I want to detect the exact time signal from an FM radio station. The signal is transmitted in the form of 5 short pulses and one long one. My task is to determine the presence of a pulse in the audio data stream and its duration. It is understood that the signal is received by the radio receiver and then the audio signal is fed to the atmega microcontroller. The pulse duration varies in the range of 100 ... 560 ms

How can you control the appearance of these pulses using a microcontroller, and also determine their duration? Below is an example of computer processing. The same is required to be implemented on a microcontroller.

  1. I have a reference recorded signal in WAV format. This recorded signal from the output of the radio receiver contains both the speaker's speech, music, and the exact time signal: enter image description here

  2. I know the time signal is centered at 1 kHz, so apply filtering to this signal. The filter has the following frequency response: enter image description here

The signal after the filter looks like: enter image description here

  1. It remains to apply the threshold device and duration selection. However, I do all this in an audio editor to understand the ongoing processes. In fact, I need to implement this on an arduino, so I really hope for your tips, because I don't know how to implement a filter on an arduino for one frequency, and I also don't know how to make a pulse detector and a meter for its parameters.

Below is the result of homodyne, the method suggested in the comments. On the test signal, the algorithm was able to identify the required pulses! It remains to write a sketch where there is a detection threshold and definition of the duration! I was able to write a condition, when the threshold is exceeded, the LED turns on, and turns off otherwise. But I don't know how to make the definition of duration yet ...enter image description here

Added after a few days:

I was able to assemble the entire structure, the sketch below does the following:

  1. Listens to the audio signal coming to the analog input of the arduino from the radio receiver on the RDA5807M microcircuit, through an amplifier on one transistor, which provides signal amplification and offset to the positive range.

  2. Performs homodyne processing: selects only the signal with a frequency of 1 kHz, all other frequencies are suppressed

  3. Selects the optimal detection threshold for finding the exact time signal of 1 kHz frequency at the background level.

  4. Calculates the duration of each pulse

  5. Finds the exact time by the last impulse using the recalculation formula.

Thus, at the moment I get the exact time digit in the serial port monitor.

But the end result I want to get is setting the resulting digit on a seven-segment display. Having looked at the sketches of his work, I found that the values ​​in the loop are constantly processed there, which again slows down the microcontroller, but for me this is unacceptable. As I understand it, you need to write a separate function into which you pass the value of the hour and set this value on the indicator only once an hour. But I don’t know how to implement it. I ask for help!

/ * Last edited on January 25, 2021.
   1 kHz homodyne signal detection

  This program continuously samples analog input A0 and uses a homodyne
  detection circuitry for signal identification at 1 kHz (+/- 24 Hz at -3 dB).

  The A / D converter is set to "free running mode" and takes one sample every 104 µs.
  The samples are multiplied by two generated 1 kHz signals ("local oscillator") in quadrature to each other.
  The products are then low-pass filtered with a time constant of 64 sampling periods (6.656 ms),
  which gives (I, Q) signals with a bandwidth of 24 Hz.
  Finally, the signal strength is calculated as I ^ 2 + Q ^ 2.

  The program is designed for Arduino Uno and will probably work on any AVR based Arduino,
  having an ADC and operating at a frequency of 16 MHz.

  For a detailed explanation, see

  Posted by: 2016 Edgar Bonet Orozco.
* /

// Here is everything related to the parameters of the RDA5807M receiver:
#include <Wire.h>

// registers
#define RDA5807M_REG_CONFIG 0x02
#define RDA5807M_REG_TUNING 0x03
#define RDA5807M_REG_VOLUME 0x05
#define RDA5807M_REG_RSSI 0x0B
// flags
#define RDA5807M_FLG_DHIZ 0x8000
#define RDA5807M_FLG_DMUTE 0x4000
#define RDA5807M_FLG_BASS 0x1000
#define RDA5807M_FLG_ENABLE word (0x0001)
#define RDA5807M_FLG_TUNE word (0x0010)
// masks
#define RDA5807M_CHAN_MASK 0xFFC0
#define RDA5807M_CHAN_SHIFT 6
#define RDA5807M_VOLUME_MASK word (0x000F)
#define RDA5807M_VOLUME_SHIFT 0
#define RDA5807M_RSSI_MASK 0xFE00
#define RDA5807M_RSSI_SHIFT 9

uint8_t volume = 7; // 0..15
uint16_t freq = 938; // 107.3FM
uint16_t reg02h, reg03h, reg05h, reg0Bh;
// Receiver end

unsigned long startTime; // Turn on two timers, this
unsigned long elapsedTime; // and this one
int threshold = 300; // Set the maximum noise level
boolean timing = false; // I do not know...

