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Lab Exercise:

A DTMF Tone Generator

This exercise requires use of the 'C542 kits in the lab along with the C54x Code Explorer software. You will also need to use the dskplasm assembler to generate .obj files that could be downloaded to the 'C542 kit thru Code Explorer.

DTMF Tones

A DTMF tone is the sum of two tones:

• one tone of a low frequency f₁, and
• one tone of a high frequency f₂

DTMF Tone = tone of frequency f₁ + tone of frequency f₂.

The low frequency f₁ is one of 697 Hz, 770 Hz, 852 Hz, and 941 Hz.
The high frequency f₂ is one of 1209 Hz, 1336 Hz, 1477 Hz, and 1633 Hz.

Hence, there are 16 possible DTMF tones. One DTMF tone each is associated with the 10 digits, the symbols * and #, and the letters A, B, C, D as follows:

	f1 (Hz)	f2 (Hz)
1	697	1209
2	697	1336
3	697	1477
4	770	1209
5	770	1336
6	770	1477
7	852	1209
8	852	1336
9	852	1477
0	941	1336
*	941	1209
#	941	1477
A	697	1633
B	770	1633
C	852	1633
D	941	1633

DTMF tones are extensively used in the telephone system.

Tone Generation Exercise

For this lab exercise, you are to use the 'C542 kits in the lab to generate the DTMF tone assigned to you [tone assignments].

Use the analog interface settings as provided by the delay example in c:\dskplus\demos\delay. In particular, assume an operating frequency of 5787 Hz for the analog interface. Use circular buffers to store tone samples that will still be needed in subsequent computations. You may choose whatever phase you find convenient; and you may choose whatever amplitude you find convenient.

Be able to show that the tone samples generated are really those required. Do this by simulating in Excel the computations involved, and by inserting code somewhere before interrupts are enabled, and before your firmware goes into an infinite loop. The inserted code fragment must generate at least the first 10 tone samples using the same computations that will be used later in the actual tone generation. It should be possible, by stepping thru this code fragment, to compare the samples generated by this code fragment with the results of a simulation of the same computations in Excel.

NOTE: The code in dly_ac01.asm in the delay demo apparently sets the analog interface to operate at 5787 Hz.

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