Is there DTMF API or standard that includes letters as well as standard numeric values?

Question

I have not been able to find any literature on the subject.

Answer 1

"DTMF "Touch" Tones are defined in CCITT Volume VI: General Recommendations on Telephone Switching and Signalling Recommendation Q.23: Technical Features of Push-Button Telephone Sets.". This document and its related standard documents will tell you more than you would ever want to know about DTMF tones. "

This quote is from here. That webpage covers all the basics.

Answer 2

DTMF only covers the digits 0 to 9 and letters #, *, A, B, C, and D. So if your question is does there exist anything that supports letters after D, then the answer is no.

Answer 3

Well the following function will generate a 16-bit, signed, linear PCM, DTMF tone in Java.

public byte[] generateTone(float a, float b)
{
    byte samples[] = new byte[16000];    // Tone data buffer.
    int frames = samples.length / 2;     // Number of frames that fit in the buffer.

    /* Fill the buffer with the tone data. */
    for(int i = 0; i < frames; i++)
    {
        /* The 8000 value is the sample rate. */
        short value = (short)(32768 + 63 * Math.sin(i * 2 * Math.PI * a / 8000) + 63 * Math.sin(i * 2 * Math.PI * b / 8000));
        samples[i + i] = (byte)(value >>> 8);
        samples[i + (i + 1)] = (byte)value;
    }

    return samples;
}

I hope this helps... Just plug in the two frequencies as parameters a and b and out you get a tone. For example number one would be generated as:

byte tone[] = generateTone(697, 1209);