The DTMF encoding

The DTMF encoding system has become very common in phones where it has replaced the old pulse dialer, but even on the air the DTMF encoding is widely used, for example, for the tone squelch.
DTMF means Dual Tone Modulation Frequency. This systems is intended to transmit keys pressed on a keyboard through an audio channel such as a telephone line or a radio connection. For each pressed key two audio frequencies are emitted: one (higher) corresponds to the column in which the key is in, and the other (lower) corresponds to the row. This encoding handles a maximum of 4 rows and 4 columns, that means 16 keys (from 0 to 9, *, #, and from A to D). The lower frequency (corresponding to the row) is emitted with less amplitude than the higher one, and precisely of 2 dB (that means that the amplitude is 1.259 times smaller).

This table shows graphically the layout of the keys on the DTMF keyboard with the corresponding frequencies and the minimum and maximum values for each tone. Wave files containing the corresponding tones for each key can also be directly downloaded.

	dtmf.pas ( 11k bytes)	Source file in Borland Pascal 7.0 and executable file to automatically generate all .wav files. Different sample frequencies, number of bit and number of channels can be obtained by modifying constants in the source code end recompiling.
	dtmf.exe ( 19k bytes)
	dtmf.zip ( 64k bytes)	Compressed file zip or tgz (tar & gzip) containing the 16 .wav files corresponding to the 16 keys.
	dtmf.tgz ( 62k bytes)
	dial.pas ( 12k bytes)	Source file in Borland Pascal 7.0 and executable file to generate DTMF tones directly on the speakers of your PC if you have an AdLib or compatible sound card; it's very useful to dial telephone numbers. Includes the ability to use aliases if defined in the text file dial.pbk.
	dial.exe ( 10k bytes)
	dial.pbk ( 1k bytes)
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