Cd-Player, D/A-Converter; Quantization Noise; Reduction Of Jitter - AVM inspiration C6m Operating Instructions Manual

1.5 CD player / D/A- converter
The inspiration C6m is equipped with upsampling circuitry and highly precise a/d converters. The theory of
function will be described in the following text. If You are not interested in technical details, skip these
chapters and simply listen to the music coming from the C6m. You will discover Your CD collection anew!
And that is what we want to achieve. Because application of new technologies is not just a gimmick but
offers audible and measurable advantages to the listener.

1.5.1 Quantization noise

The quantity of information on a CD is defined by the audio format of 44,1 kHz sampling rate and 16 bits of
resolution. Additional information (i.e. higher resolution or bandwidth) cannot be created by any electronic
circuitry playing back such a CD. It is a fact that conventional d-/a converter systems do not fully reproduce
the given information. This has several reasons: Converting a digital signal to an analogue signal produces
analogue noise. This is because the digital (quantized) values which represent the signal are discrete with a
very fine – but nevertheless limited - resolution. Therefore exist slight deviations in respect to the analogue
original signal which was continuous (means infinite resolution). These deviations are random and cause an
additional noise to the original signal when it is converted from the digital domain to the analogue domain.
This kind of noise is called quantization noise.
The characteristic of this noise is that it has an energy which depends on the resolution used to quantize the
original signal and which is continuously spread over the whole range of the sampling frequency bandwidth.
It is obvious that this noise can mask fine details of the originally recorded music.
For physical reasons it is not possible to avoid quantization noise. Also a reduction of the total noise energy
is not possible because the noise has been created when the signal was recorded. An elegant solution of
this problem is to increase sampling frequency when re-converting the signal from digital to analogue. The
upsampling converter installed in the C6m can increase sampling frequency from 44,1 kHz up to 96 kHz.
When re-converting the upsampled signal the upsampling converter produces the same amount of noise
energy as a conventional converter.
The difference is that the noise energy is spread over a much broader frequency band. So the part of noise
energy which is within the audible spectrum decreases. You can imagine that like if You have a certain
volume of fluid in a small glass. If You fill the fluid in a glass which has much more diameter the quantity of
fluid doesn't change but height of the fluid surface will be lower than in the small glass. In the same way the
increasing of sampling frequency (called upsampling) broadens the noise bandwidth and reduces the noise
level. Most of the noise energy now is located in a frequency region beyond the audible range and can easily
be filtered out without affecting the music signal.

1.5.2 Reduction of jitter

Jitter means slight, varying deviations in the sampling frequency of a digital signal. These deviations come
from deviations in speed of the CD when it is played back (a natural effect, which can be reduced by
mechanical means, but never fully eliminated). They can additionally come from electronic circuits through
which the signal must pass. When such a signal is converted to analogue the samples arrive sometimes a
little bit too early, sometimes a little bit too late at the DAC. This leads to modulations in the analogue signal
which can affect the quality of the reproduced music. The spatial image is not precise, You cannot exactly
locate the instruments, the sound is a bit roughened.
The solution for this problem is upsampling. Upsampling does not only mean multiplying of sampling
frequency by a fixed factor like it is done by the oversampling technique used in former times. Upsampling
technique is more similar to recording the original digital signal anew with a different sampling frequency (re-
clocking). That means that the sampling frequency of the original signal and the upsampled signal are fully
independent of each other. Thus if the upsampling converter has a stable jitter free clock the upsampled
signal contains less jitter than the original digital signal.
The musical advantages of re-clocking are the second reason why the AVM inspiration C6m is equipped with
a brand-new upsampling circuitry and an additional stable oscillator circuit.
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