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 5000 bit/s work in progress samples & source: rowetel.com/codec2.html

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

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

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

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

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

Octave Demo

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

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

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 ....

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

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

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)

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

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