By John Siau
Variable-pitch features add versatility to CD players. Unfortunately, these features usually create jitter problems. Benchmark’s UltraLock™ jitter-attenuation system provides a unique solution that is fully compatible with variable-pitch transports.
Variable pitch is a popular feature on professional and semi-professional CD players. The variable pitch can be used to vary the tempo of the music by as much as +/- 15% on many players. In DJ applications the pitch control can be used as a bend control, or for tempo or pitch matching when fading from one song to the next. In live sound applications, small pitch adjustments can allow mixing of CD accompaniments with live instruments where the tuning of the live instruments may differ slightly from the recording. Dance groups often practice new performances at a reduced tempo when they are learning a new choreography. These are just some of the applications for variable pitch, and it is easy to see why this has become such a popular feature on professional CD players.
Unfortunately, the variable pitch circuitry almost always adds high levels of jitter to the output of the CD player. High jitter levels are often present even when the pitch control is set to “0”. Variable-pitch CD players use a master oscillator that can be pulled +/- 15% (or more) from the nominal sample rate. This oscillator usually has much higher jitter than the fixed-frequency crystal oscillator used in non-variable pitch players. Users of professional CD players expect high performance from these premium-priced machines, but these machines often under-perform because of jitter problems. The jitter performance of most professional players is not specified and consequently it is very hard to make an intelligent decision when selecting a player. Users who don’t need variable pitch will discover that it is very difficult to find a professional transport that does not have a variable-pitch feature.
The Benchmark DAC1 and DAC2 family of converters offers a unique solution to the jitter produced by variable-pitch machines. DAC1 converters use Benchmark’s proprietary UltraLock™ jitter-attenuation system. DAC2 converters feature Benchmark new UltraLock2™ jitter-attenuation. Both systems remove virtually all of the jitter on any digital input signal.
Variable pitch systems change the digital sample rate by as much as +/- 15% in order to shift the pitch of the music. This means that most external converters will not operate if the variable pitch feature is activated. In contrast, Benchmark DAC1 and DAC2 converters accept any input frequency between 24 and 195 kHz and are fully compatible with variable-pitch features. Most competing systems are restricted to the standard sample rates (44.1, 48, 88.2, 96, 176.4, and 192 kHz). Any slight deviation (+/- 0.005%) from these frequencies will cause most competing systems to lose lock and/or lose their ability to attenuate jitter. Competing systems that can accept non-standard rates usually have little or no jitter attenuation. In contrast, the Benchmark systems are frequency agile and jitter immune.
The DAC1 or DAC2 must be connected to a digital output from the CD player. Fortunately, most professional CD players have digital outputs, and in most cases, these outputs remain active over the entire pitch control range. When this is the case, the DAC1 or DAC2 bypass all of the analog circuitry in the player, removing the jitter, and providing mastering-quality balanced and unbalanced analog outputs. A DAC1 or DAC2 converter will provide a very significant upgrade to any CD player that is equipped with a digital output. This upgrade can be even more significant when performance of the stock player is degraded by the presence of a variable-pitch circuit.
UltraLock™ and UltraLock2™ remove the problems created by variable-pitch circuits without restricting the useful range of the pitch-variation feature.
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