04-01-12: FmnpFrom this white paper by the noted audio designer Charles Hansen of Ayre:
I don´t know what are the technical reason's that you mentioned but I'll be glad to read them.
... the standard S/PDIF (Sony/Philips Digital Interface Format) digital connection used with two-box solutions is flawed, unavoidably and needlessly introducing jitter into the audio playback system.Also see the following paper by the distinguished authority and academician Malcolm Hawksford:
A low-jitter master audio clock is essential for attaining high performance levels of digital audio reproduction. But that low jitter only matters at one critical point—at the D/A chip itself. Jitter-induced timing errors create artifacts that audibly degrade the music signal. Therefore a well-designed one-box disc player places a fixed-frequency master audio clock right next to the D/A chip for the best possible performance.
In contrast, a two-box system splits the system into a disc transport box and a D/A converter box. The two are normally connected with the industry-standard S/PDIF connection which places the master audio clock in the transport box, where it is mixed together with synchronization codes and the audio data and transmitted to the D/A converter box. The D/A converter box must then attempt to recover the critical master audio clock from this jumble of signals for delivery to the D/A chip itself.
The standard solution for a two-box disc player is to use a PLL (Phase-Locked Loop) to control a VCO (Voltage-Controlled Oscillator) in the D/A box, generating the master audio clock. The VCO varies its frequency in order to lock onto the incoming signal sent from the transport box. Unfortunately, a variable-frequency oscillator simply cannot achieve the low-jitter performance of a fixed-frequency crystal oscillator.
Over the years many schemes have been implemented by various manufacturers in attempts to improve the jitter performance of the S/PDIF connection, including dual PLL’s, VCXO’s (Voltage-Controlled Crystal Oscillators), frequency synthesizers, FIFO (First-In, First-Out) buffers for the audio data, external re-clocking (”jitter reduction”) devices, and so forth. While all of these methods are able to reduce the jitter levels, they cannot eliminate the jitter that is inherently added by the S/PDIF connection.
Another approach to reduce jitter that has become increasingly popular in recent years is to use an ASRC (Asynchronous Sample Rate Converter) chip. The idea is that the original audio data is replaced with newly calculated data that represents what the audio data would have been if the incoming signal had most of the jitter filtered out. The technical theory behind this method is sound, as demonstrated by the measured performance, which is generally quite good. However the audible performance of these devices is controversial, and Ayre has avoided this approach as it completely discards the original audio data.
Is The AESEBU/SPDIF Digital Audio Interface Flawed?
And these articles as well:
The Jitter Game
Jitter in Digital Audio Data Streams
In addition to the problems that are inherent in extracting a jitter-free clock from the single S/PDIF signal that combines clock and data and other information, the precision with which the DAC can detect the timing of the signal transitions (between the voltage states corresponding to 0's and 1's) that ultimately are used to perform that extraction will be limited by noise on the signal waveform, by distortion of the waveform, and by the bandwidth limitations of the interface (resulting in slow risetimes and falltimes). All of these things will contribute, to some degree, to fluctuations from one clock interval to the next in the timing with which the start and end of the interval is sensed by the DAC.
Toslink has inherently limited bandwidth, and correspondingly slow risetimes and falltimes. Risetimes and falltimes of electrical S/PDIF signals are intentionally limited in order to reduce RFI emissions. Noise is always present to some degree on electrical signals, and in the case of coaxial S/PDIF will be contributed to by ground loop effects between the connected components, as well as by noise generated in the connected components which couples onto the signal, and by pickup in the cabling. Waveform distortion will arise in coaxial S/PDIF connections as a result of reflections caused by impedance mismatches between the cable and the components that it connects. Also, anecdotal indications seem to be that Toslink commonly suffers from low transducer quality, which contributes to jitter.
Regards,
-- Al