Digital audio quality - DAQ - is the ability of an audio system to recreate the characteristics of an original digital sound signal. Digital audio and transmission system impairments include codec (compression) type, packet loss, packet corruption and echo (for two-way systems). Some of the measures of audio quality include fidelity, frequency response, total harmonic distortion, crosstalk, noise level and signal to noise ratio.

Fidelity - Audio fidelity is the degree to which a system, or a portion of a system, accurately reproduces at its output the essential characteristics of the signal impressed upon its input.

Frequency response - Frequency response is the ability of a system or device to process audio across a specified bandwidth. The lower the frequency response range, the smaller the amount of bandwidth is needed to transfer the audio signal.

Total harmonic distortion - THD - Total harmonic distortion is a ratio of the combined amplitudes of all signals related harmonically to the amplitude of a fundamental signal. THD is typically expressed as a percentage of signal level.

Crosstalk - Crosstalk is the undesired leakage of a signal from one communications channel to another, which may be expressed as a measure of isolation between a desired channel and another channel.

Noise level - Noise level is a measure of the energy of unwanted signals over an entire frequency range.

Echo - Echo is a type of audio transmission distortion which is caused when a signal is reflected back to the originating source.

This figure shows some of the causes and effects of audio distortion in IP audio systems. This example shows that packet loss results in the temporary muting of the audio signal. Packet corruption results in the creation of a different altered sound than the sound that was previously transmitted. Echo results from some of the caller's audio signal being sent back (audio feedback) by the receiver of the call.