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2435ee780f
* Small fine tuning xOVS selection for BW 25k * Comments , same growing order ending with dot.
75 lines
3.4 KiB
C++
75 lines
3.4 KiB
C++
/*
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* Copyright (C) 2023 Kyle Reed, zxkmm
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*
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* This file is part of PortaPack.
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*
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* This program is free software; you can redistribute it and/or modify
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* it under the terms of the GNU General Public License as published by
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* the Free Software Foundation; either version 2, or (at your option)
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* any later version.
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*
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* This program is distributed in the hope that it will be useful,
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* but WITHOUT ANY WARRANTY; without even the implied warranty of
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* MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
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* GNU General Public License for more details.
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*
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* You should have received a copy of the GNU General Public License
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* along with this program; see the file COPYING. If not, write to
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* the Free Software Foundation, Inc., 51 Franklin Street,
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* Boston, MA 02110-1301, USA.
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*/
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/* Helpers for handling oversampling and interpolation. */
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#ifndef __OVERSAMPLE_H__
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#define __OVERSAMPLE_H__
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#include "message.hpp"
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#include "utility.hpp"
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/* TODO:
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* The decision to oversample/interpolate should only be a baseband concern.
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* However, the baseband can't set up the radio (M0 code), so the apps also
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* need to know about the "actual" sample rate so the radio settings can be
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* applied correctly. Ideally the baseband would tell the apps what the
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* actual sample rate is. Currently the apps are telling the baseband and
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* that feels like a separation of concerns problem. */
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/* HackRF suggests a minimum sample rate of 2M so a oversample rate is applied
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* to the sample rate (pre-scale) to get the sample rate closer to that target.
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* The baseband needs to know how to correctly decimate (or interpolate) so
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* the set of allowed scalars is fixed (See OversampleRate enum).
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* In testing, a minimum rate of 400kHz seems to the functional minimum.
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*
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* There are several different concepts or terms related to Capture and Replay,
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* (1) Oversampling (x8, x16, ...) and Decimation (/8, /16...)
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* In Capture App, the ADC can not directly handle low sample rates.
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* We need to apply oversampling (x8, x16, ...), collecting more "x number" additional real samples.
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* To write it to SD card with the desired sample rate, we apply Decimation ("/ number").
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*
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* (2) Up-sampling or re-scale or interpolation. (x8, x16, ...).
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* In Replay-list App, when the bit rate data is too low for HackRF.
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* We need to upsample or interpolate to increase those low bit rates
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* to a rate higher than the hardware nminimum to be transmitted by Hackrf.
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*/
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/* Gets the oversample rate for a given sample rate.
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* The oversample rate is used to increase the sample rate to improve SNR and quality.
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* This is also used as the interpolation rate when replaying captures. */
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inline OversampleRate get_oversample_rate(uint32_t sample_rate) {
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if (sample_rate < 25'000) return OversampleRate::x64; // 12k5, 16k.
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if (sample_rate < 80'000) return OversampleRate::x32; // 25k, 32k, 50k, 75k.
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if (sample_rate < 250'000) return OversampleRate::x16; // 100k, 150k.
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if (sample_rate < 1'250'000) return OversampleRate::x8; // 250k, 500k, 600k, 750k, 1Mhz.
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return OversampleRate::x4; // Top range (1.25Mhz ... 5.5Mhz).
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}
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/* Gets the actual sample rate for a given sample rate.
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* This is the rate with the correct oversampling rate applied. */
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inline uint32_t get_actual_sample_rate(uint32_t sample_rate) {
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return sample_rate * toUType(get_oversample_rate(sample_rate));
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}
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#endif /*__OVERSAMPLE_H__*/
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