HiFiiPod

The tests are done using Rightmark Audio Analyser. This excellent software allows for testing almost any audio devices capable of recording or playing audio. The result of the tests are then shown as the values and charts.

For testing iPods I am using MOTU 828 audio interface which records the signal from iPod into computer where I then use RMAA to compare the recorded signal with the original.
There are 7 important factors of the tested device which are determined in those tests:

- Frequency response
- Noise level
- Dynamic range
- THD
- IMD + Noise
- Stereo crosstalk
- IMD at 10 kHz

(When looking at those tests it is also important to know the characteristics of the MOTU 828 because all the tests are slightly biased according to this.)

For more test results of various audio soundcards visit Learjeff’s site

Here is the explanation for each term from wikipedia:

Frequency Response

Frequency response is the measure of any system’s response at the output to a signal of varying frequency (but constant amplitude) at its input. It is usually referred to in connection with electronic amplifiers and similar systems. The frequency response is typically characterized by the magnitude of the system’s response, measured in dB, versus frequency. The frequency response of a system can be measured by:

* applying an impulse to the system and measuring its response (see impulse response)
* sweeping a constant-amplitude pure tone through the bandwidth of interest and measuring the output level
* applying a maximum length sequence

Once a frequency response has been measured (e.g., as an impulse response), providing the system is linear and time-invariant, its characteristic can be approximated with arbitrary accuracy by a digital filter. Similarly, if a system is demonstrated to have a poor frequency response, a digital or analog filter can be applied to the signals prior to their reproduction to compensate for these deficiencies.

Frequency responses curves are often used to indicate the accuracy of amplifiers and speakers for reproducing audio. As an example, a high fidelity amplifier may be said to have a frequency response of 20 Hz - 20,000 Hz �1 dB. This means that the system amplifies all frequencies within that range within the limits quoted. ‘Good frequency response’ therefore does not guarantee a specific fidelity, but only indicates that a piece of equipment meets the basic frequency response requirements.

Noise Level

In telecommunication, noise level is the noise power, usually relative to a reference.

Note: Noise level is usually measured in dB for relative power or picowatts for absolute power. A suffix is added to denote a particular reference base or specific qualities of the measurement. Examples of noise-level measurement units are dBa, dBa(F1A), dBa(HA1), dBa0, dBm, dBm(psoph), dBm0, dBm0P, dBrn, dBrnC, dBrn(f 1 - f2), dBrn(144-line), pW, pWp, and pWp0.

Source: from Federal Standard 1037C and from MIL-STD-188

Noise levels are usually viewed in opposition to signal levels and so are often seen as part of a signal-to-noise ratio. Telecommunication systems strive to increase the signal level to noise level ratio in order to effectively transmit data. In practice, if the transmitted signal falls below the level of the noise (often designated as the noise floor) in the system, data can no longer be decoded at the receiver. Noise levels in telecommunication systems are a product of both internal and external sources to the system including shot noise, thermal noise, and ambient electromagnetic interference.

Dynamic Range

Dynamic range is a term used frequently in numerous fields to describe the ratio between the smallest and largest possible values of a changeable quantity.

Audio engineers often use dynamic range to describe the ratio of the loudest possible relatively-undistorted sound to the quietest or to the noise level, say of a microphone or loudspeaker. In digital audio, the maximum possible dynamic range is given by the bit resolution.

Electronics engineers apply the term to:

* the ratio of a specified maximum level of a parameter, such as power, current, voltage or frequency, to the minimum detectable value of that parameter. (See Audio system measurements.)
* In a transmission system, the ratio of the overload level (the maximum signal power that the system can tolerate without distortion of the signal) to the noise level of the system.
* In digital systems or devices, the ratio of maximum and minimum signal levels required to maintain a specified bit error ratio.

In audio and electronics applications, the ratio involved is often so huge that it is converted to a logarithm and specified in decibels.

In music, dynamic range is the difference between the quietest and loudest volume of an instrument, part or piece of music.

THD

The total harmonic distortion, or THD, of a signal is a measurement of the harmonic distortion present, that is, any departure of the output signal waveform from that which should result from the input signal waveforms being operated on by the system’s specified or ideal transfer function. In most cases, this ideal transfer function is linear and time-invariant. When a signal passes through a non-linear device, additional content is added at the harmonics of the original frequencies. This is a measurement of the extent of that distortion.

IMD

Intermodulation distortion: Nonlinear distortion characterized by the appearance, in the output of a device, of frequencies that are linear combinations of the fundamental frequencies and all harmonics present in the input signals.

Note: Harmonic components themselves are not usually considered to characterize intermodulation distortion. When the harmonics are included as part of the distortion, a statement to that effect should be made.

Source: From Federal Standard 1037C and from MIL-STD-188

Stereo Crosstalk

In telecommunication, the term crosstalk (XT) has the following meanings:

1. Undesired capacitive, inductive, or conductive coupling from one circuit, part of a circuit, or channel, to another.
2. Any phenomenon by which a signal transmitted on one circuit or channel of a transmission system creates an undesired effect in another circuit or channel.

Note: In telephony, crosstalk is usually distinguishable as speech or signaling tones.
* In this case the crosstalk is measured between 2 audio channels which together form stereo signal.