Work in progress by Oscar Steila IK1XPV
A Fork from Hermes-Lite by Steve Haynal KF7O
Following the project goals: * Maintain enough compatibility with Hermes to use existing Hermes SDR front-end software with no or minor modification * Cost of less than $150 (cost includes FPGA board) for hobbyist who build their own,
I investigated the possibility to add a low cost audio interface to the Hermes-Lite forum.
The idea was to use or design a small PCB that contains a TLV320AIC23B (like Hermes hardware solution) or another ready audio board interfaces via I2S.
The advantage of the audio interface embedded into the SDR is that latency may be lower and there are not skewing clock problems.
1st order sigma delta DAC
I tested also the possibility to use a direct sigma delta output.
The audio output digital stream is made of samples at 48 kHz. The idea to decrease complexity was to synthesize them directly via the 1 bit DAC running at 73.728 MHz. The advantages are: The clocks (48, 73.728) are synchronous.
The Nyquist alias of the 48 kHz sampling are over 24 kHz and are not listened by my (old) ears.
I implemented the DACs in the rtl code with some some minor change to re-activate the audio path.
A RC filtering (300 ohm/ 4n7/ 10uF) is placed at the ouput.
An earphones 3.5mm plug plus rc filter has been cabled.
How it sounds ?
Some .wav recording has been made using a 2nd PC running Audacity.
Here some listening made in city with 10m random wire antenna in Torino, Italy about 21 UTC
Radio China on 60m AM_cuSDR64_SAM_HermesLite_DAC.wav
Here a 80m QSO audio LSB_80m_QSO_HermesLite_ADC.wav and in a screen shot:
The audio seems good enough for use as receiving audio.
Now a microphone ADC solution is required. Nevertheless it is difficult to find a simple solution for the ADC converter. Assuming to sample the microphone signal at 73.728 MHz it will generate a high speed stream data that requires a lot of rtl to be decimated to the 48KHz required one.
Possibly the external CODEC board solution is more effective.