The compact disc format has been available for over four decades, and in that time the engineering understanding of what determines CD player performance has advanced considerably beyond the early debates about error correction and transport mechanisms. What matters in a CD player or digital transport today is well understood — and some of it is counterintuitive.
The disc reading mechanism — the transport — does matter, but not primarily for the reasons often cited. Modern optical mechanisms read disc data with very high reliability, and data errors in the digital domain are corrected before they ever affect the output. A disc that is scratched or dirty may increase the error correction burden on the mechanism, but the output data remains bit-perfect unless the errors exceed the system’s correction capability. The transport’s primary contribution to sound quality is therefore not data integrity — it is the mechanical environment in which the disc spins and the electrical noise it introduces into the player’s circuitry.
Jitter — timing irregularity in the digital data stream — is the most significant and most debated parameter in digital source performance. When digital audio data is converted to analogue, the conversion must occur at precise, equally spaced intervals. Any variation in the timing of these intervals — however small — produces an error in the analogue output that manifests as noise and distortion, concentrated in the audio band. Jitter is measured in picoseconds and is typically inaudible at very low levels, but at higher levels it introduces a characteristic graininess and loss of low-level resolution that experienced listeners can identify.
The analogue output stage of a CD player — the circuitry that receives the DAC’s output and buffers it for the next component in the chain — is frequently the most significant determinant of sonic character among players whose digital sections are of similar quality. A well-designed, low-noise analogue output stage with appropriate bandwidth and output impedance will transmit the DAC’s output faithfully; a poorly designed one will add its own character.
What does not matter — or matters far less than its prominence in product marketing suggests — is the number of DAC chips, the bit depth and sample rate of the DAC section, or the weight and rigidity of the transport mechanism beyond a functional minimum. Bit-perfect data delivered to a low-jitter conversion circuit through a well-designed output stage is the engineering goal. The mechanism by which this is achieved is less important than whether it is achieved.