Over the years, various enhancements to the CD format have been marketed with promises of higher sound quality through the use of superior materials. One of the more recent entrants is UHQCD. We examine the performance of a CD player while playing such a disc.
A UHQCD differs, according to the manufacturer, from a regular CD in two ways. Firstly, the reflective layer is composed of “a unique and expensive alloy” rather than the more commonly used aluminium. Secondly, a low viscosity polymer layer supposedly improves the shaping of the pits during the stamping process.
These changes are purported to improve the high-frequency waveform from the optical pickup. The provided figure even suggests that the amplitude is increased, though no explanation is offered as to how this might translate into improved sound quality.
In a CD player, the laser light reflected by the disc is detected by a photodiode and converted to an electrical signal and amplified. This is the high-frequency, or HF, signal. Digital bits are encoded through the timing of edges in this waveform; the precise level between edges is unimportant.
To test the UHQCD claims, we will first attempt to reproduce the oscilloscope images depicting the HF signal using a Philips CD150 player. This old player ought, if anything, be more sensitive to disc quality than a modern one. It is also easy to access various signals for measurements. Regular CDs are represented by a cheap disc from Naxos and a more expensive Deutsche Grammophon issue.
Figures 3-5 show the HF signal produced by each of the three test discs. Although there is clear variation between the images, it looks nothing like the comparison on the official website (Fig. 2). Particularly, the marked increase in signal amplitude is nowhere to be seen. To its credit, the UHQCD does produce somewhat sharper traces and a more consistent amplitude. It should be noted, however, that a prettier scope image at this stage does not necessarily mean the sound will be better. All three waveforms are plenty clean enough to allow successful digital data recovery in the subsequent processing stages.
The demodulator uses an edge detector and a PLL to recover the clock from the HF signal. Although this clock is not used by the DAC, its quality may still tell us something about how easy or difficult the disc is to read. Specifically, irregularly spaced pits will likely manifest as increased jitter in the recovered clock. The following figures show a single edge of the recovered clock with a delay of 10 periods from the trigger point.
Some spread here is expected due to the impossibility of maintaining a perfectly constant linear velocity of the disc track. What is interesting here is that the Naxos disc shows somewhat greater peak to peak variation than the other two. The distribution of the zero-crossings, shown in the histogram at the top of the scope images, differs somewhat between the Deutsche Grammophon disc and the UHQCD. Calling either of them better than the other would, however, be a bit of a stretch.
What matters in the end is the quality of the clock signal driving the conversion in the DAC chip. This CD player uses a pair of TDA1540P chips running at 176.4 kHz. Conversion is simply triggered by a pulse causing the chip to latch the sample value from a shift register. With the scope delay set to one sample interval, we first look at this signal while the player is idle, feeding the DAC section silence. This provides a baseline against which to compare the same signal while playing different discs.
Even with no disc playing, the DAC clock exhibits some jitter with the edges concentrated around two locations not quite 1 ns apart. A possible explanation for this is that the circuit generating this signal from the reference clock has two paths with different propagation delays.
As seen in the figures above, playing a disc causes the DAC clock jitter to increase somewhat. The two lines seen in the idle state become more blurry although the distance between their centres remains unchanged. Although the waveform snapshots differ slightly, the histograms are virtually identical, and none of the images exhibit any feature setting them apart from the others.
In summary, the UHQCD produces a cleaner high frequency waveform from the optical pickup. While the initial recovered clock signal has a little less jitter than that from a cheap CD, it does not differ appreciably when compared to a more expensive disc. By the time the playback process reaches the DAC chip, the three test discs are indistinguishable. Spending extra money on these discs for the better materials would appear to be a waste.