Momentum-Apps/signal.c

/* Copyright (C) 2022-2023 Salvatore Sanfilippo -- All Rights Reserved
 * See the LICENSE file for information about the license. */

#include "app.h"

/* =============================================================================
 * Raw signal detection
 * ===========================================================================*/

/* Return the time difference between a and b, always >= 0 since
 * the absolute value is returned. */
uint32_t duration_delta(uint32_t a, uint32_t b) {
    return a > b ? a - b : b - a;
}

/* This function starts scanning samples at offset idx looking for the
 * longest run of pulses, either high or low, that are among 10%
 * of each other, for a maximum of three classes. The classes are
 * counted separtely for high and low signals (RF on / off) because
 * many devices tend to have different pulse lenghts depending on
 * the level of the pulse.
 *
 * For instance Oregon2 sensors, in the case of protocol 2.1 will send
 * pulses of ~400us (RF on) VS ~580us (RF off). */
#define SEARCH_CLASSES 3
uint32_t search_coherent_signal(RawSamplesBuffer *s, uint32_t idx) {
    struct {
        uint32_t dur[2];     /* dur[0] = low, dur[1] = high */
        uint32_t count[2];   /* Associated observed frequency. */
    } classes[SEARCH_CLASSES];

    memset(classes,0,sizeof(classes));
    uint32_t minlen = 40, maxlen = 4000; /* Depends on data rate, here we
                                            allow for high and low. */
    uint32_t len = 0; /* Observed len of coherent samples. */
    s->short_pulse_dur = 0;
    for (uint32_t j = idx; j < idx+500; j++) {
        bool level;
        uint32_t dur;
        raw_samples_get(s, j, &level, &dur);
        if (dur < minlen || dur > maxlen) break; /* return. */

        /* Let's see if it matches a class we already have or if we
         * can populate a new (yet empty) class. */
        uint32_t k;
        for (k = 0; k < SEARCH_CLASSES; k++) {
            if (classes[k].count[level] == 0) {
                classes[k].dur[level] = dur;
                classes[k].count[level] = 1;
                break; /* Sample accepted. */
            } else {
                uint32_t classavg = classes[k].dur[level];
                uint32_t count = classes[k].count[level];
                uint32_t delta = duration_delta(dur,classavg);
                if (delta < classavg/10) {
                    /* It is useful to compute the average of the class
                     * we are observing. We know how many samples we got so
                     * far, so we can recompute the average easily.
                     * By always having a better estimate of the pulse len
                     * we can avoid missing next samples in case the first
                     * observed samples are too off. */
                    classavg = ((classavg * count) + dur) / (count+1);
                    classes[k].dur[level] = classavg;
                    classes[k].count[level]++;
                    break; /* Sample accepted. */
                }
            }
        }

        if (k == SEARCH_CLASSES) break; /* No match, return. */

        /* If we are here, we accepted this sample. Try with the next
         * one. */
        len++;
    }

    /* Update the buffer setting the shortest pulse we found
     * among the three classes. This will be used when scaling
     * for visualization. */
    for (int j = 0; j < SEARCH_CLASSES; j++) {
        for (int level = 0; level < 2; level++) {
            if (classes[j].dur[level] == 0) continue;
            if (classes[j].count[level] < 3) continue;
            if (s->short_pulse_dur == 0 ||
                s->short_pulse_dur > classes[j].dur[level])
            {
                s->short_pulse_dur = classes[j].dur[level];
            }
        }
    }
    return len;
}

/* Search the buffer with the stored signal (last N samples received)
 * in order to find a coherent signal. If a signal that does not appear to
 * be just noise is found, it is set in DetectedSamples global signal
 * buffer, that is what is rendered on the screen. */
void scan_for_signal(ProtoViewApp *app) {
    /* We need to work on a copy: the RawSamples buffer is populated
     * by the background thread receiving data. */
    RawSamplesBuffer *copy = raw_samples_alloc();
    raw_samples_copy(copy,RawSamples);

