Why should we carefully increase the THD of the audio processing system?

THD (Total Harmonic Distortion) is an indicator of signal harmonic distortion, expressed as the ratio of the sum of the power of all harmonic components to the power of the fundamental frequency signal. The lower total harmonic distortion makes the audio, electronic amplifier or microphone and other equipment produce more accurate, less harmonic, and close to the original sampled output signal.

In order to obtain higher fidelity of the audio system, the article will introduce a new concept. Many systems, especially those applied to the home theater / mini-mini band market, are careful to add distortion to the output signal. Although this seems to be inconsistent with our common sense, there are reasons why designers consider doing so. The main purpose of this technique is to maximize the average power output while limiting the occurrence of peaks.

Some customers use the same power amplifier IC in a number of products. This allows them to purchase a device in larger quantities, thereby reducing costs and simplifying inventory. They may use a small power supply to save costs. Customers will use a closed loop, fixed gain amplifier with a low power supply. It limits the output voltage swing (by limiting the output), which can protect the low-power power supply from damage due to overcurrent conditions. However, a simple attenuator can make the system quieter. Distorting the output slightly can greatly increase the perceived RMS power. Care must be taken when determining the degree of distortion, not too much!

For other customers, limiting the voltage output of their signals can help limit speaker drift. However, care should be taken in this case because the high RMS power entering the speaker may cause reliability problems.

In a digital processing system, THD can be introduced to the signal by saturating the digital samples. That is, using sufficient gain to shift the most significant bit beyond the digital sample size. For example, if you have a 24-bit word and your sample is 0x900000. With a gain of 12 Db, the highest audio bit exceeds the most significant bit (MSB) of the sample.

After that, lower the data to the audio output level you need. Therefore, it can be summarized as:

The amplified signal increases THD for clipping

Figure 1. The amplified signal increases THD for clipping, and then reduces the output to produce a more average power with a specific peak-to-peak voltage

This sounds simple, but many audio processors are not actually the most significant bits = full-range audio. For example, some TI audio processors use a data format called 9.23. This kind of sampling data can represent 16-bit or 24-bit data in the following ways:

Map standard 16-bit or 24-bit audio samples to 32-bit or 48-bit memory locations

Figure 2 Maps standard 16-bit or 24-bit audio samples to 32-bit or 48-bit memory locations

As you can see, MSB and LSB have added some padding. LSB is easy to understand-if you cut a 16-bit word (using a CD player), you still have some bits that can be copied without deleting.

At the top, there are 9 bits in total to prevent accidental saturation of audio data. For example, if you use a 24-dB boosted equalizer (EQ), and you enter a "full-scale" 16-bit word, you may unintentionally saturate the signal, that is, increase the distortion, which is different from ours. The direction of efforts runs counter to each other.

There is amplitude loss during clipping, so THD (back) may allow a small amount of gain to pass through the THD manager. 10% distortion clipping brings about –1dB output level loss.

In our example, the system has a 9.23 audio path. We want to produce 10% THD at –12 dB output. The average input is –10 dBFS (–10 dB refers to a 24-bit full-scale audio source).

We need to zoom in to full range and above (9 digits for "overflow bit"). Therefore, in a booster module, we add 10 dB to the original source to reach full scale, and then add 27 dB to fill the 9 overflow bits. Now, increase the gain by 3dB to clip the signal. In total, we need to increase the gain by 40dB.

Now, we have a signal that fills the MSB of the audio path and requires a cut so that the content can be output at –12 dB. This means cutting 39dB. The resulting output has 10% distortion and the output level is –12 dB. Look! We have now increased the RMS power (by increasing the distortion) at –12 dB output, and at the same time made the work of the power supply and the speaker easier and more comfortable.


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