Vocoder Redux: the AMBE-2020

A Digital Voice Systems AMBE-2020 voice coder (vocoder) has been implemented as a functional replacement for the obsolete AMBE-1000. The effort, in part, was a capstone senior design project in Electrical and Computer Engineering at Temple University, which fosters advanced technical topics in undergraduate education using amateur radio. The implementation is open-source and complies with the data communication protocol of the original G4GUO AMBE-1000 vocoder design. This AMBE-2020 vocoder is offered as a reasonable standard for amateur radio digital voice experimentation. Further documentation for the AMBE-2020 vocoder is available at www.temple.edu/k3tu. Aborted Beta Testing Charles Brain G4GUO and Andy Talbot G4JNT awaken the amateur radio technical community with their voice coder (vocoder) implementation that introduced high-quality digital voice communication on HF within a 3.3 KHz bandwidth. A nominal 3 KHz voice signal sampled at 8000 samples/sec with only 8 bits of resolution, the parameters of a pulse code modulation (PCM) telephone circuit at 64 Kb/sec, would require at least a +64 dB SNR for a 3 KHz transmission bandwidth. A vocoder was chosen to lower the data communication rate to 3.6 Kb/sec, but, and more importantly, the minimum SNR required then is only +15.5 dB. They investigated several candidate vocoders, including LPC10e, MELP and CLEP, before settling on the Digital Voice Systems (DVS, www.dvsinc.com) Advanced Multi-Band Excitation (AMBE) vocoder. The selected DVS AMBE-1000 vocoder was based on a Lucent DSP16 microprocessor. The vocoder provided a large ensemble of possible data rates and forward error correction (FEC) options. A voice data rate of 2.4 Kb/sec with 1.2 Kb/sec of forward error correction (FEC) was adopted for their tests. An auxiliary microprocessor, a Microchip PIC 17C44JW, was used to initialize the AMBE-1000 vocoder and provide process control. A Motorola MC14LC5480 8-bit, μ-Law serial coder/decoder (codec) digitizes the analog voice input signal and converts binary PCM data back to an analog voice output signal. The G4GUO/TAPR Vocoder was produced and offered for beta testing by TAPR (Figure 1). Several experimenters obtained the device and a TAPR Vocoder Development List was organized. Unfortunately, by early in 2002 the DVS AMBE-1000 was discontinued (“not recommended for new designs”) and replaced by the interoperable but not compatible DVS AMBE-2000/2020. Thus the technological momentum for amateur radio digital voice experimentation was seemingly thwarted, as is so often the case, by obsolescence.