SPMA084 December 2021 TM4C1230C3PM , TM4C1230C3PM , TM4C1230D5PM , TM4C1230D5PM , TM4C1230E6PM , TM4C1230E6PM , TM4C1230H6PM , TM4C1230H6PM , TM4C1231C3PM , TM4C1231C3PM , TM4C1231D5PM , TM4C1231D5PM , TM4C1231D5PZ , TM4C1231D5PZ , TM4C1231E6PM , TM4C1231E6PM , TM4C1231E6PZ , TM4C1231E6PZ , TM4C1231H6PGE , TM4C1231H6PGE , TM4C1231H6PM , TM4C1231H6PM , TM4C1231H6PZ , TM4C1231H6PZ , TM4C1232C3PM , TM4C1232C3PM , TM4C1232D5PM , TM4C1232D5PM , TM4C1232E6PM , TM4C1232E6PM , TM4C1232H6PM , TM4C1232H6PM , TM4C1233C3PM , TM4C1233C3PM , TM4C1233D5PM , TM4C1233D5PM , TM4C1233D5PZ , TM4C1233D5PZ , TM4C1233E6PM , TM4C1233E6PM , TM4C1233E6PZ , TM4C1233E6PZ , TM4C1233H6PGE , TM4C1233H6PGE , TM4C1233H6PM , TM4C1233H6PM , TM4C1233H6PZ , TM4C1233H6PZ , TM4C1236D5PM , TM4C1236D5PM , TM4C1236E6PM , TM4C1236E6PM , TM4C1236H6PM , TM4C1236H6PM , TM4C1237D5PM , TM4C1237D5PM , TM4C1237D5PZ , TM4C1237D5PZ , TM4C1237E6PM , TM4C1237E6PM , TM4C1237E6PZ , TM4C1237E6PZ , TM4C1237H6PGE , TM4C1237H6PGE , TM4C1237H6PM , TM4C1237H6PM , TM4C1237H6PZ , TM4C1237H6PZ , TM4C123AE6PM , TM4C123AE6PM , TM4C123AH6PM , TM4C123AH6PM , TM4C123BE6PM , TM4C123BE6PM , TM4C123BE6PZ , TM4C123BE6PZ , TM4C123BH6PGE , TM4C123BH6PGE , TM4C123BH6PM , TM4C123BH6PM , TM4C123BH6PZ , TM4C123BH6PZ , TM4C123BH6ZRB , TM4C123BH6ZRB , TM4C123FE6PM , TM4C123FE6PM , TM4C123FH6PM , TM4C123FH6PM , TM4C123GE6PM , TM4C123GE6PM , TM4C123GE6PZ , TM4C123GE6PZ , TM4C123GH6PGE , TM4C123GH6PGE , TM4C123GH6PM , TM4C123GH6PM , TM4C123GH6PZ , TM4C123GH6PZ , TM4C123GH6ZRB , TM4C123GH6ZRB , TM4C1290NCPDT , TM4C1290NCPDT , TM4C1290NCZAD , TM4C1290NCZAD , TM4C1292NCPDT , TM4C1292NCPDT , TM4C1292NCZAD , TM4C1292NCZAD , TM4C1294KCPDT , TM4C1294KCPDT , TM4C1294NCPDT , TM4C1294NCPDT , TM4C1294NCZAD , TM4C1294NCZAD , TM4C1297NCZAD , TM4C1297NCZAD , TM4C1299KCZAD , TM4C1299KCZAD , TM4C1299NCZAD , TM4C1299NCZAD , TM4C129CNCPDT , TM4C129CNCPDT , TM4C129CNCZAD , TM4C129CNCZAD , TM4C129DNCPDT , TM4C129DNCPDT , TM4C129DNCZAD , TM4C129DNCZAD , TM4C129EKCPDT , TM4C129EKCPDT , TM4C129ENCPDT , TM4C129ENCPDT , TM4C129ENCZAD , TM4C129ENCZAD , TM4C129LNCZAD , TM4C129LNCZAD , TM4C129XKCZAD , TM4C129XKCZAD , TM4C129XNCZAD , TM4C129XNCZAD
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Digital audio is a representation of sound recorded in digital form. In a digital audio system, an analog signal representing the sound can be converted, with an analog-to-digital converter (ADC), into a digital signal using the pulse code modulation (PCM) method. In PCM, the amplitude of an analog signal is sampled regularly at uniform intervals and each sample is quantized into digital steps. The uniform interval is called the sampling rate and the number of discrete levels a signal can be quantized into is called the bit depth. For example, CD audio achieves a sampling rate of 44.1 kHz with a bit depth of 16 bits. What this means is that an analog audio signal is sampled 44100 times in a second and the amplitude of the signal is quantized into 216 or 65536 steps.
