Understanding THD+N
Total Harmonic Distortion plus Noise, or THD+N, is one of the most common and
widely used measurements made on audio devices. Virtually every type of device used
to produce, transmit, or process audio signals has a THD+N specification.
Fundamentally, THD+N is a measurement of sound that is not supposed to be there. If
your device is creating a sound, you want it to create that sound and nothing else.
However, all devices create other sounds as well.
The unwanted sounds break down into four basic categories:
. Harmonic Distortion - unwanted sound that is related to the desired sound.
. Random Noise - unwanted sound caused by random fluctuations in electronic
components.
. Interference - unwanted sound related to other signals within the device or
environment.
. Hum - unwanted sound related to the frequency of the main supply voltage.
Harmonic distortion is caused by slight imperfections in the creation or recreation of the
sound. These imperfections generally cause unwanted sounds to be generated at
whole-number frequency multiples of the desired sound.
Random noise is created by almost every type of electronic component. Noise sounds
generally sound like ‘ffff...’ and are generated regardless of whether there is a desired
sound being created or not. Some noise exists at every frequency.
Hum is created by the main supply voltage leaking into the audio signal. It exists at the
frequency of the supply voltage and whole-number multiples of this frequency.
Interference is caused by other signals leaking into the sound signal. Crosstalk is a
certain type of interference caused by sound leaking in from other audio channels within
the device.
The THD+N measurement includes all four types of unwanted sound. If any of these
are excessive, it will be noticeable in a THD+N reading. In this way, a single THD+N
reading can be a good indication that a device is operating properly.
The THD+N reading alone does not tell you which type of unwanted sound is
dominant. If the reading is unexpectedly high, you may need further testing to
determine the source of the unwanted sound. More detail on determining which type of
unwanted sound is causing the problem is given in the section ‘Isolating Sources of
THD+N’ on page 4-20.
4 Functions
Function Descriptions THD+N (Total Harmonic Distortion plus Noise)
ATS-1 Dual Domain User's Manual 4-11
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