#include <util / atomic.h>

// The analog input used must be between A0 and A5.
const uint8_t analog_in = 0;

// The frequency we want to define, in Hz.
const float SIGNAL_FREQ = 1000.0;

// Time bits.
const float SAMPLING_FREQ = F_CPU / (128 * 13.0); // 9.615 kHz
const long PHASE_INC = round (SIGNAL_FREQ / SAMPLING_FREQ * (1L << 16));
const int LOG_TAU = 6; // tau = 64 / SAMPLING_FREQ = 6.656 ms

// Set the ADC to constant conversion mode "free running mode"
static void configure_adc ()
  ADMUX = _BV (REFS0) // ref = AVCC
           | _BV (ADLAR) // left adjust result
           | analog_in; // input channel
  ADCSRB = 0; // free running mode
  ADCSRA = _BV (ADEN) // enable
           | _BV (ADSC) // start conversion
           | _BV (ADATE) // auto trigger enable
           | _BV (ADIF) // clear interrupt flag
           | _BV (ADIE) // interrupt enable
           | 7; // prescaler = 128

// Demodulated (I, Q) amplitudes.
volatile int16_t signal_I, signal_Q;

// Interrupt handler is called every time the ADC is ready to read.
ISR (ADC_vect)
  // Read the ADC and convert to a signed number.
  int8_t sample = ADCH - 128;

  // Update the phase of the local oscillator.
  static uint16_t phase;
  phase + = PHASE_INC;

  // Multiply samples by square waves in quadrature.
  int8_t x = sample;
  if (((phase >> 8) + 0x00) & 0x80) x = -1 - x;
  int8_t y = sample;
  if (((phase >> 8) + 0x40) & 0x80) y = -1 - y;

  // First order low pass filter.
  signal_I + = x - (signal_I >> LOG_TAU);
  signal_Q + = y - (signal_Q >> LOG_TAU);

/ * Return the power reading. * /
static uint16_t get_power_reading ()
  int16_t I, Q;
    I = signal_I;
    Q = signal_Q;
  return sq ((int8_t) (I >> LOG_TAU)) + sq ((int8_t) (Q >> LOG_TAU));

// *********************************************** ***********************
void setup ()
  configure_adc ();
  Serial.begin (9600);

  // Receiver start:
  Serial.begin (9600);
  Wire.begin ();
  // Register 02h - enable, settings
  reg02h = RDA5807M_FLG_ENABLE | RDA5807M_FLG_DHIZ | RDA5807M_FLG_DMUTE;
  setRegister (RDA5807M_REG_CONFIG, reg02h);

  // And then we decided to further strengthen the bass: (I don't need this, I need to somehow remove it)
  reg02h | = RDA5807M_FLG_BASS;
  setRegister (RDA5807M_REG_CONFIG, reg02h);

  // Register 03h - select radio station
  // After reset in register 03h, the default value is 0
  // Thus BAND = 00 (87..108MHz), STEP = 00 (100kHz). Let's leave them as they are
  reg03h = (freq - 870) << RDA5807M_CHAN_SHIFT; // chan = (freq - band) / space
  setRegister (RDA5807M_REG_TUNING, reg03h | RDA5807M_FLG_TUNE);
// Register 05h. Set the volume, do not change the rest of the beats
  reg05h = getRegister (RDA5807M_REG_VOLUME); // Read the current value
  reg05h & = ~ RDA5807M_VOLUME_MASK; // Reset the VOLUME bits
  reg05h | = volume << RDA5807M_VOLUME_SHIFT; // Set a new volume
  setRegister (RDA5807M_REG_VOLUME, reg05h);
  // Receiver end:

void loop ()
  // Print power readings every 8ms.
  static const uint16_t print_period = 8;
  static uint16_t last_print;
  uint16_t now = millis ();
  if (now - last_print> = print_period) {
    //Serial.println (get_power_reading ()); // Build a graph by which you can determine the optimal threshold
    // If the signal has exceeded the threshold, start timing:
    if (get_power_reading ()> threshold &&! timing)
      startTime = millis ();
      timing = true;
    // If the signal is below the threshold, stop the countdown and display the amount of time passed - the pulse duration:
    if (get_power_reading () <= threshold && timing)
      elapsedTime = millis () - startTime;
      timing = false;
      if (elapsedTime> = 80 && elapsedTime <= 580)
        int h = floor ((elapsedTime - 100) / 20); // Get the time in hours, but there are 6 pulses, but only the last one is needed.
        // "valround" --- Round to the nearest integer,
        // "floor (x)" --- Round down to integer
        if (h> = 1 && h <= 24) // If the time is defined as abnormal, beyond reasonable limits, then simply forget it:
          Serial.println ("Exact time:");
          Serial.print (h);
          Serial.println (": 00");
    last_print + = print_period;

