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Text File | 1989-12-17 | 14KB | 331 lines

When designing products for the consumer market, cost is one of the most critical design criteria. Usually one of the best ways to reduce product cost is to move as much functionality as possible into the product's firmware, reducing the unit cost of the hardware. In a telephone design, generating Dual Tone Multiple Frequency (DTMF) signalling is usually done by specialized hardware dialler chips. However, these devices typically add more than a dollar to the manufacturing cost of the phone; it would be desirable to be able to do this in software, with minimal hardware support. Another benefit of a software implementation is the added flexibility of generating tones other than the standard DTMF tone pairs. This might be useful for special signalling with propietary equipment, or other tones such as audible feedback when the user presses keys, etc. Also, there applications other than telephony which could benefit from software sinewave generation. This report examines the requirements for DTMF dialling, and shows how these requirements may be met at very low cost with a software implementation for both the 68HC05 and 68HC11 family of microcontrollers. DTMF Requirements DTMF signalling represents 16 binary digit codes through the use of sine wave tone pairs, organized into a high group (1200-1700 Hz) and a low group (600-1000 Hz). There are four tones in each group. As shown below, the tones are associated with a particular row or column on the telephone keypad, which is shown below with the associated frequencies. Column four is defined but is not usually implemented on a telephone. Signalling is accomplished by transmitting one tone from each group for a minimum of 50 ms, followed by a silent period of at least 50 ms. Column 1 Column 2 Column 3 Column 4 1209 Hz 1336 Hz 1477 Hz 1633 Hz Row 1 "1" "2" "3" "A" 697 Hz Row 1 "4" "5" "6" "B" 770 Hz Row 1 "7" "8" "9" "C" 852 Hz Row 1 "*" "0" "#" "D" 941 Hz Generating network-quality DTMF is not a trivial task. Industry specifications generally require frequency errors to be less than 1%, and total harmonic distortion (THD) to be less than 10%. Additionally, the frequency response of the telephone line will generally roll off at high frequencies, requiring the high group of tones to be transmitted at a higher amplitude than the low group. The telephony buzzword for this characteristic is twist. Software Approach The basic technique to generate these tones in software involves a table lookup of two sine waves, adding them together, and outputting them through a D/A converter. In order to reduce harmonic content to a minimum, they must be sampled at a high enough rate so that the noise introduced at the sampling frequency can be easily filtered from the output. A sample period of 128 us was chosen for this example; this produces sampling noise at 7.8 Khz. In order to save memory, a single 256 sample sine wave table is used. If we just picked out one sample per period and output that data to the D/A, the output frequency would be 1/(256*128 us) or 30.5 Hz. However, we need the flexibility to generate many different frequencies, none of which are 30.5 Hz. To do this, we can skip several samples after one is output, wrapping around to the start of the table when we skip off the end. This has the effect of multiplying the output frequency by the number of samples skipped. For example, if we skipped past two samples to arrive at the next one to be output, we would be outputting only every third sample. The output frequency would be three times the base frequency or 3/(256 * 128 us) = 91.55 Hz. So the lowest frequency we can generate is 30.5 Hz, and the highest is 128/(256 * 128 us) = 3.9 Khz, Since we cannot go above 1/2 the sample frequency due to Nyquist's sampling theory. Our frequency resolution is 30.5 Hz. So in order to generate DTMF, we must use two pointers stepping through this 256 byte table in memory; at each sample period, two values are retrieved from the table, summed, and output to a DAC. The pointers are then updated by adding a value to each to step through the table. This value is called the pointer interval, and is different for each frequency that we must generate. Calculating the Pointer Intervals The general form of our equation is Fout = Interval/(Table Size * Sample Period) So the interval is calculated by flipping that to become Interval = Table Size * Sample Period * Fout Let's run through an example. The frequency for row 1 on the DTMF keypad is 697 Hz. If we use a sine wave table of 256 entries, Interval = 256 * 128 us * 697 Hz = 22.83 We can't step through the table by fractional intervals, so we round the interval to 23. If we plug that number back into the frequency equation, our actual Fout is: Fout = 23/(256 * 128 us) = 701.9 Hz. This gives us a frequency error of 0.70% which is acceptable. Below is a table of DTMF frequencies which were calculated using the above equations. The actual frequencies are also shown with the percentage error calculated. A Lotus 1-2-3 compatible spreadsheet file, DTMF.WKS, is available which calculates these intervals based on the sine wave table size, sample period, and desired frequency, as well as actual frequencies and error. Software DTMF Dialler Using Fixed Sample Rate Sample Period (uS) 128 Sample Table Size 256 Frequency -> 697 770 852 941 Interval 23 25 28 31 Actual 701.90 762.94 854.49 946.04 Error (%) 0.70 -0.92 0.29 0.54 Frequency -> 1209 1336 1477 1633 Interval 40 44 48 54 Actual 1220.70 1342.77 1464.84 1647.95 Error (%) 0.97 0.51 -0.82 0.92 Implementing High Tone Pre Emphasis In order to compensate for the rolloff characteristic of most telephone lines, the high group of tones must be about 1 to 3 db higher power than the low group. This equates to a voltage multiplication of about 1.12 to 1.41. A convenient value might be 1.25 (1 and 1/4th), since 1/4th is a binary fraction which can be achieved by shifting instead of a full multiply. To implement this, the sine table contains values for the low frequency row tones. When the high group sample is read from the sine table, it is shifted right 2 bits (divide by 4), and then the same sine value is added again into the accumulator, producing the 1.25 multiplication. The pre emphasis is therefore db = 20 log (V1/V2) = 20 log (1.25) = 1.94 db. Calculating Sine Values to Avoid Overflow The values in the sine wave table must calculated to avoid overflow errors when the two samples are summed and output to the DAC. In this example, an 8 bit D/A converter is used, so our maximum data values are +127 to -128. If two samples are retrieved from the table, added, and output to the DAC, the values in the table must be between +63 and -64 to avoid overflow. In practice, they will be somewhat smaller since the column tones are pre emphasized. The DAC output value is calculated by MaxOutput = MaxSineValue * (1 + PreEmphasis) To solve for the MaxSineValue, rearrange the equation: MaxSineValue = MaxOutput / (1 + PreEmphasis) = 127 / (1 + 1.25) = 56.4 So the sine values in the table must vary between +/- 56. A C program called MAKESINE.C is available which will output a sine table based on user-defined table size and maximum output value. The output format is compatible with most cross assemblers for Motorola microcontrollers. Benchmarking the Assembly Language Code Two programs are included, DTMF05.ASM and DTMF11.ASM, which implement the algorithm for 68HC05 and 68HC11 processors. Both programs use the timer to generate an interrupt at 128 us intervals; at each interrupt, the program calculates the next value to be output, and then updates the row and column pointers based on pre-calculated pointer intervals. Looking at the code, we see that the 68HC05 takes 116 clocks and 464 bytes of code to generate DTMF; the 68HC11 takes 119 clocks and 457 bytes of code. The additional clock cycles for the HC11 is caused by the requirement to always manipulate 16-bit addresses, whereas the HC05 can get away with byte-size address calculations applied to 16-bit offsets. The HC05, operating at 2 MHz bus speed, uses 116/256 clock cycles (45%) to service the DTMF interrupt. The HC11 is slightly higher at 46%. THD Performance Harmonic Distortion performance has not been measured at this time. I feel that a simple two-pole filter will be sufficient to reduce harmonic content to the point that it is below the 10% limit for network operation.