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Why I2S?
Essensio - Review
Why I2S?
The Model 192 work fine with a different CD-Player or CD-Transport?
When connected with a different CD-Player or CD-Transport we need to use the standard connections S/PDIF and AES/EBU. The Extremo DAC input section allows to reduce the jitter of incoming signal with an anti-jitter circuitry before the analog conversion. This section is based on a digital receiver, the Crystal CS8416-CS; this IC is very important to reach the maximum performances in the digital domain: the digital receiver need to lock correctly the incoming signal (also if jitter affected) and reconstruct, from this digital serial signal, four parallel signals: Master clock, Serial Clock, Left-Right Clock, Serial Data. Inside the DAC the ICs communicate with these four digital signals. The stability of Master Clock and the synchronism of the others signals with this signal is the secret to perform the maximum jitter reduction. To reach this goal, also when the incoming signal is very high jitter noise affected, the Extremo input section reconstruct the Master Clock signal with an anti-jitter circuitry.
The signal outgoing from the CS8416-CS go into the Analog Device AD1896; this IC perform the upsampling from the incoming input frequency to 192 kHz and another anti-jitter treatment with an high precision reference clock; now the signal is ready to be sent at the DAC.
But the new 192kHz/24bit signal is different from the incoming 44.1kHz-16bit signal?
The differences are only in the reduced jitter and the more high sampling frequency; the quantity of the information of the 44.1kHz signal are not changed, no spectral information are added up to 20kHz. But if anything is added to the signal why I need to upsample it?
The upsampled signal have a sampling frequency of 192kHz and a spectrum until 20kHz, the same information (of 44.1kHz signal) in a different form. The digital to analogue conversion of a more high sampling frequency signal is more easy. In fact the analogue stage is a low pass filter with a corner frequency near to the half of sampling frequency (22,050kHz for 44,1kHz signals and 96kHz for 192kHz signals). With a 44.1kHz signal direct converted to the analogue domain we need to design an analogue filter with a corner frequency very near to the maximum frequency present in the musical signal ; with the 192kHz signals it is possible to design the low pass analogue stage with a corner frequency (96kHz) very high with respect the maximum frequency present in to the musical signal (20kHz for the CD), this allowing the reduction of phase rotation in the high frequency of signals and, in this way, a correct reproduction of harmonics of signal.

North Star Design was one of the first to use the upsampling function inside the digital products.

M192 CD-Transport.
The M192 CD-Transport is a specialized CD, CD-R, CD-RW-disc reader. The mechanism is the hi-end Philips CD-PRO2, used only in high-cost CD-Player and CD-Transport. For us the better way to improve the sonic performances is an error free reading of the signal present on the CD-disc.
The CD-PRO2 was designed to work correctly also in the worst conditions of temperature and humidity and in a 90 degree position with respect a plain surface.
The effort for North Star Design was to include this high end precision mechanism in a high value for money product. But this choice is only one of secret of the very high performances of this CD-Transport. The others are the anti-jitter system and the built-in upsampling function.
Like the Extremo, the CD-Transport can operate an anti-jitter function on the signal incoming from the CD-PRO2; the native signal from the mechanism is in I2S form and with low jitter noise; now the same anti jitter system work on a signal witch have a different form (parallel) with respect the digital signal in S/PDIF,AES/EBU format (serial) and don't need to extract the Master Clock from the incoming signal, the Master Clock and the other signals are generated directly from CD-PRO2 mechanism.
The upsampling function inside the CD-Transport allow to have a 96kHz digital signal on a standard AES/EBU output connection.

But the most relevant feature is the I2S external connection between the CD-Transport and the DAC M192.

The North Star design digital combo was designed to obtain the maximum sonic performance from the synergy of I2S connection.

The goal of the I2S external connection is to transmit the signal from the CD-Transport to the DAC in the I2S native format; in this way we can connect directly the CD-Transport to the DAC avoiding the path trough the digital receiver and to going directly at the DAC chip input. This connection was optimized to obtain a very low jitter transmission with a very high frequency signals (the Master Clock frequency is 24,576MHz).

The most important advantage of this connection is to avoid the double conversion from I2S to S/PDIF or AES/EBU inside the CD-Transport and vice-versa inside the DAC; with the I2S connection the Master Clock is generated inside the CD-Transport and all the signals are synchronized with this and sent directly to the DAC. In this way only one well defined clock synchronize the digital combo operations.
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