Created Digital voice DV over LoRa (markdown)

Digital-voice-DV-over-LoRa.md

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+- This modem could be used in conjunction with [Android Codec2 Walkie-Talkie](https://github.com/sh123/codec2_talkie), when application connects to the modem, instead of sending AX25 APRS packets it sends Codec2 speech encoded frames. This enables digital voice communicaiton between one or multiple modems. 
+- Select appropriate lora spread factor `cfg.LoraSf` and bandwidth `cfg.LoraBw` depending on Codec2 speech rate from 450-3200 bps. For example, if you are using 450 bps mode and 20 kHz bandwidth then set spreading factor to 6 or 7. See data rate table above.
+- When using modem for voice communication  `Loraprs::Service::CfgCsmaPersistence` must be set to maximum 255 value to disable CSMA, otherwise real time voice communication won't be guaranteed. Android codec2_talkie application automatically sets this parameter to 255 by using KISS P command code.
+- Also, it might be useful to disable CRC check for LoRa packets with `cfg.LoraEnableCrc` parameter equal to `false`. Some broken bits in one speech frame will cause audio being scrambled, it might be better then longer gap when complete packet is dropped.