    /* Try to seek on data that looks to have a regular high low high low
     * pattern. */
    uint32_t minlen = 13;           /* Min run of coherent samples. Up to
                                       12 samples it's very easy to mistake
                                       noise for signal. */

    uint32_t i = 0;
    while (i < copy->total-1) {
        uint32_t thislen = search_coherent_signal(copy,i);
        if (thislen > minlen && thislen > app->signal_bestlen) {
            app->signal_bestlen = thislen;
            raw_samples_copy(DetectedSamples,copy);
            DetectedSamples->idx = (DetectedSamples->idx+i)%
                                   DetectedSamples->total;
            FURI_LOG_E(TAG, "Displayed sample updated (%d samples)",
                (int)thislen);
        }
        i += thislen ? thislen : 1;
    }
    raw_samples_free(copy);
}

/* =============================================================================
 * Decoding
 * ===========================================================================*/

/* Set the 'bitpos' bit to value 'val', in the specified bitmap
 * 'b' of len 'blen'.
 * Out of range bits will silently be discarded. */
void set_bit(uint8_t *b, uint32_t blen, uint32_t bitpos, bool val) {
    uint32_t byte = bitpos/8;
    uint32_t bit = bitpos&7;
    if (byte >= blen) return;
    if (val)
        b[byte] |= 1<<bit;
    else
        b[byte] &= ~(1<<bit);
}

/* Get the bit 'bitpos' of the bitmap 'b' of 'blen' bytes.
 * Out of range bits return false (not bit set). */
bool get_bit(uint8_t *b, uint32_t blen, uint32_t bitpos) {
    uint32_t byte = bitpos/8;
    uint32_t bit = bitpos&7;
    if (byte >= blen) return 0;
    return (b[byte] & (1<<bit)) != 0;
}

/* Take the raw signal and turn it into a sequence of bits inside the
 * buffer 'b'. Note that such 0s and 1s are NOT the actual data in the
 * signal, but is just a low level representation of the line code. Basically
 * if the short pulse we find in the signal is 320us, we convert high and
 * low levels in the raw sample in this way:
 *
 * If for instance we see a high level lasting ~600 us, we will add
 * two 1s bit. If then the signal goes down for 330us, we will add one zero,
 * and so forth. So for each period of high and low we find the closest
 * multiple and set the relevant number of bits.
 * 
 * In case of a short pulse of 320us detected, 320*2 is the closest to a
 * high pulse of 600us, so 2 bits will be set.
 *
 * In other terms what this function does is sampling the signal at
 * fixed 'rate' intervals.
 *
 * This representation makes it simple to decode the signal at a higher
 * level later, translating it from Marshal coding or other line codes
 * to the actual bits/bytes.
 *
 * The 'idx' argument marks the detected signal start index into the
 * raw samples buffer. The 'count' tells the function how many raw
 * samples to convert into bits. The function returns the number of
 * bits set into the buffer 'b'. The 'rate' argument, in microseconds, is
 * the detected short-pulse duration. We expect the line code to be
 * meaningful when interpreted at multiples of 'rate'. */
uint32_t convert_signal_to_bits(uint8_t *b, uint32_t blen, RawSamplesBuffer *s, uint32_t idx, uint32_t count, uint32_t rate) {
    if (rate == 0) return 0; /* We can't perform the conversion. */
    uint32_t bitpos = 0;
    for (uint32_t j = 0; j < count; j++) {
        uint32_t dur;
        bool level;
        raw_samples_get(s, j+idx, &level, &dur);

        uint32_t numbits = dur / rate; /* full bits that surely fit. */
        uint32_t rest = dur % rate;    /* How much we are left with. */
        if (rest > rate/2) numbits++;  /* There is another one. */
        while(numbits--) set_bit(b,blen,bitpos++,s[j].level);
    }
    return bitpos;
}