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home | newest check | boards | help index | log | ps | userlogin | send sysop | slog | status forward | bcm news | users | version | remove cookieHP2DFA > TECHNI 01.11.26 23:04l 43 Lines 2069 Bytes #7 (0) @ WW BID : 6696_HP2DFA Subj: UNMODIFIED C62 DECODES TETRA: BK4819 SDR HACK & OPENRTX PORT Path: JH4XSY<IW0QNL<HB9ON<DK0WUE<PD0LPM<OK2PEN<GB7CIP<VK2RZ<W0ARP<HP2DFA Sent: 261101/1351Z 6696@HP2DFA.#COL.PAN.CEAM BPQ6.0.24 AMATEUR RADIO TECHNICAL PAPERS AND EMBEDDED SOFTWARE NEWS AUTHOR: WOJCIECH KACZMARSKI (OPENRTX / C62 SDR EXPERIMENTS) DISTRIBUTED VIA BPQ32 CENTRAL NODE HP2DFA-5 BY HP2DFA ====================================================================== AN UNMODIFIED C62 CANNOT DECODE TETRA... OR CAN IT? ====================================================================== Am I crazy? Without a doubt, but at least I focus my craziness on a good purpose. My goal was testing if these assumptions were viable: 1. Demodulating pi/4-DQPSK is possible with a plain analog FM demod. 2. Beken BK4819 chip can run wide enough to demodulate 25kHz channels. 3. CSK6011B MCU can run the full TETRA stack plus vocoder on top. The answer is窶ｦ yes to all. STEP-BY-STEP BREAKDOWN: ---------------------------------------------------------------------- 1. DIFFERENTIAL MODULATION: Because modulation is differential (D in pi/4-DQPSK), only phase transitions matter. Phase transitions are frequency changes, so pi/4-DQPSK acts as 4-level FSK right out of the FM discriminator. 2. BEKEN BK4819 CONFIGURATION: The BK4819 handles 25kHz-wide channels by toggling a single register. The frequency response is not flat, but a lightweight FIR filter handles equalization cleanly in real time. 3. EMBEDDED STACK & VOCODER REWRITE: Osmo-tetra and ETSI reference vocoder code were replaced with libtetra and OpenACELP. Both adhere to ETSI EN 300 3xx-x and run optimized on 32-bit MCUs (Cortex-M4/M33 inside OpenRTX platforms). IMPLEMENTATION DETAILS: ---------------------------------------------------------------------- Built on top of turnrye窶冱 OpenRTX fork (M17 focused, 48kHz baseband DSP sampling rate). TETRA mode is toggled via the quick menu; key [5] cycles between M17, legacy FM, and TETRA. Validated using TMO samples courtesy of Manoel ON6RF and the HamTETRA project group. ====================================================================== SYSOP: HP2DFA | GRID: FJ09 (PANAMA) | EMAIL: HP2DFA@GMAIL.COM 73 DE HP2DFA / AMATEUR RADIO STATION PANAMA
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