Once the audio signal is digitized, it can be stored digitally as a file and it can be edited, modified, and manipulated using digital machines such as a computer or MCU. To play the digital audio file on a speaker, a reverse process is performed. The analog audio signal is simply recreated by using a digital-to-analog converter (DAC) to convert the digital signal back into an analog signal. The analog signal is then typically sent through an audio amplifier to boost the signal before reaching the speaker.
Figure 1-1 illustrates a generic digital audio synthesis and playback system. The DAC shown in the figure can be: an integrated component to the MCU, or a discrete component external to the MCU, or emulated by using PWM signal. As TM4C12x MCUs do not have an integrated DAC, this applicable report will demostrate playing back digital audio using both PWM emulation of a DAC and an external discrete DAC.
This application report focuses more on how the TM4C12x MCU can be used to playback rather than recording the digital music stored in an audio file.
Not all microcontrollers have an integrate DAC on chip, but almost all have the capability to generate PWM signals either with a dedicated peripheral or with a timer module. PWM is a low-cost way to control analog devices (a speaker) with a digital output. PWM emulates an analog signal by means of controlling the duty cycle. Suppose a 100% duty cycle PWM is produced from a 3.3 V MCU, it is easily expected the PWM will produce a 3.3 V DC level signal. A 0% duty cycle PWM is expected to produce a 0 V DC signal as well. What about a 10% duty cycle periodic PWM? If a multimeter is used to measure the PWM output, it is not surprised the multimeter measures 3.3 V * 10% = 0.33 V. A 50% duty cycle PWM will measure 3.3 V * 50% = 1.65 V. This relationship between duty cycle and measured voltage is fairly intuitive. This can be concluded with a simple equation as shown in Equation 1:
Earlier in the Section 1, it was mentioned that each time an analog audio signal is sampled, the signal is recorded as a binary number and the length of the binary numbers is called the resolution. In another word, the resolution expresses the number of discrete steps an analog signal can be represented. If the length of binary numbers is 16, this is denoted as 16-bit resolution. A 16-bit binary value can produce 216 = 65536 discrete steps of a signal. Dividing 3.3 V by 65536 is equal to about 50 µV for each step.
Figure 2-1 shows an example analog signal that is digitized with 12 evenly spaced samples at 3-bit resolution. With only 3 bits of resolution, there are only 8 discrete steps. At this resolution, severe quantization error can result which is the difference between the analog signal and the closest available digital value at each sampling instant from the ADC. In Figure 2-1, samples 3, 4 and 5 are converted to the same digital value when they are different in its analog form at the respective sample points.
To recreate the analog signal in Figure 2-1 using PWM, the duty cycle is obtained by dividing the digital value at each sample point by the bit depth which is 23 = 8.
Figure 2-2 illustrates the reproduction of the analog signal based on the sampling rate and resolution shown in Figure 2-1. Although it resembles the original analog signal but it does not come close to true reproduction. Improvements can be made by increasing the sampling rate and resolution. Imagine the original analog signal was digitized at 8 kHz (8000 samples per second) and 8 bit (256 discrete steps) resolution or higher, the original analog signal will be reproduced by the PWM DAC method at much higher accuracy.