Play notification (frequency x) sound in Qt - easiest way?

Question

Qt 5.1 or later:

I need to play a notification sound of frequency x for n milliseconds. It'd also be nice if I could combine tones like this: 1000Hz for 2 secs, then 3000Hz for 1 sec, ..

The easiest way is to use files (WAV, MP3, ..), e.g. as described here: How to play sound with Qt But then I have to generate such files for each of my scenarios.

Qt's audio output example generates such tones in memory ( Generator::generateData(const QAudioFormat &format, qint64 durationUs, int sampleRate) ). I could write such a generator for my purpose. But do I have to?

So what is the easiest way to just play frequency x for n milliseconds?

Answer 1

To generate a tone in Qt, we can pass our own QBuffer to the QAudioOutput to be played.

Take a look at the first example on the QAudioOutput page.

What I did was create my waveform in a QByteArray. Remember that sin(2 * pi * frequency * i / sample_rate) will give you a sin tone of the desired frequency:

#define FREQ_CONST ((2.0 * M_PI) / SAMPLE_RATE)

QByteArray* bytebuf = new QByteArray();
buf->resize(seconds * SAMPLE_RATE);

for (int i=0; i<(seconds * SAMPLE_RATE); i++) {
    qreal t = (qreal)(freq * i);
    t = t * FREQ_CONST;
    t = qSin(t);
    // now we normalize t
    t *= TG_MAX_VAL;
    (*bytebuf)[i] = (quint8)t;
}

Then we can take that buffer and do something like this to play it:

// Make a QBuffer from our QByteArray
QBuffer* input = new QBuffer(bytebuf);
input->open(QIODevice::ReadOnly);

// Create an output with our premade QAudioFormat (See example in QAudioOutput)
QAudioOutput* audio = new QAudioOutput(format, this);
audio->start(input);

If you need more example code, you can see how I did it in a little project I just started here.

Answer 2

You could use the "window.h" package (not a QT one). Then you just use the Beep(frequency, miliseconds). For your example it should be Beep(1000,2000) and Beep(3000,1000) respectively.