We wrote this to try to explain why comparison tests are mostly valid for analogue cables, but are system and environment specific for digital cables:
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A comparison of signal and noise transmission in Analogue vs Digital audio.
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Effects in analogue transmission are easy to understand.
The signal is transmitted in the same form in which it will be directly converted to sound. Any ‘in-band’ alteration to this signal directly affects the resultant sound in the same way as the alteration to the signal. This means that such alterations may potentially be heard in the same way as the alteration is made – this directness results on listening test giving reasonably consistent results.
An exception to that is out of band signals (such as spurious ultrasonic frequency components, such as from early CD players). These signals affect the way amplifiers behave when amplifying the ‘in-band’ signal. This means that out of band signals will have different effects dependent on the interaction of the signal and the design of the amplifier(s). In the case of analogue transmission this indirect effect is usually smaller than the direct effect.
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Effects in digital transmission are more difficult to understand.
The signal is not transmitted in a form that is directly related to the sound produced; rather being transmitted as a set of numbers encoded on an electrical signal. If the signal itself is altered, i.e. the numbers are changed, the system will correct or resend and the signal is restored. However, the numbers are transmitted using an electrical signal, and this signal also carries with it electrical noise, usually mostly at frequencies outside the audio band. This signal can affect the following analogue amplifiers (including the DAC) in a manner similar to out of bound signals in the case of analogue transmission.
Since this electrical signal isn’t the same signal as the sound that will be produced by decoding, the effect of changes to the electrical signal are not directly connected to changes in the sound produced; the effects are due to interaction of the interference caused by the electrical signal affecting the way the DAC and amplifiers behave. As this is not a direct effect on the signal, there is no way to predict how the effects on the sound will change if either the electrical signal or the amplifiers are changed. There are, however, two effects that are likely to be predictable to some degree, provided the reliability of data transmission is maintained:
- Reducing the amplitude of the noise on the electrical signal will reduce the unwanted effects.
- Reducing the amplitude of the electrical signal will also reduce unwanted effects.
Note that reducing the amplitude of the noise on the electrical signal by analogue filtering will also reduce the sharpness of the edges on which digital data transmission relies for decoding; excessive filtering will reduce noise, but also degrade the quality digital transmission, and this may cause more serious unwanted effects.