Voice codec eval board is Raspberry Pi HAT

The codec, the CMX655D, has been built into the EV6550DHAT – a board compliant with Raspberry Pi’s HAT (hardware attached on top) extension board standard.

“The Raspberry Pi single board computer is widely recognised by makers, enthusiasts and professional developers as the platform of choice for rapid hardware and software development,” according to CML. “The EV6550DHAT builds on this growing ecosystem by bringing CMX655D voice codec to the Raspberry Pi community.”

CMX655D has been developed for digital voice and sensor applications.

It integrates the entire signal chain (see above diagram), from dual matched digital MEMS microphone interfaces to a 1W Class D speaker driver, incorporating decimation, filtering, signal processing and digital gain, supported by an I2S/PCM audio interface and SPI control interface.

“It represents a single-chip, low power solution to adding advanced voice coding/decoding to any application”, claims CML.

On the power consumption, the data sheet shows this between 220 µA (stand-by, main clock enabled) to 9mW when line driving) When not required to drive anything, recording modes appear to need 500μA or less.

The evaluation HAT is compatible with any Raspberry Pi with a 40-pin extended GPIO connector (almost all) and is powered by the Raspberry Pi – external power can be applied if other HATs or peripherals are limiting the total power available from the host USB interface.

Associated open-source software is accessible via third-party apps and projects including GTK3+, Geanie and Glade, which means “the GUI used to access the EV6550DHAT and all of the features are easily modifiable and accessible to the engineering community”, said CML.

Features include access to pre-recorded sample files, as well as the ability to record and playback audio files in the .wav format – allowing play-back of pre-recorded samples at different rates and levels to compare audio quality.

Configurable parameters include sample rate (8, 16, 32 or 48ksample/s), volume and level (dB).

The GUI allows the audio input gain from -12dB to +3dB, and playback between -90dB to 0dB. Muting and something called ‘smoothing’ is also supported.

EV6550DHAT is available now from Digi-Key and Mouser.

“The EV6550DHAT is our first HAT, and the entire team has been excited by the prospect of bringing voice codec capability to the Pi,” said CML product manager David Brooke.

Essex fabless chip firm CML Microcircuits designs, develops and supplies low-power analogue, digital and mixed-signal ICs for telecommunications, focusing on narrow-band comms – its chips are found in digital/analogue PMR/LMR, wireless data, satellite comms and marine radio applications, such as TETRA, DMR, dPMR and SCADA systems.