Advanced Lecture

Audio Distortion  Distortion is the name given to anything that alters a pure input signal in any way other than changing its magnitude.  The most common forms of distortion are unwanted components or artifacts added to the original signal, including random and hum-related noise.

Seeing Distortion  A spectral analyzer shows unwanted components in an output.  If the OUTPUT = INPUT, there is no distortion

 THD is a measurement of the “purity” of an amplifier in a numerical format  An amplifier can never make an EXACT copy of an Input  For example, a sine wave will never reach the output in exactly the same form as its input

More on THD  A change in the waveform adds a series of "harmonics" of the fundamental frequency.  THD is the RMS voltage equivalent of total harmonic distortion power  The THD number is measured as a PERCENTAGE of the total output RMS voltage.

 What is tested? We look for unwanted signal that is added that is harmonically related to the original signal.  How is it measured? Use a simple SINE WAVE and look at the output to see what is added  This method ONLY looks at harmonics. It is more challenging to eliminate influence of NOISE Measuring THD

 The level of Harmonic Distortion is divided by the fundamental level, and cited as the total harmonic distortion (expressed in percent).  Measuring individual harmonics with precision is difficult, tedious, and not commonly done; consequently, THD+N (see below) is the more common test. Level of THD

What is tested?  Similar to the THD test above, except instead of measuring individual harmonics this tests measures everything added to the input signal.  Tests EVERYTHING including THD and any other NOISE  This can include harmonics, hum, noise, RFI, buzz... everything.

Measuring THD+N How is it measured?  In a SINE WAVE, remove the FUNDAMENTAL and measure ALL other sound that is left  Distortion analyzers make this measurement by using a NOTCH FILTER and measuring what's left  The remainder contains harmonics as well as random noise and other artifacts

 THD+N is always going to be a LARGER number than just plain THD.  Most manufacturers show specs on only THD rather than THD+N even though it is more difficult to measure  It is ultimately to market their product Marketing based on Specs

An example of THD Measurement  American Audio VLP300 Power Amplifier Features:  Output Power: 150W RMS Per 4 Ohms, 1kHz, 0.1% THD / 100W RMS Per 8 Ohms, 1kHz, 0.1% THD / (Bridge Mode, Mono)  300W 8 Ohms, 1K Hz, 1% THD  Total Harmonic Distortion: Less Than 0.1% (20Hz - 8 Ohms)  Frequency response: output Power, 8 Ohms): 20Hz-20KHz

Required Conditions.  Since individual harmonic amplitudes are measured, the manufacturer must state the test signal frequency, its level, and the gain conditions set on the tested unit, as well as the number of harmonics measured.

 THD is often not described in enough detail  It is easy to SKEW the FACTS!  Think of the example of RMS vs PEAK  Correct: THD (5th-order) less than 0.01%, +4 dBu, kHz, unity gain  Wrong: THD less than 0.01% Truth in Advertising?

 Go with a trusted brand first and foremost  You can’t afford to have an amp fail often!  Be sure to understand and compare THD specs on your equipment  The more gear, the more of a chance there is to add THD!  Remember: The SHORTEST distance from one point to another is usually the best! KEY PONTS TO REMEMBER