// Receiver start:
void setRegister (uint8_t reg, const uint16_t value) {
  Wire.beginTransmission (0x11);
  Wire.write (reg);
  Wire.write (highByte (value));
  Wire.write (lowByte (value));
  Wire.endTransmission (true);

uint16_t getRegister (uint8_t reg) {
  uint16_t result;
  Wire.beginTransmission (RDA5807M_RANDOM_ACCESS_ADDRESS);
  Wire.write (reg);
  Wire.endTransmission (false);
  Wire.requestFrom (0x11, 2, true);
  result = (uint16_t) Wire.read () << 8;
  result | = Wire.read ();
  return result;
// receiver end
  • 2
    Start by conditioning the signal to a voltage within 0 – 1.5 V (no pulse) or 3 – 5 V (during the pulse). Capture that on a scope and post the trace here. It should be way easier to help you if we can see what the conditioned signal really looks like. Jan 19, 2021 at 16:14
  • can you actually hear the time signal on the radio? ... how often is it transmitted? ... if you can hear it, then what does it sound like?
    – jsotola
    Jan 19, 2021 at 16:27
  • Are you referring to the WWV/WWVH time broadcast signal?
    – jwh20
    Jan 19, 2021 at 16:45
  • or to the 1 kHz beeps
    – Juraj
    Jan 19, 2021 at 16:54
  • I am receiving a signal from a broadcasting station, and the time pulses sound at an audio frequency of 1 kHz. There are 6 of them, sometimes 5, when there are overlays on the presenter's broadcasting. The signal is transmitted at the end of every hour. Is not WWV/WWVH
    – Антон
    Jan 20, 2021 at 2:43

1 Answer 1


If the voice and music are superimposed on top of the signal you are looking for, that will make matters quite difficult. The first thing you have to do is filter out all the unwanted sound. The limit on how aggressively you can filter is given by the width of the pulses. If you want to detect 100 ms pulses, your filter width should probably be more than 10 Hz. The filter you used in your test is way larger than that.

Given the limited processing power of your typical Arduino, it may be preferable to do all the signal conditioning in the analog domain, prior to digitizing it on the Arduino. If you really have to do everything in the Arduino, you will still have to adjust the amplitude and the offset of the signal in the analog domain so that it fits the range of the ADC.

For implementing a narrow-band filter on the Arduino itself, I generally recommend to use an homodyne detection scheme. This can give you very narrow filters at a minimal processing cost. See this other answer where I discuss the working of the scheme, and provide a link to an implementation.

If you have something similar to an Uno, you should be able to load that sketch directly, although you may want to tweak the time constant in order to make the filter narrower. You can then send it your test signal, collect back the power readings sent by the sketch through the serial port, and plot those. This should give you a new test signal for prototyping the next stages of your processing pipeline. If all goes well, you would just have to set a threshold for detecting the pulses, and then have some condition on the length of the detected pulses to sort out spurious pulses from legit ones.

Edit: I suggested using a threshold on the detected power for deciding when there is a pulse and when there is not. However, if the power signal is noisy, this simple scheme may not be good enough.

One possible way to reduce the noise is to increase the time constant of the filter by increasing LOG_TAU. Do not set it above 8, otherwise it may overflow some variables. Also note that, the larger the time constant, the lower the bandwidth, which can be an issue if the frequencies are not very accurate.

Another option is to low-pass filter the power readings. This can be done on a sub-sampled signal in order to lower the processing cost.

Finally, for deciding whether there is a pulse or not, it can be useful to add some hysteresis to the detection, basically simulating a Schmitt trigger. This is simply accomplished by replacing the detection threshold with two distinct thresholds: a high one for the “no pulse” → “pulse” transition, and a lower one for the reverse transition:

const uint16_t LOW_THRESHOLD  = ...;
const uint16_t HIGH_THRESHOLD = ...;

// within loop:
static enum { NO_PULSE, PULSE } state = NO_PULSE;
uint16_t power = get_power_reading();
if (state == NO_PULSE && power >= HIGH_THRESHOLD) {
    state = PULSE;
if (state == PULSE && power < LOW_THRESHOLD) {
    state = NO_PULSE;
  • Thank you! I used your code unchanged and it works! Only I used the signal from the generator, since I do not yet have an amplified signal after the receiver. Now output the result to the serial port. I only see huge numbers when I apply 1 kHz! Tell me what can I do next? After all, the survey is carried out quite rarely.
    – Антон
    Jan 20, 2021 at 9:48
  • 1
    @Антон: Next step would be to define a threshold for pulse detection, but you cannot do that without a realistic test signal (with realistic amplitude): send the test signal to the Arduino, plot the numbers that come out, and select a threshold in between the lowest pulse level and the highest non-pulse level. Jan 20, 2021 at 10:23
  • I ran a test, this time using a signal from the phone generator. I changed the frequency of the signal and watched the result. I think the optimal threshold is 25. I cannot give a test signal because Arduino rarely plots points and my short impulses simply does not fix. I attached the graph in the post above
    – Антон
    Jan 20, 2021 at 10:47
  • 1
    @Антон: A sampling rate of “115200 baud” does not make sense. A sampling rate of 155200 samples per second is more than what the Arduino can handle: processing the samples would require 127% of the available CPU power. You can increase the sampling rate, but that will not help detecting short pulses. You instead have to increase the rate at which samples are printed to the serial port, i.e. decrease print_period. Increasing the baud rate will be needed only if you set print_period to less than 8 ms, which is unlikely to be useful. Jan 21, 2021 at 8:55
  • 1
    @Антон: Every low-pass filter I know about introduces some lag. But the lag is the same for the rising and falling edges of a pulse, so it does not interfere with the pulse length measurement. Re “the signal is very jagged”: see amended answer. Jan 28, 2021 at 8:24

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