phase L Frequency L

7. Octave Demo

8. Bit Allocation Alpha codec, subject to rapid change
56 bits per 40ms frame, or 1400 bit/s
Parameter
Bits/frame
Amplitudes 36
Frame Energy + Pitch 16
Voicing 4
Total 56

9. Sinusoidal Model Samples
Original
Sinusoidal Model
Sinusoidal Model - Low Freq Amplitudes
Sinusoidal Model - High Freq Amplitudes Only
Sinusoidal Model – All Voiced
Sinusoidal Model – All Un-Voiced
Sinusoidal Model – Double Pitch

10. Voice over Digital Radio
RF spectrum is extremely limited, noise, bit errors
traditional analog speech systems (FM, SSB) are really efficient in power and bandwidth use
but there is interest in using digital techniques .. if the right codec is available
compressed speech requires less bit/s over channel
this means less bandwidth
and maybe improved speech quality ....

11. Power Efficiency: Tighter Compression = Better Speech
1 Watt
Noise
1400 bit/s
2 x 1400 = 2800 bit/s
You also use less Bandwidth...

12. Demo: Digital Voice Radio System
codec2 enc
FDMDV
mod
mic
HF/VHF radio
codec2 dec
FDMDV
demod
spkr

13. Demo Input Speech (300 to 2600 Hz) Modem signal at Tx
Modem signal at Rx (SNR = 5dB, 50Hz offset)
Decoded Speech using Codec 2
SSB over same channel
Decoded Speech (high SNR)

14. Voice is not like data
A single error in a data packet make the entire packet is useless
With a few errors in a voice packet it probably sounds OK (previous demo was 1% BER, NO FEC)
If you discard or lose a voice packet every now and again it's probably OK
In speech packets some bits are more important than others
You may not need FEC, or only require it on some bits

15. Roadmap 2500 b/s 1400 b/s Quality improvements FDMDV modem GUI VOIP
VHF Digital Radio
FDMDV modem
GUI